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THE DIGESTIVE SYSTEM
• AIM
• After completion of this system the students should be able to gain knowledge in depth anatomy and physiology of digestive system.
• SPECIFIC OBJECTIVES
• List out the main organs of the alimentary tract.
• List out the accessory organs of the digestive system.
• Describe general structure of alimentary canal.
• Describe the distribution of the peritoneum.
• Describe the structure and functions of the mouth.
• Describe the structure and functions of the salivary gland.
• Describe the structure and functions of the oesophagus.
• Describe the structure and functions of the stomach.
• Describe the structure and functions of the small intestine.
• Identify the different sections of the large intestine.
• Describe the structure and functions of the large intestine.
• Describe the structure and functions of the accessory organs.
• List out the principal digestive enzyme and their actions.
• Explain general principles of metabolism.
• Define terms of metabolism, anabolism, catabolism, unit of energy and metabolic rate.
• The digestive system
• The digestive system is the collective name used to describe the alimentary canal, some accessory organs and a variety of digestive processes which take place at different levels in the canal to prepare food eaten in the diet for absorption.
• The alimentary canal begins at the mouth, passes through the thorax, abdomen and pelvis and ends at the anus .It has a general structure which is modified at different levels to provide for the processes occurring at each level . The complex of digestive processes gradually breaks down the foods eaten until they are in a form suitable for absorption
• After absorption, nutrients are used to synthesis body constituents. They provide the raw materials for the manufacture of new cells, hormones and enzymes, and the energy needed for these and other processes and for the disposal of waste materials.
• What are the activities of digestive system?
• Ingestion. This is the process of taking food into the alimentary tract.
• Propulsion. This moves the contents along the alimentary tract.
• Digestion. This consists of:
• • mechanical breakdown of food by, e.g. mastication(chewing)
• • chemical digestion of food by enzymes present in secretions produced by glands and accessory organs of the digestive system.
• Absorption. This is the process by which digested food substances pass through the walls of some organs of the alimentary canal into the blood and lymph capillaries for circulation round the body.
• Elimination. Food substances which have been eaten but cannot be digested and absorbed are excreted by the bowel as faeces
• ORGANS OF THE DIGESTIVE SYSTEM
• Mouth
• Pharynx
• Oesophagus
• Small intestine
• Large intestine
• Rectum and anal canal
• ACCESSORY ORGANS
• Salivary gland
• The pancreas
• The liver and biliary tract
• General structure of alimentary tract
• Adventitia or serosa –outer covering
• Muscle layer
• Sub mucosa
• Mucosa lining
• Adventitia (outer covering)
• In the thorax this consists of loose fibrous tissue and in the abdomen the organs are covered by a serous membrane called peritoneum.
• Peritoneum (serosa)
• The peritoneum is the largest serous membrane of the body . It consists of a closed sac, containing a small amount of serous fluid, within the abdominal cavity. It is richly supplied with blood and lymph vessels, and contains a considerable number of lymph nodes.
• It provides a physical barrier to local spread of infection, and can isolate an infective focus such as appendicitis, preventing involvement of other abdominal structures.
It has two layers:
• the parietal layer, which lines the abdominal wall
• the visceral layer, which covers the organs (viscera) within the abdominal and pelvic cavities.
• The arrangement of the peritoneum is such that the organs are invaginated into the closed sac from below, behind and above so that they are at least partly covered by the visceral layer.
• What is the meaning of that?
• pelvic organs are covered only on their superior surface
• the stomach and intestines, deeply invaginated from behind, are almost completely surrounded by peritoneum and have a double fold (the mesentery) that attaches them to the posterior abdominal wall. The fold of peritoneum enclosing the stomach extends beyond the greater curvature of the stomach, and hangs down in front of the abdominal organs like an apron. This is the greater omentum, and it stores fat, which provides both insulation and along-term energy store
• the pancreas, spleen, kidneys and adrenal glands are invaginated from behind but only their anterior surfaces are covered and are therefore retroperitoneal
• the liver is invaginated from above and is almost completely covered by peritoneum which attaches it to the inferior surface of the diaphragm
• the main blood vessels and nerves pass close to the posterior abdominal wall and send branches to the organs between folds of peritoneum
• The two layers of peritoneum are actually in contact and friction between them is prevented by the presence of serous fluid secreted by the peritoneal cells, thus the peritoneal cavity is only a potential cavity. A similar arrangement is seen with the membranes covering the lungs, the pleura. In the male it is completely closed but in the female the uterine tubes open into it and the ovaries are the only structures inside
• Muscle layer (muscularis)
• With some exceptions this consists of two layers of smooth (involuntary) muscle. The muscle fibres of the outer layer are arranged longitudinally, and those of the inner layer encircle the wall of the tube. Between these two muscle layers are blood vessels, lymph vessels and a plexus (network) of sympathetic and parasympathetic nerves, called the myenteric or Auerbach's plexus. These nerves supply the adjacent smooth muscle and blood vessels.
• Contraction and relaxation of these muscle layers occurs in waves which push the contents of the tract onwards. This type of contraction of smooth muscle is called peristalsis .Muscle contraction also mixes food with the digestive juices. Onward movement of the contents of the tract is controlled at various points by sphincters consisting of an increased number of circular muscle fibres. They also act as valves preventing backflow in the tract. The control allows time for digestion and absorption to take place.
• Submucosa
• This layer consists of loose connective tissue with some elastic fibres. Within this layer are plexuses of blood vessels and nerves, lymph vessels and varying amounts of lymphoid tissues. The blood vessels consist of arterioles, venules and capillaries. The nerve plexus is the submucosal or Meissner's plexus, consisting of sympathetic and parasympathetic nerves which supply the mucosal lining.
• Mucosa
• This consists of three layers of tissue:
1. mucous membrane
2. lamina propria
3. muscularis mucosa
• mucous membrane formed by columnar epithelium is the innermost layer and has three main functions: protection, secretion and absorption
• lamina propria consisting of loose connective tissue which supports the blood vessels that nourish the inner epithelial layer, and varying amounts of lymphoid tissue that has a protective function
• muscularis mucosa, a thin outer layer of smooth muscle that provides involutions of the mucosa layer, e.g. gastric glands, villi.
• Mucous membrane
• In parts of the tract which are subject to great wear and tear or mechanical injury this layer consists of stratified squamous epithelium with mucus-secreting glands just below the surface. In areas where the food is already soft and moist and where secretion of digestive juices and absorption occur, the mucous membrane consists of columnar epithelial cells interspersed with mucus-secreting goblet cells
• Mucus lubricates the walls of the tract and protects them from digestive enzymes. Below the surface in the regions lined with columnar epithelium are collections of specialized cells, or glands, which pour their secretions into the lumen of the tract
• What are the secretions?
• saliva from the salivary glands
• gastric juice from the gastric glands
• intestinal juice from the intestinal glands
• pancreatic juice from the pancreas
• bile from the live
• Nerve supply
• The parasympathetic supply. This supply to most of the alimentary tract is provided by one pair of cranial nerves, the vagus nerves. Stimulation causes smooth muscle contraction and the secretion of digestive juices. The most distal part of the tract is supplied by sacral nerves.
• The sympathetic supply. This is provided by numerous nerves which emerge from the spinal cord in the thoracic and lumbar regions. These form plexuses in the thorax, abdomen and pelvis, from which nerves pass to the organs of the alimentary tract. Their action is to reduce smooth muscle contraction and glandular secretion.
• Within the walls of the canal there are two nerve plexuses from which both sympathetic and parasympathetic fibres are distributed.
• The myenteric or Auerbach's plexus lies between the two layers of smooth muscle that it supplies, and influences peristalsis.
• The submucosal or Meissner's plexus lies in the submucosa and supplies the mucous membrane and secretory glands.
• Blood supply
• In the thorax. The oesophagus is supplied by paired oesophageal arteries, branches from the thoracic aorta
• In the abdomen and pelvis. The alimentary tract, pancreas, liver and biliary tract are supplied by the unpaired branches from the aorta: the coeliac artery and the superior and inferior mesenteric arteries and. The coeliac artery divides into three branches which supply the stomach, duodenum, pancreas, spleen, liver, gall bladder and bile ducts.
• The superior mesenteric artery supplies the whole of the small intestine, the caecum, ascending colon and most of the transverse colon.
• The inferior mesenteric artery supplies a small part of the transverse colon, the descending colon, sigmoid colon and most of the rectum.
• The distal part of the rectum and the anus are supplied by the middle and inferior rectal arteries, branches of the internal iliac arteries.
• The arteries supplying the stomach and intestines pass between the layers of peritoneum from the posterior abdominal wall to the organs
• Venous drainage
• In the thorax. Venous blood from the oesophagus passes in the oesophageal veins to the azygos and hemiazygos veins. The azygos vein joins the superior vena cava near the heart, and the hemiazygos joins the left brachiocephalic vein.Some blood from the lower part of the oesophagus drains into the left gastric vein. There are anastomotic vessels between the azygos, hemiazygos and left gastric veins.
• In the abdomen and pelvis. The veins that drain blood from the lower part of the oesophagus, the stomach, pancreas, small intestine, large intestine and most of the rectum join to form the portal vein. This blood, containing a high concentration of absorbed nutritional materials, is conveyed first to the liver then to the inferior vena cava.
• Blood from the lower part of the rectum and the anal canal drains into the internal iliac veins. This blood is delivered directly into the inferior vena cava, hence bypassing the hepatic portal circulation.
• MOUTH
• The mouth or oral cavity is bounded by muscles and bones:
• Anteriorly —by the lips
• Posteriorly —it is continuous with the oropharynx
• Laterally —by the muscles of the cheeks
• Superiorly —by the bony hard palate and muscularsoft palate
• Inferiorly —by the muscular tongue and the soft tissues of the floor of the mouth.
• The oral cavity is lined throughout with mucous containing small mucus-secreting glands
• The part of the mouth between the gums (alveolarridges) and the cheeks is the vestibule and the remainder of the cavity is the mouth proper. The mucous membrane lining of the cheeks and the lips is reflected on to the gums or alveolar ridges and is continuous with the skin of the face.
• The palate forms the roof of the mouth and is divided into the anterior hard palate and the posterior soft palate. The bones forming the hard palate are the maxilla and the palatine bones. The soft palate is muscular, curves downwards from the posterior end of the hard palate and blends with the walls of the pharynx at the sides.
• The uvula is a curved fold of muscle covered with mucous membrane, hanging down from the middle of the free border of the soft palate. Originating from the upper end of the uvula there are four folds of mucous membrane, two passing downwards at each side to form membranous arches. The posterior folds, one on each side, are the palatopharyngeal arches and the two anterior folds are the palatoglossal arches. On each side, between the arches, is a collection of lymphoid tissue called the palatine tonsil.
• Tongue
• The tongue is a voluntary muscular structure which occupies the floor of the mouth. It is attached by its base to the hyoid bone and by a fold of its mucous membrane covering, called the fermium, to the floor of the mouth. The superior surface consists of stratified squamous epithelium, with numerous papillae (little projections), containing nerve endings of the sense of taste, sometimes called the taste buds.
• Types of papillae
• Vallate papillae
• Fungiform papillae
• Filiform papillae
• Vallate papillae
• Vallate papillae, usually between 8 and 12 altogether, are arranged in an inverted V shape towards the base of the tongue. These are the largest of the papillae and are the most easily seen.
• Fungiform papillae
• Fungiform papillae are situated mainly at the tip and the
• edges of the tongue and are more numerous than the vallate
• papillae.
• Filiform papillae
• Filiform papillae are the smallest of the three types. They are most numerous on the surface of the anterior two-thirds of the tongue
• Blood supply
• The main arterial blood supply to the tongue is by the lingual branch of the external carotid artery. Venous drainage is by the lingual vein which joins the internal jugular vein
• Nerve supply
• the hypoglossal nerves (12th cranial nerves) which supply the voluntary muscle tissue
• the lingual branch of the mandibular nerves which are the nerves of somatic (ordinary) sensation, i.e. pain, temperature and touch
• the facial and glossophanryngeal nerves (7th and 9thcranial nerves) which are the nerves of the special sensation of taste.
• Functions of the tongue
• mastication (chewing)
• deglutition (swallowing)
• speech
• taste
• Nerve endings of the sense of taste are present in the papillae and widely distributed in the epithelium of the tongue, soft palate, pharynx and epiglottis
• Teeth
• The teeth are embedded in the alveoli or sockets of the alveolar ridges of the mandible and the maxilla. Each individual has two sets, or dentitions, the temporary or deciduous teeth and the permanent teeth . At birth the teeth of both dentitions are present in immature form in the mandible and maxilla
• There are 20 temporary teeth, 10 in each jaw. They begin to erupt when the child is about 6 months old, and should all be present after 24 months
• The permanent teeth begin to replace the deciduous teeth in the 6th year of age and this dentition, consisting of 32 teeth, is usually complete by the 24th year
• Functions of the teeth
• The incisor and canine teeth are the cutting teeth and are used for biting off pieces of food, whereas the premolar and molar teeth, with broad, flat surfaces, are used for grinding or chewing food
• Structure of a tooth
• Although the shapes of the different teeth vary, the structure is the same and consists of:
• the crown — the part which protrudes from the gum
• the root — the part embedded in the bone
• the neck — the slightly narrowed region where the crown merges with the root.
• In the centre of the tooth is the pulp cavity containing blood vessels, lymph vessels and nerves, and surrounding this is a hard ivory-like substance called dentine. Outside the dentine of the crown is a thin layer of very hard substance, the enamel. The root of the tooth, on the other hand, is covered with a substance resembling bone, called cement, which fixes the tooth in its socket. Blood vessels and nerves pass to the tooth through a small foramen at the apex of each root.
• Blood supply
• Most of the arterial blood supply to the teeth is by branches of the maxillary arteries. The venous drainage is by a number of veins which empty into the internal jugular veins
• Nerve supply
• The nerve supply to the upper teeth is by branches of the maxillary nerves and to the lower teeth by branches of the mandibular nerves. These are both branches of the trigeminal nerves (5th cranial nerves).
• SALIVARY GLANDS
• Salivary glands pour their secretions into the mouth. There are three pairs:
• parotid glands
• submandibular glands
• sublingual glands.
• Parotid glands
• These are situated one on each side of the face just below the external acoustic meatus. Each gland has a parotid duct opening into the mouth at the level of the second upper molar tooth.
• Submandibular glands
• These lie one on each side of the face under the angle of the jaw. The two submandibular ducts open on the floor of the mouth, one on each side of the frenulum of the tongue.
• Sublingual glands
• These glands lie under the mucous membrane of the floor of the mouth in front of the submandibular glands. They have numerous small ducts that open into the floor of the mouth.
• Structure of the salivary glands
• The glands are all surrounded by a fibrous capsule. They consist of a number of lobules made up of small acini lined with secretory cells. The secretions are poured into ductules which join up to form larger ducts leading into the mouth.
• Nerve supply
• The glands are supplied by parasympathetic and sympathetic nerve fibres. Parasympathetic stimulation increases secretion, whereas sympathetic stimulation decreases it.
• Blood supply
• Arterial supply is by various branches from the external carotid arteries and venous drainage is into the external jugular veins.
• Composition of saliva
• water
• mineral salts
• enzyme: salivary amylase
• mucus
• lysozyme
• immunoglobulins
• blood-clotting factors
• Secretion of saliva
• Secretion of saliva is under autonomic nerve control. Parasympathetic stimulation causes vasodilatation and profuse secretion of watery saliva with a relatively low content of enzymes and other organic substances. Sympathetic stimulation causes vasoconstriction and secretion of small amounts of saliva rich in organic material, especially from the submandibular glands. Reflex secretion occurs when there is food in the mouth and the reflex can easily become conditioned so that the sight, smell and even the thought of food stimulates the flow of saliva.
• Functions of saliva
• Chemical digestion of polysaccharides
• Lubrication of food
• Cleansing and lubricating
• Non-specific defence
• Taste.
• PHARYNX
• The pharynx is divided for descriptive purpose into three parts, the nasopharynx, oropharynx and laryngopharynx. The nasopharynx is important in respiration. The oropharynx and laryngopharynx are passages common to both the respiratory and the digestive systems. Food passes from the oral cavity into the pharynx then to the oesophagus below, with which it is continuous. The walls of the pharynx are built of three layers of tissue
• The lining membrane (mucosa) is stratified squamous epithelium, continuous with the lining of the mouth atone end and with the oesophagus at the other.
• The middle layer consists of fibrous tissue which becomes thinner towards the lower end and contains blood and lymph vessels and nerves.
• The outer layer consists of a number of involuntary constrictor muscles which are involved in swallowing. When food reaches the pharynx swallowing is no longer under voluntary control.
• Blood supply
• The blood supply to the pharynx is by several branches of the facial arteries. Venous drainage is into the facial veins and the internal jugular veins.
• Nerve supply
• This is from the pharyngeal plexus and consists of parasympathetic and sympathetic nerves. Parasympathetic supply is mainly by the glossopharyngeal and vagus nerves and sympathetic from the cervical ganglia
• OESOPHAGUS
• The oesophagus is about 25 cm long and about 2 cm in diameter and lies in the median plane in the thorax in front of the vertebral column behind the trachea and the heart. It is continuous with the pharynx above and just below the diaphragm it joins the stomach.
• It passes between muscle fibres of the diaphragm behind the central tendon at the level of the 10th thoracic vertebra. Immediately the oesophagus has passed through the diaphragm it curves upwards before opening into the stomach. This sharp angle is believed to be one of the factors which prevents the regurgitation (backward flow) of gastric contents into the oesophagus.
• The upper and muscles. The upper cricopharyngeal sphincter prevents air passing into the oesophagus during inspiration and the aspiration of oesophageal contents. The cardiac or lower esophageal sphincter prevents the reflux of acid gastric contents into the oesophagus. There is no thickening of the circular muscle in this area and this sphincter is therefore' physiological', i.e. this region can act as a sphincter without the presence of the anatomical features.
• When intra abdominal pressure is raised, e.g. during inspiration and defaecation, the tone of the lower sphincter muscle increases. There is an added pinching effect by the contracting muscle fibres of the diaphragm.
• Structure
• There are four layers of tissue as shown in Figure. As the oesophagus is almost entirely in the thorax the outer covering, the adventitia, consists of elastic fibrous tissue. The proximal third is lined by stratified squamous epithelium and the distal third by columnar epithelium. The middle third is lined by a mixture of the two.
• Blood supply
• Arterial. The thoracic region of the oesophagus is suppliedmainly by the oesophageal arteries, branches from the aorta. The abdominal region is supplied by branches from the inferior phrenic arteries and the left gastric branch of the coeliac artery.
• Venous drainage. From the thoracic region venousdrainage is into the azygos and hemiazygos veins. The abdominal part drains into the left gastric vein. There is a venous plexus at the distal end that links the upward and downward venous drainage, i.e. the general and portal circulations.
• Nerve supply
• Sympathetic and parasympathetic nerves terminate in them enteric and submucosal plexuses. Parasympathetic fibres are branches of the vagus nerves
• Functions of the mouth, pharynx and
oesophagus
• Formation of a bolus. When food is taken into the mouth it is masticated or chewed by the teeth and moved round the mouth by the tongue and muscles of the cheeks . It is mixed with saliva and formed into a soft mass or bolus ready for deglutition or swallowing. The length of time that food remains in the mouth depends, to a large extent, on the consistency of the food Some foods need to the chewed longer than others befor the individual feels that the mass is ready for swallowing.
• Deglutition or swallowing This occurs in three stages after mastication is complete and the bolus has been formed. It is initiated voluntarily but completed by a reflex (involuntary) action
1.The mouth is closed and the voluntary muscles of the tongue and cheeks push the bolus backwards into the pharynx.
2. The muscles of the pharynx are stimulated by a reflex action initiated in the walls of the oropharynx and coordinated in the medulla and lower pons in the brain stem. Contraction of these muscles propels the bolus down into the oesophagus. All other routes that the bolus could possibly take are closed. The soft palate rises up and closes off the nasopharynx; the tongue and the pharyngeal folds block the way back into the mouth; and the larynx is lifted up and forward so that its opening is occluded by the overhanging epiglottis preventing entry into the airway.
3. The presence of the bolus in the pharynx stimulates a wave of peristalsis which propels the bolus through the oesophagus to the stomach.
• Peristaltic waves pass along the oesophagus only after swallowing . Otherwise the walls are relaxed. Ahead of a peristaltic wave, the cardiac sphincter guarding the entrance to the stomach relaxes to allow the descending bolus to pass into the stomach. Usually, constriction of the cardiac sphincter prevents reflux of gastric acid into the oesophagus.
• What are the Other factors preventing gastric reflux
• the attachment of the stomach to the diaphragm by the peritoneum
• the maintenance of an acute angle between the esophagus and the fundus of the stomach, i.e. an acute cardio-oesophageal angle
• increased tone of the cardiac sphincter when intra abdominalpressure is increased and the pinching effect of diaphragm muscle fibres
• STOMACH
• The stomach is a J-shaped dilated portion of the alimentary tract situated in the epigastric, umbilical and left hypochondriac regions of the abdominal cavity.
• What are the Organs associated with the stomach
• Anteriorly —left lobe of liver and anterior abdominal wall
• Posteriorly —abdominal aorta, pancreas, spleen, leftkidney and adrenal gland
• Superiorly — diaphragm, oesophagus and left lobe of liver
• Inferiorly — transverse colon and small intestine
• To the left — diaphragm and spleen
• To the right—liver and duodenu
• Structure of the stomach
• The stomach is continuous with the oesophagus at the cardiac sphincter and with the duodenum at the pyloric sphincter. It has two curvatures. The lesser curvature is short, lies on the posterior surface of the stomach and is the downwards continuation of the posterior wall of the oesophagus. Just before the pyloric sphincter it curves upwards to complete the J shape. Where the oesophagus joins the stomach the anterior region angles acutely upwards, curves downwards forming the greater curvature then slightly upwards towards the pyloric sphincter.
• The stomach is divided into three regions:
1. the fundus,
2.the body
3.the antrum.
• At the distal end of the pyloric antrum is the pyloric sphincter, guarding the opening between the stomach and the duodenum. When the stomach is inactive the pyloric sphincter is relaxed and open and when the stomach contains food the sphincteris closed.
• Walls of the stomach
• The four layers of tissue that comprise the basic structure of the alimentary canal are found in the stomach but with some modifications.
• Muscle layer -This consists of three layers of smooth muscle fibres.
• an outer layer of longitudinal fibres
• a middle layer of circular fibres
• an inner layer of oblique fibres
• In this respect, the stomach is different from other regions of the alimentary tract as it has three layers of muscle instead of two.
• This arrangement allows for the churning motion characteristic of gastric activity, as well as peristaltic movement. Circular muscle is strongest in the pyloric antrum and sphincter.
• Mucosa. When the stomach is empty the mucous membrane lining is thrown into longitudinal folds or rugae, and when full the rugae are 'ironed out' and the surface has a smooth, velvety appearance. Numerous gastric glands are situated below the surface in the mucous membrane. They consist of specialised cells that secrete gastric juice into the stomach
• Blood supply
• Arterial blood is supplied to the stomach by branches of the coeliac artery and venous drainage is into the portal vein. Nerve supply
• The sympathetic supply to the stomach is mainly from the coeliac plexus and the parasympathetic supply is from the vagus nerves. Sympathetic stimulation reduces the motility of the stomach and the secretion of gastric juice; vagal stimulation has the opposite effect
• Gastric juice and functions of the
stomach
• Stomach size varies with the volume of food it contains, which may be 1.5 litres or more in an adult. When a meal has been eaten the food accumulates in the stomach in layers, the last part of the meal remaining in the fundus for some time. Mixing with the gastric juice takes place gradually and it may be some time before the food is sufficiently acidified to stop the action of salivary amylase.
• Gastric muscle contraction consists of a churning movement that breaks down the bolus and mixes it with gastric juice, and peristaltic waves that propel the stomach contents towards the pylorus. When the stomach is active the pyloric sphincter closes. Strong peristaltic contraction of the pyloric antrum forces gastric contents, after they are sufficiently liquefied, through the pylorus into the duodenum in small spurts.
• Gastric juice
• About 2 litres of gastric juice are secreted daily by special secretory glands in the mucosa . It consists of:
• water
• mineral salts
• mucus secreted by goblet cells in the glands and on the stomach surface
• hydrochloric acid
• intrinsic factor
• inactive enzyme precursors: pepsinogens secreted by chief cells in the glands
• Functions of gastric juice
• Water further liquefies the food swallowed.
• Hydrochloric acid:
— acidifies the food and stops the action of salivary amylase
— kills ingested microbes
— provides the acid environment needed for effective digestion by pepsins
• Pepsinogens are activated to pepsins by hydrochloric acid and by pepsins already present in the stomach. They begin the digestion of proteins, breaking them into smaller molecules. Pepsins act most effectively at pH 1.5 to 3.5.
• Intrinsic factor (a protein) is necessary for the absorption of vitamin B12 from the ileum
• Mucus prevents mechanical injury to the stomach wall by lubricating the contents. It prevents chemical injury by acting as a barrier between the stomach wall and the corrosive gastric juice. Hydrochloric acid is present in potentially damaging concentrations and pepsins digest protein
• Secretion of gastric juice
• There is always a small quantity of gastric juice present in the stomach, even when it contains no food. This is known as fasting juice. Secretion reaches its maximum level about 1 hour after a meal then declines to the fasting level after about 4 hours.
• There are three phases of secretion of gastric juice
• 1. Cephalic phase
• 2. Gastric phase
• 3. Intestinal phase
• Cephalic phase.
• This flow of juice occurs before food reaches the stomach and is due to reflex stimulation of the vagus nerves initiated by the sight, smell or taste of food. When the vagus nerves have been cut (vagotomy) this phase of gastric secretion stops.
• Gastric phase
• . When stimulated by the presence of food the enteroendocrine cells in the pyloric antrum and duodenum secrete gastrin, a hormone which passes directly into the circulating blood. Gastrin, circulating in the blood which supplies the stomach, stimulate the gastric glands to produce more gastric juice. In this way the secretion of digestive juice is continued after the completion of the meal and the end of the cephalic phase. Gastrin secretion is suppressed when the pH in the pyloric antrum falls to about 1.5.
• Intestinal phase
• . When the partially digested contents of the stomach reach the small intestine, a hormone complex enterogastrone* is produced by endocrine cells in the intestinal mucosa, which slows down the secretion of gastric juice and reduces gastric motility. Two of the hormones forming this complex are secretin and cholecystokinin (CCK).
• By slowing the emptying rate of the stomach, the contents of the duodenum become more thoroughly mixed with bile and pancreatic juice. This phase of gastric secretion is most marked when the meal has had a high fat content.
• The rate at which the stomach empties depends to a large extent on the type of food eaten. A carbohydrate meal leaves the stomach in 2 to 3 hours, a protein meal remains longer and a fatty meal remains in the stomach longest.
• What are the functions of the stomach?
• temporary storage allowing time for the digestive enzymes, pepsins, to act
• chemical digestion — pepsins convert proteins to polypeptides
• mechanical breakdown — the three smooth muscle layers enable the stomach to act as a churn, gastric juice is added and the contents are liquefied to chyme
• limited absorption of water, alcohol and some lipid solubledrugs
• non-specific defence against microbes — provided by hydrochloric acid in gastric juice. Vomiting may be a response to ingestion of gastric irritants, e.g. microbes or chemicals
• preparation of iron for absorption further along the tract — the acid environment of the stomach solubilises iron salts, which is required before iron can be absorbed
• production of intrinsic factor needed for absorption of vitamin B12 in the terminal ileum
• regulation of the passage of gastric contents into the duodenum. When the chyme is sufficiently acidified and liquefied, the pyloric antrum forces small jets of gastric contents through the pyloric sphincter into the duodenum
• SMALL INTESTINE
• The small intestine is continuous with the stomach at the pyloric sphincter and leads into the large intestine at the ileocaecal valve. It is a little over 5 metres long and lies in the abdominal cavity surrounded by the large intestine. In the small intestine the chemical digestion of food is completed and most of the absorption of nutrients takes place.
• What are the main section of small intestine
• The duodenum
• The jejunum
• The ileum
• The duodenum
• The duodenum is about 25 cm long and curves around the head of the pancreas. Secretions from the gall bladder and pancreas are released into the duodenum through a common structure, the hepato pancreatic ampulla, and the opening into the duodenum is guarded by the hepatopancreatic sphincter
• The jejunum
• The jejunum is the middle section of the small intestine and is about 2 metres long.
• The ileum
• The ileum, or terminal section, is about 3 metres long and ends at the ileocaecal valve, which controls the flow of material from the ileum to the caecum, the first part of the large intestine, and prevents regurgitation
• Structure of the small intestine
• Peritoneum. A double layer of peritoneum called the mesentery attaches the jejunum and ileum to the posterior abdominal wall . The attachment is quite short in comparison with the length of the small intestine, therefore it is fan-shaped. The large blood vessels and nerves lie on the posterior abdominal wall and the branches to the small intestine pass between the two layers of the mesentery.
• Mucosa. The surface area of the small intestine mucosa is greatly increased by permanent circular folds, villi and microvilli. The permanent circular folds, unlike the rugae of the stomach, are not smoothed out when the small intestine is distended. They promote mixing of chymeas it passes along.
• The villi are tiny finger-like projections of the mucosal layer into the intestinal lumen, about 0.5 to 1 mm long. Their walls consist of columnar epithelial cells, or enterocytes, with tiny microvilli (1 μm long) on their free border. Goblet cells that secrete mucus are interspersed between the enterocytes. These epithelial cells enclose a network of blood and lymph capillaries. The lymph capillaries are called lacteals because absorbed fat gives the lymph a milky appearance. Absorption and some final stages of digestion of nutrients take place in the enterocytes before entering the blood and lymph capillaries.
• The intestinal glands are simple tubular glands situated below the surface between the villi. The cells of the glands migrate upwards to form the walls of the villi replacing those at the tips as they are rubbed off by the intestinal contents. The entire epithelium is replaced every 3 to 5 days. During migration the cells form digestive enzymes that lodge in the microvilli and, together with intestinal juice, complete the chemical digestion of carbohydrates, protein and fats.
• Numerous lymph nodes are found in the mucosa at irregular intervals throughout the length of the small intestine. The smaller ones are known as solitary lymphatic follicles, and about 20 or 30 larger nodes situated towards the distal end of the ileum are called aggregated lymphatic follicles (Peyer's patches). These lymphatic tissues, neutralise ingested antigens
• Blood supply
• The superior mesenteric artery supplies the whole of the small intestine, and venous drainage is by the superior mesenteric vein which joins other veins to form the portal vein.
• Nerve supply
• Innervations of the small intestine is both sympathetic and parasympathetic.
• Intestinal juice
• About 1500 ml of intestinal juice are secreted daily by the glands of the small intestine. It consists of:
• • water
• • mucus
• • mineral salts
• • enzyme: enterokinase (enteropeptidases).
• The pH of intestinal juice is usually between 7.8 and 8.0.
• Functions of the small intestine
• onward movement of its contents which is produced by peristalsis
• secretion of intestinal juice
• completion of chemical digestion of carbohydrates, protein and fats in the enterocytes of the villi
• protection against infection by microbes that have survived the antimicrobial action of the hydrochloric acid in the stomach, by the solitary lymph follicles and aggregated lymph follicles
• secretion of the hormones cholecystokinin (CCK) and secretin
• absorption of nutrients.
• Chemical digestion in the small
intestine
• When acid chyme passes into the small intestine it is mixed with pancreatic juice, bile and intestinal juice, and is in contact with the enterocytes of the villi. In the small intestine the digestion of all the nutrients is completed:
• carbohydrates are broken down to monosaccharides
• proteins are broken down to amino acids
• fats are broken down to fatty acids and glycerol.
• Pancreatic juice
• Pancreatic juice enters the duodenum at the hepatopancreatic ampulla and consists of:
• water
• mineral salts
• enzymes:
— amylase
— lipase
• inactive enzyme precursors:
— trypsinogen
— chymotrypsinogen
— procarboxypeptidase.
• Pancreatic juice is alkaline (pH 8) because it contains significant quantities of bicarbonate ions, which are alkaline in solution. When acid stomach contents enter the duodenum they are mixed with pancreatic juice and bile and the pH is raised to between 6 and 8. This is the pH at which the pancreatic enzymes, amylase and lipase, act most effectively.
• Functions
• Digestion of proteins. Trypsinogen and chymotrypsinogen are inactive enzyme precursors activated by enterokinase (enteropeptidase), an enzyme in the microvilli, which converts them into the active proteolytic enzymes trypsin and chymotrypsin. These enzymes convert polypeptides to tripeptides, dipeptides and amino acids. It is important that they are produced as inactive precursors and are activated only upon arrival in the duodenum, otherwise they would digest the pancreas.
• Digestion of carbohydrates. Pancreatic amylase converts all digestible polysaccharides (starches) not acted upon by salivary amylase to disaccharides.
• Digestion of fats. Lipase converts fats to fatty acids and glycerol. To aid the action of lipase, bile salts emulsify fats, i.e. reduce the size of the globules, increasing their surface area.
• Control of secretion
• The secretion of pancreatic juice is stimulated by secretin and CCK, produced by endocrine cells in the walls of the duodenum. The presence in the duodenum of acid material from the stomach stimulates the production of these hormones
• Bile
• Bile, secreted by the liver, is unable to enter the duodenum when the hepatopancreatic sphincter is closed; therefore it passes from the hepatic duct along the cystic duct to the gall bladder where it is stored .Bile has a pH of 8 and between 500 and 1000 ml are secreted daily.
• What are the consists of bile?
• water
• mineral salts
• mucus
• bile salts
• bile pigments, mainly bilirubin
• cholesterol.
• What are the functions of the bile
• The bile salts, sodium taurocholate and sodium glycocholate, emulsify fats in the small intestine.
• The bile pigment, bilirubin, is a waste product of the breakdown of erythrocytes and is excreted in the bile rather than in the urine because of its low solubility in water. Bilirubin is altered by microbes in the large intestine. Some of the resultant urobilinogen, which is highly water soluble, is reabsorbed and then excreted in the urine, but most is converted to stercobilin and excreted in the faeces.
• Fatty acids are insoluble in water, which makes them very difficult to absorb through the intestinal wall. Bile salts make fatty acids soluble, enabling both these and fat-soluble vitamins (e.g. vitamin K) to be readily absorbed.
• Stercobilin colours and deodorises the faeces
• Release from the gall bladder
• When a meal has been eaten the hormone CCK is secreted by the duodenum during the intestinal phase of secretion of gastric juice .This stimulates contraction of the gall bladder and relaxation of the hepatopancreatic sphincter, enabling the bile and pancreatic juice to pass into the duodenum together. A more marked activity is noted if chyme entering the duodenum contains a high proportion of fat.
• Intestinal secretions
• The principal constituents of intestinal secretions are:
• water
• mucus
• mineral salts
• enzyme: enterokinase (enteropeptidase)
• Most of the digestive enzymes in the small intestine are contained in the enterocytes of the walls of the villi. Digestion of carbohydrate, protein and fat is completed by direct contact between these nutrients and the microvilli and within the enterocytes. The enzymes involved in completing the chemical digestion of food in the enterocytes of the villi are:
• peptidases
• lipase
• sucrase, maltase and lactase.
• Chemical digestion associated with enterocytes
• Alkaline intestinal juice (pH 7.8 to 8.0) assists in raising the pH of the intestinal contents to between 6.5 and 7.5.
• Enterokinase activates pancreatic peptidases such as trypsin which convert some polypeptides to amino acids and some to smaller peptides. The final stage of break down to amino acids of all peptides occurs inside the enterocytes.
• Lipase completes the digestion of emulsified fats to fatty acids and glycerol partly in the intestine and partly in the enterocytes.
• Sucrase, maltase and lactase complete the digestion of carbohydrates by converting disaccharides such as sucrose, maltose and lactose to monosaccharides inside the enterocytes
• Control of secretion
• Mechanical stimulation of the intestinal glands by chymeis believed to be the main stimulus for the secretion of intestinal juice, although the hormone secretin may also be involved
• Absorption of nutrients
• Absorption of nutrients occurs by two possible processes
• Diffusion
• Active transport
• Diffusion. Monosaccharides, amino acids, fatty acids and glycerol diffuse slowly down their concentration gradients into the enterocytes from the intestinal lumen.
• Active transport. Monosaccharides, amino acids, fatty acids and glycerol may be actively transported into the villi; this is faster than diffusion. Disaccharides, dipeptides and tripeptides are also actively transported into the enterocytes where their digestion is completed before transfer into the capillaries of the villi.
• Monosaccharides and amino acids pass into the capillaries in the villi and fatty acids and glycerol into the lacteals.
• Some proteins are absorbed unchanged, e.g. antibodies present in breast milk and oral vaccines, such as poliomyelitis vaccine. The extent of protein absorption is believed to be limited.
• Other nutrients such as vitamins, mineral salts and water are also absorbed from the small intestine into the lacteals along with fatty acids and glycerol. VitaminB12 combines with intrinsic factor in the stomach and is actively absorbed in the terminal ileum
• The surface area through which absorption takes place in the small intestine is greatly increased by the circular folds of mucous membrane and by the very large number of villi and microvilli present. It has been calculated that the surface area of the small intestine is about five times that of the whole body.
• Large amounts of fluid enter the alimentary tract each day .Of this, only about 500ml is not absorbed by the small intestine, and passes into the large intestine.
• The large intestine
• This is about 1.5 metres long, beginning at the caecum in the right iliac fossa and terminating at the rectum and anal canal deep in the pelvis. Its lumen is larger than that of the small intestine. It forms an arch round the coiled-up small intestine
• The parts of the colon
• caecum
• ascending colon
• transverse colon
• descending colon
• sigmoid colon,
• rectum
• anal canal
• The caecum
• This is the first part of the colon. It is a dilated region which has a blind end inferiorly and is continuous with the ascending colon superiorly. Just below the junction of the two the ileocaecal valve opens from the ileum. The vermiform appendix is a fine tube, closed at one end, which leads from the caecum. It is usual 13 cm long and has the same structure as the walls of the colon but contains more lymphoid tissue
• The ascending colon
• . This passes upwards from the caecum to the level of the liver where it curves acutely to the left at the hepatic flexure to become the transverse colon
• The transverse colon
• . This is a loop of colon which extends across the abdominal cavity in front of the duodenum and the stomach to the area of the spleen where it forms the splenic flexure and curves acutely down wards to become the descending colon.
• The descending colon
• . This passes down the left side of the abdominal cavity then curves towards the midline. After it enters the true pelvis it is known as the sigmoid colon
• The sigmoid colon
• . This part describes an S-shaped curve in the pelvis then continues downwards to become the rectum.
• The rectum
• . This is a slightly dilated section of the colon about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal.
• The anal canal
• . This is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior. Two sphincter muscles control the anus; the internal sphincter, consisting of smooth muscle fibres, is under the control of the autonomic nervous system and the external sphincter, formed by skeletal muscle, is under voluntary control
• Structure
• The four layers of tissue described in the basic structure of the gastrointestinal tract are present in the colon, the rectum and the anal canal. The arrangement of the longitudinal muscle fibres is modified in the colon
• They do not form a smooth continuous layer of tissue but are collected into three bands, called taeniae coli, situated at regular intervals round the colon. They stop at the junction of the sigmoid colon and the rectum. As these bands of muscle tissue are slightly shorter than the total length of the colon they give a sacculated or puckered appearance to the organ
• The longitudinal muscle fibres spread out as in the basic structure and completely surround the rectum and the anal canal. The anal sphincters are formed by thickening of the circular muscle layer
• In the submucosal layer there is more lymphoid tissue than in any other part of the alimentary tract, providing on-specific defence against invasion by resident and other microbes.
• In the mucosal lining of the colon and the upper region of the rectum are large numbers of goblet cells forming simple tubular glands, which secrete mucus. They are not present beyond the junction between the rectum and the anus.
• The lining membrane of the anus consists of stratified squamous epithelium continuous with the mucous membrane lining of the rectum above and which merges with the skin beyond the external anal sphincter. In the upper section of the anal canal the mucous membrane is arranged in 6 to 10 vertical folds, the anal columns. Each column contains a terminal branch of the superior rectal artery and vein.
• Blood supply
• Arterial supply is mainly by the superior and inferior mesenteric arteries
• The superior mesenteric artery supplies the caecum, ascending and most of the transverse colon.
• The inferior mesenteric artery supplies the remainder of the colon and the proximal part of the rectum.
• The distal section of the rectum and the anus are supplied by branches from the internal iliac arteries.
• Venous drainage is mainly by the superior and inferior mesenteric veins which drain blood from the parts supplied by arteries of the same names. These veins join the splenic and gastric veins to form the portal vein. Veins draining the distal part of the rectum and the anus join the internal iliac veins.
• Functions of the large intestine, rectum and
anal canal
• Absorption
• The contents of the ileum which pass through the ileocaecal valve into the caecum are fluid, even though some water has been absorbed in the small intestine. In the large intestine absorption of water continues until the familiar semisolid consistency of faeces is achieved. Mineral salts, vitamins and some drugs are also absorbed into the blood capillaries from the large intestine
• Microbial activity
• The large intestine is heavily colonised by certain types of bacteria, which synthesise vitamin K and folic acid. They include Escherichia coli, Enterobacter aerogenes, Streptococcus faecalis and Clostridium perfringens (welchii). These microbes are commensals in humans. They may become pathogenic if transferred to another part of the body, e.g. Escherichia coli may cause cystitis if it gains access to the urinary bladder.
Gases in the bowel consist of some of the constituents of air, mainly nitrogen, swallowed with food and drink and as a feature of some anxiety states. Hydrogen, carbondioxide and methane are produced by bacterial fermentation of unabsorbed nutrients, especially carbohydrate. Gases pass out of the bowel as flatus. Large numbers of microbes are present in the faeces.
• Mass movement
The large intestine does not exhibit peristaltic movement as it is seen in other parts of the digestive tract. Only at fairly long intervals (about twice an hour) does a wave of strong peristalsis sweep along the transverse colon forcing its contents into the descending and sigmoid colons.
This is known as mass movement and it is often precipitately the entry of food into the stomach. This combination stimulus and response is called the gastrocolic reflex
• Defaecation
Usually the rectum is empty, but when a mass movement forces the contents of the sigmoid colon into the rectum the nerve endings in its walls are stimulated by stretch. In the infant defaecation occurs by reflex (involuntary) action. However, sometime in the second or third year of life the ability to override the defaecation reflex is developed.
• In practical terms this acquired voluntary control means that the brain can inhibit the reflex until such time as it is convenient to defaecate. The external anal sphincteris under conscious control through the pudendal nerve. Thus defaecation involves involuntary contraction of the muscle of the rectum and relaxation of the internal anal sphincter.
• Contraction of the abdominal muscles and lowering of the diaphragm increase the intra-abdominal pressure (Valsalva's manoeuvre) and so assist the process of defaecation. When defaecation is voluntarily postponed the feeling of fullness and need to defaecate tends to fade until the next mass movement occurs and the reflex is initiated again. Repeated suppression of the reflex may lead to constipation.
• Constituents of faeces.
• The faeces consist of a semisolid brown mass. The brown colour is due to the presence of stercobilin .Even though absorption of water takes place in the large intestine, water still makes up about 60 to 70% of the weight of the faeces
• The remainder consists of:
• fibre (indigestible cellular plant and animal material)
• dead and live microbes
• epithelial cells from the walls of the tract
• fatty acids
• mucus secreted by the epithelial lining of the large intestine
• Mucus helps to lubricate the faeces and an adequate amount of roughage in the diet ensures that the contents of the colon are sufficiently bulky to stimulate defaecation.
• Tissues
• Tissues
• The tissue of the body consists of large numbers of cells and they are classified according to the size, shape and functions of these cells. They are four main types.
• Epithelial tissue or epithelium
• Connective tissue
• Muscle tissue
• Nervous tissue
• Epithelial tissue
• This group of tissue is found covering the body lining cavities, hollow organs tube. It is also found in glands. The structure of epithelium is closely related to its functions they are
• Protection
• secretion
• Absorptions
• filtration
• The cells are very closely packed and the intercellular substance, called mafrix is minimal. The cells usually lie on a basement membrane, which is an inter connective tissue made by the epithelial cells themselves.
• epithelial tissue
• simple epithelial -
a single layer of cells
• stratified epithelial -
several layer of cells
• simple epithelium
• simple epithelium consists of a single layer of identical cells and is divided into three main types. It is usually found on absorptive or secretary surfaces, and single layer enhances these process. These types are named according to the shape of the cells , which differs according to their functions.
• Simple squamous epithelium
• This is composed of a single layer flattened cells. Here faming a thing and very smooth membrane and diffusion very easily occurs. It lining of the following structures.
• Heart – known as endocardium
•
• B/d
• Lymp
• Alveola of the lungs.
• Collecting ducts of nephrons. In the kidney.
• Cuboidal epithelium
• This consist of cube- shaped cells fitting closely together on a basement membrane – it lining kidney tubules, and some glands cuboidal epithelium is actively involved in secretion, absorption, and excretion
• Columnar epithelium
• Columnar epithelium
• This is formed by a single layer of cells, rectrangulor in shape, on a basement membrane. It lines many organs and often has adaptations that make it well specific function. The lining of the stomach is formed from simple columnar epithelium without surface structure.
• The surface of the columnar epithelium lining the small intestine is covered with microvilli.Microivlli provide a very large surface area for absorptions of nutrients from the small intestine. In the trachea, columnar epithelium is ciliated and also contains goblet cells. That secret mucus this means that inhaled particles that stick to the mucus layer are moves towards the throat by cilia in the respiratory tract, in the uterine tube ova are propelled along by ciliary action towards the uterine.
• Stratified epithelium
• Stratified epithelium
• Consists of several layers of cells of various shapes. Continual cell division in the lower (basal) layers pushes cell above nearer and nearer to the surface. Basement membranes are usually absent.
• The main function of stratified epithelium is to protect underlying structures from mechanical wear and tear. Two main types, stratified squamous and transitional.
• Stratified squamous epithelium
• This is composed of a number of layers of cells. In the deepest layers the cells are mainly columnar and as they grow towards the surface, they become flattened and are then shed.
• Keratinized stratified epithelium
• This is found on dry surfaces subjected to wear and tear, in skin, hair and nails.
• The surface layer consists of dead epithelial cells that have lost their nuclei and contain the protein keratin.
• This forms a tough relatively waterproof protective layer that prevents drying of the live cells underneath.
• The surface layer of skin is rubbed of and is replaced from below.
• Non keratinized stratified epithelium
• This protects moist surfaces subjected to wear and tear, and prevents them from drying out Eg-: the conjunctiva of the eyes, the lining of the mouth, the pharynx, the eosophagus and the vagina.
• Transitional epithelium
• This is composed of several layers of pear- shaped cells, It is found lining the urinary bladder and allows for stretching as the bladder fills.
• Ciliated columnar epithelium.
• It special form of columnar epithelium. The free surface of each cell is surmounted by fine hair-like processes or cilia.
• The cilia bend rapidly to one side and then straighten again. ciliated epithelium is found in the respiratory system and lines the nasal cavities,
the trachea and bronchi from the deeper parts of the lungs towards the exterior. it is also found in the fallopian tube (uterine). It assists the ovum on it passage to the uterus
• Connective tissue
• Connective tissue
• The connective tissues are the most wide spread and abundant tissue in the body, exist in more varied forms than the other basic types.
• The connective tissues cells are more widely separated from each other than in epithelial tissues and intercellular substance(matrix) is present in considerably larger amounts. fibres present in the matrix.
• The fibres form a supporting network for the cells to attach to. Most types of connective tissue have a good blood supply. Major functions of connective tissue are,
– binding and structural support.
– protection .
– Transport.
– Insulation.
• Types of connective tissue
• Fibrous tissue.
• Areola tissue.
• Adipose tissue.
• Lymphoid tissue.
• Cartilage tissue.
• Bone tissue.
• Blood tissue.
• Cells in connective tissue
• Connective tissue, excluding blood is found in all organs supporting the specialize tissue.
• The different types of cell. Involved include fibroblast , fat cells , macrophages, leukocytes and mast cells.
• Fibrous tissue -
there are two types.
1. white fibrous tissue.
2. yellow elastic tissue.
• White fibrous tissue
• White fibrous tissue
• Does not stretch, consists of bundles of white fibers Containing a few cells, tendons and ligaments.
• Consist of this material also the dura mater.
• The outer layer of pericardium, the fascia and fibrous covering of organs.
• It also contains a substance called collagen. Gelatine can be extracted from this tissue.
• Yellow elastic fibrous
• Yellow elastic fibrous.
(secred by fibroblasts).
• Stretch and consist of yellow fibers. This is found in the walls of arteries, in the bronchi and alveoli of the lungs and in a few special ligaments in the spine.
• Areola connective tissue. (loose).
• Consist of various cells, elastic fibers and collagen (white fibers).
• Described as the general packing and supporting tissue of the body.
• It is found under the skin and mucous membranes and surrounding blood vessels and nerves.
• Adipose tissue.
• Adipose tissue consists of fat cells, containing large fat globules. In a matrix of areola tissue. Two types
• 1.white
• 2.brown
• white adipose tissue
• white adipose tissue
• 20% 25% of body weight in well nourished adults. It is found supporting the kidneys and the eyes, between muscle Fibers and under the skin. It act as a thermal insulator and energy store.
• Brown adipose tissue
• Brown adipose tissue
• This is present in the newborn. It has a more extensive capillary network than white adipose tissue
• . When brown tissue is metabolized, it produces less energy and considerably more heat than other fat, contributing to the maintenance of body temperatures. In some adult it is present in small amounts.
Lymphoid tissue
• This tissue also know as reticular tissue, has a semisolid matrix with fine branching retcular fibers.
• It contains reticular cells and white blood cells. (monocyte and lymphocytes
• lymphoid tissue is found in lymph nodes and all organs of the lymphatic system.
• CARTILAGE.
• Is firm flexible tissue found mainly in connection with the skeleton.
• The outer surface of cartilage is covered by a fibrous membrane, the perichondrium, which is supplied with blood vessels.
• No blood vessels how ever enter the cartilage itself ,which is nourished by tissue fluid.
• There are three types,hyaline cartilage, fibro-cartilage and elastic cartilage.
• HYALINE CARTILAGE.
• A bluish white tissue with a smooth,glassy surface
• .It is found covering the end of bones where they form joints .(articular cartilage)In the coastal cartilage trachea and larynx
• AIM
• After completion of this system the students should be able to gain knowledge in depth anatomy and physiology of digestive system.
• SPECIFIC OBJECTIVES
• List out the main organs of the alimentary tract.
• List out the accessory organs of the digestive system.
• Describe general structure of alimentary canal.
• Describe the distribution of the peritoneum.
• Describe the structure and functions of the mouth.
• Describe the structure and functions of the salivary gland.
• Describe the structure and functions of the oesophagus.
• Describe the structure and functions of the stomach.
• Describe the structure and functions of the small intestine.
• Identify the different sections of the large intestine.
• Describe the structure and functions of the large intestine.
• Describe the structure and functions of the accessory organs.
• List out the principal digestive enzyme and their actions.
• Explain general principles of metabolism.
• Define terms of metabolism, anabolism, catabolism, unit of energy and metabolic rate.
• The digestive system
• The digestive system is the collective name used to describe the alimentary canal, some accessory organs and a variety of digestive processes which take place at different levels in the canal to prepare food eaten in the diet for absorption.
• The alimentary canal begins at the mouth, passes through the thorax, abdomen and pelvis and ends at the anus .It has a general structure which is modified at different levels to provide for the processes occurring at each level . The complex of digestive processes gradually breaks down the foods eaten until they are in a form suitable for absorption
• After absorption, nutrients are used to synthesis body constituents. They provide the raw materials for the manufacture of new cells, hormones and enzymes, and the energy needed for these and other processes and for the disposal of waste materials.
• What are the activities of digestive system?
• Ingestion. This is the process of taking food into the alimentary tract.
• Propulsion. This moves the contents along the alimentary tract.
• Digestion. This consists of:
• • mechanical breakdown of food by, e.g. mastication(chewing)
• • chemical digestion of food by enzymes present in secretions produced by glands and accessory organs of the digestive system.
• Absorption. This is the process by which digested food substances pass through the walls of some organs of the alimentary canal into the blood and lymph capillaries for circulation round the body.
• Elimination. Food substances which have been eaten but cannot be digested and absorbed are excreted by the bowel as faeces
• ORGANS OF THE DIGESTIVE SYSTEM
• Mouth
• Pharynx
• Oesophagus
• Small intestine
• Large intestine
• Rectum and anal canal
• ACCESSORY ORGANS
• Salivary gland
• The pancreas
• The liver and biliary tract
• General structure of alimentary tract
• Adventitia or serosa –outer covering
• Muscle layer
• Sub mucosa
• Mucosa lining
• Adventitia (outer covering)
• In the thorax this consists of loose fibrous tissue and in the abdomen the organs are covered by a serous membrane called peritoneum.
• Peritoneum (serosa)
• The peritoneum is the largest serous membrane of the body . It consists of a closed sac, containing a small amount of serous fluid, within the abdominal cavity. It is richly supplied with blood and lymph vessels, and contains a considerable number of lymph nodes.
• It provides a physical barrier to local spread of infection, and can isolate an infective focus such as appendicitis, preventing involvement of other abdominal structures.
It has two layers:
• the parietal layer, which lines the abdominal wall
• the visceral layer, which covers the organs (viscera) within the abdominal and pelvic cavities.
• The arrangement of the peritoneum is such that the organs are invaginated into the closed sac from below, behind and above so that they are at least partly covered by the visceral layer.
• What is the meaning of that?
• pelvic organs are covered only on their superior surface
• the stomach and intestines, deeply invaginated from behind, are almost completely surrounded by peritoneum and have a double fold (the mesentery) that attaches them to the posterior abdominal wall. The fold of peritoneum enclosing the stomach extends beyond the greater curvature of the stomach, and hangs down in front of the abdominal organs like an apron. This is the greater omentum, and it stores fat, which provides both insulation and along-term energy store
• the pancreas, spleen, kidneys and adrenal glands are invaginated from behind but only their anterior surfaces are covered and are therefore retroperitoneal
• the liver is invaginated from above and is almost completely covered by peritoneum which attaches it to the inferior surface of the diaphragm
• the main blood vessels and nerves pass close to the posterior abdominal wall and send branches to the organs between folds of peritoneum
• The two layers of peritoneum are actually in contact and friction between them is prevented by the presence of serous fluid secreted by the peritoneal cells, thus the peritoneal cavity is only a potential cavity. A similar arrangement is seen with the membranes covering the lungs, the pleura. In the male it is completely closed but in the female the uterine tubes open into it and the ovaries are the only structures inside
• Muscle layer (muscularis)
• With some exceptions this consists of two layers of smooth (involuntary) muscle. The muscle fibres of the outer layer are arranged longitudinally, and those of the inner layer encircle the wall of the tube. Between these two muscle layers are blood vessels, lymph vessels and a plexus (network) of sympathetic and parasympathetic nerves, called the myenteric or Auerbach's plexus. These nerves supply the adjacent smooth muscle and blood vessels.
• Contraction and relaxation of these muscle layers occurs in waves which push the contents of the tract onwards. This type of contraction of smooth muscle is called peristalsis .Muscle contraction also mixes food with the digestive juices. Onward movement of the contents of the tract is controlled at various points by sphincters consisting of an increased number of circular muscle fibres. They also act as valves preventing backflow in the tract. The control allows time for digestion and absorption to take place.
• Submucosa
• This layer consists of loose connective tissue with some elastic fibres. Within this layer are plexuses of blood vessels and nerves, lymph vessels and varying amounts of lymphoid tissues. The blood vessels consist of arterioles, venules and capillaries. The nerve plexus is the submucosal or Meissner's plexus, consisting of sympathetic and parasympathetic nerves which supply the mucosal lining.
• Mucosa
• This consists of three layers of tissue:
1. mucous membrane
2. lamina propria
3. muscularis mucosa
• mucous membrane formed by columnar epithelium is the innermost layer and has three main functions: protection, secretion and absorption
• lamina propria consisting of loose connective tissue which supports the blood vessels that nourish the inner epithelial layer, and varying amounts of lymphoid tissue that has a protective function
• muscularis mucosa, a thin outer layer of smooth muscle that provides involutions of the mucosa layer, e.g. gastric glands, villi.
• Mucous membrane
• In parts of the tract which are subject to great wear and tear or mechanical injury this layer consists of stratified squamous epithelium with mucus-secreting glands just below the surface. In areas where the food is already soft and moist and where secretion of digestive juices and absorption occur, the mucous membrane consists of columnar epithelial cells interspersed with mucus-secreting goblet cells
• Mucus lubricates the walls of the tract and protects them from digestive enzymes. Below the surface in the regions lined with columnar epithelium are collections of specialized cells, or glands, which pour their secretions into the lumen of the tract
• What are the secretions?
• saliva from the salivary glands
• gastric juice from the gastric glands
• intestinal juice from the intestinal glands
• pancreatic juice from the pancreas
• bile from the live
• Nerve supply
• The parasympathetic supply. This supply to most of the alimentary tract is provided by one pair of cranial nerves, the vagus nerves. Stimulation causes smooth muscle contraction and the secretion of digestive juices. The most distal part of the tract is supplied by sacral nerves.
• The sympathetic supply. This is provided by numerous nerves which emerge from the spinal cord in the thoracic and lumbar regions. These form plexuses in the thorax, abdomen and pelvis, from which nerves pass to the organs of the alimentary tract. Their action is to reduce smooth muscle contraction and glandular secretion.
• Within the walls of the canal there are two nerve plexuses from which both sympathetic and parasympathetic fibres are distributed.
• The myenteric or Auerbach's plexus lies between the two layers of smooth muscle that it supplies, and influences peristalsis.
• The submucosal or Meissner's plexus lies in the submucosa and supplies the mucous membrane and secretory glands.
• Blood supply
• In the thorax. The oesophagus is supplied by paired oesophageal arteries, branches from the thoracic aorta
• In the abdomen and pelvis. The alimentary tract, pancreas, liver and biliary tract are supplied by the unpaired branches from the aorta: the coeliac artery and the superior and inferior mesenteric arteries and. The coeliac artery divides into three branches which supply the stomach, duodenum, pancreas, spleen, liver, gall bladder and bile ducts.
• The superior mesenteric artery supplies the whole of the small intestine, the caecum, ascending colon and most of the transverse colon.
• The inferior mesenteric artery supplies a small part of the transverse colon, the descending colon, sigmoid colon and most of the rectum.
• The distal part of the rectum and the anus are supplied by the middle and inferior rectal arteries, branches of the internal iliac arteries.
• The arteries supplying the stomach and intestines pass between the layers of peritoneum from the posterior abdominal wall to the organs
• Venous drainage
• In the thorax. Venous blood from the oesophagus passes in the oesophageal veins to the azygos and hemiazygos veins. The azygos vein joins the superior vena cava near the heart, and the hemiazygos joins the left brachiocephalic vein.Some blood from the lower part of the oesophagus drains into the left gastric vein. There are anastomotic vessels between the azygos, hemiazygos and left gastric veins.
• In the abdomen and pelvis. The veins that drain blood from the lower part of the oesophagus, the stomach, pancreas, small intestine, large intestine and most of the rectum join to form the portal vein. This blood, containing a high concentration of absorbed nutritional materials, is conveyed first to the liver then to the inferior vena cava.
• Blood from the lower part of the rectum and the anal canal drains into the internal iliac veins. This blood is delivered directly into the inferior vena cava, hence bypassing the hepatic portal circulation.
• MOUTH
• The mouth or oral cavity is bounded by muscles and bones:
• Anteriorly —by the lips
• Posteriorly —it is continuous with the oropharynx
• Laterally —by the muscles of the cheeks
• Superiorly —by the bony hard palate and muscularsoft palate
• Inferiorly —by the muscular tongue and the soft tissues of the floor of the mouth.
• The oral cavity is lined throughout with mucous containing small mucus-secreting glands
• The part of the mouth between the gums (alveolarridges) and the cheeks is the vestibule and the remainder of the cavity is the mouth proper. The mucous membrane lining of the cheeks and the lips is reflected on to the gums or alveolar ridges and is continuous with the skin of the face.
• The palate forms the roof of the mouth and is divided into the anterior hard palate and the posterior soft palate. The bones forming the hard palate are the maxilla and the palatine bones. The soft palate is muscular, curves downwards from the posterior end of the hard palate and blends with the walls of the pharynx at the sides.
• The uvula is a curved fold of muscle covered with mucous membrane, hanging down from the middle of the free border of the soft palate. Originating from the upper end of the uvula there are four folds of mucous membrane, two passing downwards at each side to form membranous arches. The posterior folds, one on each side, are the palatopharyngeal arches and the two anterior folds are the palatoglossal arches. On each side, between the arches, is a collection of lymphoid tissue called the palatine tonsil.
• Tongue
• The tongue is a voluntary muscular structure which occupies the floor of the mouth. It is attached by its base to the hyoid bone and by a fold of its mucous membrane covering, called the fermium, to the floor of the mouth. The superior surface consists of stratified squamous epithelium, with numerous papillae (little projections), containing nerve endings of the sense of taste, sometimes called the taste buds.
• Types of papillae
• Vallate papillae
• Fungiform papillae
• Filiform papillae
• Vallate papillae
• Vallate papillae, usually between 8 and 12 altogether, are arranged in an inverted V shape towards the base of the tongue. These are the largest of the papillae and are the most easily seen.
• Fungiform papillae
• Fungiform papillae are situated mainly at the tip and the
• edges of the tongue and are more numerous than the vallate
• papillae.
• Filiform papillae
• Filiform papillae are the smallest of the three types. They are most numerous on the surface of the anterior two-thirds of the tongue
• Blood supply
• The main arterial blood supply to the tongue is by the lingual branch of the external carotid artery. Venous drainage is by the lingual vein which joins the internal jugular vein
• Nerve supply
• the hypoglossal nerves (12th cranial nerves) which supply the voluntary muscle tissue
• the lingual branch of the mandibular nerves which are the nerves of somatic (ordinary) sensation, i.e. pain, temperature and touch
• the facial and glossophanryngeal nerves (7th and 9thcranial nerves) which are the nerves of the special sensation of taste.
• Functions of the tongue
• mastication (chewing)
• deglutition (swallowing)
• speech
• taste
• Nerve endings of the sense of taste are present in the papillae and widely distributed in the epithelium of the tongue, soft palate, pharynx and epiglottis
• Teeth
• The teeth are embedded in the alveoli or sockets of the alveolar ridges of the mandible and the maxilla. Each individual has two sets, or dentitions, the temporary or deciduous teeth and the permanent teeth . At birth the teeth of both dentitions are present in immature form in the mandible and maxilla
• There are 20 temporary teeth, 10 in each jaw. They begin to erupt when the child is about 6 months old, and should all be present after 24 months
• The permanent teeth begin to replace the deciduous teeth in the 6th year of age and this dentition, consisting of 32 teeth, is usually complete by the 24th year
• Functions of the teeth
• The incisor and canine teeth are the cutting teeth and are used for biting off pieces of food, whereas the premolar and molar teeth, with broad, flat surfaces, are used for grinding or chewing food
• Structure of a tooth
• Although the shapes of the different teeth vary, the structure is the same and consists of:
• the crown — the part which protrudes from the gum
• the root — the part embedded in the bone
• the neck — the slightly narrowed region where the crown merges with the root.
• In the centre of the tooth is the pulp cavity containing blood vessels, lymph vessels and nerves, and surrounding this is a hard ivory-like substance called dentine. Outside the dentine of the crown is a thin layer of very hard substance, the enamel. The root of the tooth, on the other hand, is covered with a substance resembling bone, called cement, which fixes the tooth in its socket. Blood vessels and nerves pass to the tooth through a small foramen at the apex of each root.
• Blood supply
• Most of the arterial blood supply to the teeth is by branches of the maxillary arteries. The venous drainage is by a number of veins which empty into the internal jugular veins
• Nerve supply
• The nerve supply to the upper teeth is by branches of the maxillary nerves and to the lower teeth by branches of the mandibular nerves. These are both branches of the trigeminal nerves (5th cranial nerves).
• SALIVARY GLANDS
• Salivary glands pour their secretions into the mouth. There are three pairs:
• parotid glands
• submandibular glands
• sublingual glands.
• Parotid glands
• These are situated one on each side of the face just below the external acoustic meatus. Each gland has a parotid duct opening into the mouth at the level of the second upper molar tooth.
• Submandibular glands
• These lie one on each side of the face under the angle of the jaw. The two submandibular ducts open on the floor of the mouth, one on each side of the frenulum of the tongue.
• Sublingual glands
• These glands lie under the mucous membrane of the floor of the mouth in front of the submandibular glands. They have numerous small ducts that open into the floor of the mouth.
• Structure of the salivary glands
• The glands are all surrounded by a fibrous capsule. They consist of a number of lobules made up of small acini lined with secretory cells. The secretions are poured into ductules which join up to form larger ducts leading into the mouth.
• Nerve supply
• The glands are supplied by parasympathetic and sympathetic nerve fibres. Parasympathetic stimulation increases secretion, whereas sympathetic stimulation decreases it.
• Blood supply
• Arterial supply is by various branches from the external carotid arteries and venous drainage is into the external jugular veins.
• Composition of saliva
• water
• mineral salts
• enzyme: salivary amylase
• mucus
• lysozyme
• immunoglobulins
• blood-clotting factors
• Secretion of saliva
• Secretion of saliva is under autonomic nerve control. Parasympathetic stimulation causes vasodilatation and profuse secretion of watery saliva with a relatively low content of enzymes and other organic substances. Sympathetic stimulation causes vasoconstriction and secretion of small amounts of saliva rich in organic material, especially from the submandibular glands. Reflex secretion occurs when there is food in the mouth and the reflex can easily become conditioned so that the sight, smell and even the thought of food stimulates the flow of saliva.
• Functions of saliva
• Chemical digestion of polysaccharides
• Lubrication of food
• Cleansing and lubricating
• Non-specific defence
• Taste.
• PHARYNX
• The pharynx is divided for descriptive purpose into three parts, the nasopharynx, oropharynx and laryngopharynx. The nasopharynx is important in respiration. The oropharynx and laryngopharynx are passages common to both the respiratory and the digestive systems. Food passes from the oral cavity into the pharynx then to the oesophagus below, with which it is continuous. The walls of the pharynx are built of three layers of tissue
• The lining membrane (mucosa) is stratified squamous epithelium, continuous with the lining of the mouth atone end and with the oesophagus at the other.
• The middle layer consists of fibrous tissue which becomes thinner towards the lower end and contains blood and lymph vessels and nerves.
• The outer layer consists of a number of involuntary constrictor muscles which are involved in swallowing. When food reaches the pharynx swallowing is no longer under voluntary control.
• Blood supply
• The blood supply to the pharynx is by several branches of the facial arteries. Venous drainage is into the facial veins and the internal jugular veins.
• Nerve supply
• This is from the pharyngeal plexus and consists of parasympathetic and sympathetic nerves. Parasympathetic supply is mainly by the glossopharyngeal and vagus nerves and sympathetic from the cervical ganglia
• OESOPHAGUS
• The oesophagus is about 25 cm long and about 2 cm in diameter and lies in the median plane in the thorax in front of the vertebral column behind the trachea and the heart. It is continuous with the pharynx above and just below the diaphragm it joins the stomach.
• It passes between muscle fibres of the diaphragm behind the central tendon at the level of the 10th thoracic vertebra. Immediately the oesophagus has passed through the diaphragm it curves upwards before opening into the stomach. This sharp angle is believed to be one of the factors which prevents the regurgitation (backward flow) of gastric contents into the oesophagus.
• The upper and muscles. The upper cricopharyngeal sphincter prevents air passing into the oesophagus during inspiration and the aspiration of oesophageal contents. The cardiac or lower esophageal sphincter prevents the reflux of acid gastric contents into the oesophagus. There is no thickening of the circular muscle in this area and this sphincter is therefore' physiological', i.e. this region can act as a sphincter without the presence of the anatomical features.
• When intra abdominal pressure is raised, e.g. during inspiration and defaecation, the tone of the lower sphincter muscle increases. There is an added pinching effect by the contracting muscle fibres of the diaphragm.
• Structure
• There are four layers of tissue as shown in Figure. As the oesophagus is almost entirely in the thorax the outer covering, the adventitia, consists of elastic fibrous tissue. The proximal third is lined by stratified squamous epithelium and the distal third by columnar epithelium. The middle third is lined by a mixture of the two.
• Blood supply
• Arterial. The thoracic region of the oesophagus is suppliedmainly by the oesophageal arteries, branches from the aorta. The abdominal region is supplied by branches from the inferior phrenic arteries and the left gastric branch of the coeliac artery.
• Venous drainage. From the thoracic region venousdrainage is into the azygos and hemiazygos veins. The abdominal part drains into the left gastric vein. There is a venous plexus at the distal end that links the upward and downward venous drainage, i.e. the general and portal circulations.
• Nerve supply
• Sympathetic and parasympathetic nerves terminate in them enteric and submucosal plexuses. Parasympathetic fibres are branches of the vagus nerves
• Functions of the mouth, pharynx and
oesophagus
• Formation of a bolus. When food is taken into the mouth it is masticated or chewed by the teeth and moved round the mouth by the tongue and muscles of the cheeks . It is mixed with saliva and formed into a soft mass or bolus ready for deglutition or swallowing. The length of time that food remains in the mouth depends, to a large extent, on the consistency of the food Some foods need to the chewed longer than others befor the individual feels that the mass is ready for swallowing.
• Deglutition or swallowing This occurs in three stages after mastication is complete and the bolus has been formed. It is initiated voluntarily but completed by a reflex (involuntary) action
1.The mouth is closed and the voluntary muscles of the tongue and cheeks push the bolus backwards into the pharynx.
2. The muscles of the pharynx are stimulated by a reflex action initiated in the walls of the oropharynx and coordinated in the medulla and lower pons in the brain stem. Contraction of these muscles propels the bolus down into the oesophagus. All other routes that the bolus could possibly take are closed. The soft palate rises up and closes off the nasopharynx; the tongue and the pharyngeal folds block the way back into the mouth; and the larynx is lifted up and forward so that its opening is occluded by the overhanging epiglottis preventing entry into the airway.
3. The presence of the bolus in the pharynx stimulates a wave of peristalsis which propels the bolus through the oesophagus to the stomach.
• Peristaltic waves pass along the oesophagus only after swallowing . Otherwise the walls are relaxed. Ahead of a peristaltic wave, the cardiac sphincter guarding the entrance to the stomach relaxes to allow the descending bolus to pass into the stomach. Usually, constriction of the cardiac sphincter prevents reflux of gastric acid into the oesophagus.
• What are the Other factors preventing gastric reflux
• the attachment of the stomach to the diaphragm by the peritoneum
• the maintenance of an acute angle between the esophagus and the fundus of the stomach, i.e. an acute cardio-oesophageal angle
• increased tone of the cardiac sphincter when intra abdominalpressure is increased and the pinching effect of diaphragm muscle fibres
• STOMACH
• The stomach is a J-shaped dilated portion of the alimentary tract situated in the epigastric, umbilical and left hypochondriac regions of the abdominal cavity.
• What are the Organs associated with the stomach
• Anteriorly —left lobe of liver and anterior abdominal wall
• Posteriorly —abdominal aorta, pancreas, spleen, leftkidney and adrenal gland
• Superiorly — diaphragm, oesophagus and left lobe of liver
• Inferiorly — transverse colon and small intestine
• To the left — diaphragm and spleen
• To the right—liver and duodenu
• Structure of the stomach
• The stomach is continuous with the oesophagus at the cardiac sphincter and with the duodenum at the pyloric sphincter. It has two curvatures. The lesser curvature is short, lies on the posterior surface of the stomach and is the downwards continuation of the posterior wall of the oesophagus. Just before the pyloric sphincter it curves upwards to complete the J shape. Where the oesophagus joins the stomach the anterior region angles acutely upwards, curves downwards forming the greater curvature then slightly upwards towards the pyloric sphincter.
• The stomach is divided into three regions:
1. the fundus,
2.the body
3.the antrum.
• At the distal end of the pyloric antrum is the pyloric sphincter, guarding the opening between the stomach and the duodenum. When the stomach is inactive the pyloric sphincter is relaxed and open and when the stomach contains food the sphincteris closed.
• Walls of the stomach
• The four layers of tissue that comprise the basic structure of the alimentary canal are found in the stomach but with some modifications.
• Muscle layer -This consists of three layers of smooth muscle fibres.
• an outer layer of longitudinal fibres
• a middle layer of circular fibres
• an inner layer of oblique fibres
• In this respect, the stomach is different from other regions of the alimentary tract as it has three layers of muscle instead of two.
• This arrangement allows for the churning motion characteristic of gastric activity, as well as peristaltic movement. Circular muscle is strongest in the pyloric antrum and sphincter.
• Mucosa. When the stomach is empty the mucous membrane lining is thrown into longitudinal folds or rugae, and when full the rugae are 'ironed out' and the surface has a smooth, velvety appearance. Numerous gastric glands are situated below the surface in the mucous membrane. They consist of specialised cells that secrete gastric juice into the stomach
• Blood supply
• Arterial blood is supplied to the stomach by branches of the coeliac artery and venous drainage is into the portal vein. Nerve supply
• The sympathetic supply to the stomach is mainly from the coeliac plexus and the parasympathetic supply is from the vagus nerves. Sympathetic stimulation reduces the motility of the stomach and the secretion of gastric juice; vagal stimulation has the opposite effect
• Gastric juice and functions of the
stomach
• Stomach size varies with the volume of food it contains, which may be 1.5 litres or more in an adult. When a meal has been eaten the food accumulates in the stomach in layers, the last part of the meal remaining in the fundus for some time. Mixing with the gastric juice takes place gradually and it may be some time before the food is sufficiently acidified to stop the action of salivary amylase.
• Gastric muscle contraction consists of a churning movement that breaks down the bolus and mixes it with gastric juice, and peristaltic waves that propel the stomach contents towards the pylorus. When the stomach is active the pyloric sphincter closes. Strong peristaltic contraction of the pyloric antrum forces gastric contents, after they are sufficiently liquefied, through the pylorus into the duodenum in small spurts.
• Gastric juice
• About 2 litres of gastric juice are secreted daily by special secretory glands in the mucosa . It consists of:
• water
• mineral salts
• mucus secreted by goblet cells in the glands and on the stomach surface
• hydrochloric acid
• intrinsic factor
• inactive enzyme precursors: pepsinogens secreted by chief cells in the glands
• Functions of gastric juice
• Water further liquefies the food swallowed.
• Hydrochloric acid:
— acidifies the food and stops the action of salivary amylase
— kills ingested microbes
— provides the acid environment needed for effective digestion by pepsins
• Pepsinogens are activated to pepsins by hydrochloric acid and by pepsins already present in the stomach. They begin the digestion of proteins, breaking them into smaller molecules. Pepsins act most effectively at pH 1.5 to 3.5.
• Intrinsic factor (a protein) is necessary for the absorption of vitamin B12 from the ileum
• Mucus prevents mechanical injury to the stomach wall by lubricating the contents. It prevents chemical injury by acting as a barrier between the stomach wall and the corrosive gastric juice. Hydrochloric acid is present in potentially damaging concentrations and pepsins digest protein
• Secretion of gastric juice
• There is always a small quantity of gastric juice present in the stomach, even when it contains no food. This is known as fasting juice. Secretion reaches its maximum level about 1 hour after a meal then declines to the fasting level after about 4 hours.
• There are three phases of secretion of gastric juice
• 1. Cephalic phase
• 2. Gastric phase
• 3. Intestinal phase
• Cephalic phase.
• This flow of juice occurs before food reaches the stomach and is due to reflex stimulation of the vagus nerves initiated by the sight, smell or taste of food. When the vagus nerves have been cut (vagotomy) this phase of gastric secretion stops.
• Gastric phase
• . When stimulated by the presence of food the enteroendocrine cells in the pyloric antrum and duodenum secrete gastrin, a hormone which passes directly into the circulating blood. Gastrin, circulating in the blood which supplies the stomach, stimulate the gastric glands to produce more gastric juice. In this way the secretion of digestive juice is continued after the completion of the meal and the end of the cephalic phase. Gastrin secretion is suppressed when the pH in the pyloric antrum falls to about 1.5.
• Intestinal phase
• . When the partially digested contents of the stomach reach the small intestine, a hormone complex enterogastrone* is produced by endocrine cells in the intestinal mucosa, which slows down the secretion of gastric juice and reduces gastric motility. Two of the hormones forming this complex are secretin and cholecystokinin (CCK).
• By slowing the emptying rate of the stomach, the contents of the duodenum become more thoroughly mixed with bile and pancreatic juice. This phase of gastric secretion is most marked when the meal has had a high fat content.
• The rate at which the stomach empties depends to a large extent on the type of food eaten. A carbohydrate meal leaves the stomach in 2 to 3 hours, a protein meal remains longer and a fatty meal remains in the stomach longest.
• What are the functions of the stomach?
• temporary storage allowing time for the digestive enzymes, pepsins, to act
• chemical digestion — pepsins convert proteins to polypeptides
• mechanical breakdown — the three smooth muscle layers enable the stomach to act as a churn, gastric juice is added and the contents are liquefied to chyme
• limited absorption of water, alcohol and some lipid solubledrugs
• non-specific defence against microbes — provided by hydrochloric acid in gastric juice. Vomiting may be a response to ingestion of gastric irritants, e.g. microbes or chemicals
• preparation of iron for absorption further along the tract — the acid environment of the stomach solubilises iron salts, which is required before iron can be absorbed
• production of intrinsic factor needed for absorption of vitamin B12 in the terminal ileum
• regulation of the passage of gastric contents into the duodenum. When the chyme is sufficiently acidified and liquefied, the pyloric antrum forces small jets of gastric contents through the pyloric sphincter into the duodenum
• SMALL INTESTINE
• The small intestine is continuous with the stomach at the pyloric sphincter and leads into the large intestine at the ileocaecal valve. It is a little over 5 metres long and lies in the abdominal cavity surrounded by the large intestine. In the small intestine the chemical digestion of food is completed and most of the absorption of nutrients takes place.
• What are the main section of small intestine
• The duodenum
• The jejunum
• The ileum
• The duodenum
• The duodenum is about 25 cm long and curves around the head of the pancreas. Secretions from the gall bladder and pancreas are released into the duodenum through a common structure, the hepato pancreatic ampulla, and the opening into the duodenum is guarded by the hepatopancreatic sphincter
• The jejunum
• The jejunum is the middle section of the small intestine and is about 2 metres long.
• The ileum
• The ileum, or terminal section, is about 3 metres long and ends at the ileocaecal valve, which controls the flow of material from the ileum to the caecum, the first part of the large intestine, and prevents regurgitation
• Structure of the small intestine
• Peritoneum. A double layer of peritoneum called the mesentery attaches the jejunum and ileum to the posterior abdominal wall . The attachment is quite short in comparison with the length of the small intestine, therefore it is fan-shaped. The large blood vessels and nerves lie on the posterior abdominal wall and the branches to the small intestine pass between the two layers of the mesentery.
• Mucosa. The surface area of the small intestine mucosa is greatly increased by permanent circular folds, villi and microvilli. The permanent circular folds, unlike the rugae of the stomach, are not smoothed out when the small intestine is distended. They promote mixing of chymeas it passes along.
• The villi are tiny finger-like projections of the mucosal layer into the intestinal lumen, about 0.5 to 1 mm long. Their walls consist of columnar epithelial cells, or enterocytes, with tiny microvilli (1 μm long) on their free border. Goblet cells that secrete mucus are interspersed between the enterocytes. These epithelial cells enclose a network of blood and lymph capillaries. The lymph capillaries are called lacteals because absorbed fat gives the lymph a milky appearance. Absorption and some final stages of digestion of nutrients take place in the enterocytes before entering the blood and lymph capillaries.
• The intestinal glands are simple tubular glands situated below the surface between the villi. The cells of the glands migrate upwards to form the walls of the villi replacing those at the tips as they are rubbed off by the intestinal contents. The entire epithelium is replaced every 3 to 5 days. During migration the cells form digestive enzymes that lodge in the microvilli and, together with intestinal juice, complete the chemical digestion of carbohydrates, protein and fats.
• Numerous lymph nodes are found in the mucosa at irregular intervals throughout the length of the small intestine. The smaller ones are known as solitary lymphatic follicles, and about 20 or 30 larger nodes situated towards the distal end of the ileum are called aggregated lymphatic follicles (Peyer's patches). These lymphatic tissues, neutralise ingested antigens
• Blood supply
• The superior mesenteric artery supplies the whole of the small intestine, and venous drainage is by the superior mesenteric vein which joins other veins to form the portal vein.
• Nerve supply
• Innervations of the small intestine is both sympathetic and parasympathetic.
• Intestinal juice
• About 1500 ml of intestinal juice are secreted daily by the glands of the small intestine. It consists of:
• • water
• • mucus
• • mineral salts
• • enzyme: enterokinase (enteropeptidases).
• The pH of intestinal juice is usually between 7.8 and 8.0.
• Functions of the small intestine
• onward movement of its contents which is produced by peristalsis
• secretion of intestinal juice
• completion of chemical digestion of carbohydrates, protein and fats in the enterocytes of the villi
• protection against infection by microbes that have survived the antimicrobial action of the hydrochloric acid in the stomach, by the solitary lymph follicles and aggregated lymph follicles
• secretion of the hormones cholecystokinin (CCK) and secretin
• absorption of nutrients.
• Chemical digestion in the small
intestine
• When acid chyme passes into the small intestine it is mixed with pancreatic juice, bile and intestinal juice, and is in contact with the enterocytes of the villi. In the small intestine the digestion of all the nutrients is completed:
• carbohydrates are broken down to monosaccharides
• proteins are broken down to amino acids
• fats are broken down to fatty acids and glycerol.
• Pancreatic juice
• Pancreatic juice enters the duodenum at the hepatopancreatic ampulla and consists of:
• water
• mineral salts
• enzymes:
— amylase
— lipase
• inactive enzyme precursors:
— trypsinogen
— chymotrypsinogen
— procarboxypeptidase.
• Pancreatic juice is alkaline (pH 8) because it contains significant quantities of bicarbonate ions, which are alkaline in solution. When acid stomach contents enter the duodenum they are mixed with pancreatic juice and bile and the pH is raised to between 6 and 8. This is the pH at which the pancreatic enzymes, amylase and lipase, act most effectively.
• Functions
• Digestion of proteins. Trypsinogen and chymotrypsinogen are inactive enzyme precursors activated by enterokinase (enteropeptidase), an enzyme in the microvilli, which converts them into the active proteolytic enzymes trypsin and chymotrypsin. These enzymes convert polypeptides to tripeptides, dipeptides and amino acids. It is important that they are produced as inactive precursors and are activated only upon arrival in the duodenum, otherwise they would digest the pancreas.
• Digestion of carbohydrates. Pancreatic amylase converts all digestible polysaccharides (starches) not acted upon by salivary amylase to disaccharides.
• Digestion of fats. Lipase converts fats to fatty acids and glycerol. To aid the action of lipase, bile salts emulsify fats, i.e. reduce the size of the globules, increasing their surface area.
• Control of secretion
• The secretion of pancreatic juice is stimulated by secretin and CCK, produced by endocrine cells in the walls of the duodenum. The presence in the duodenum of acid material from the stomach stimulates the production of these hormones
• Bile
• Bile, secreted by the liver, is unable to enter the duodenum when the hepatopancreatic sphincter is closed; therefore it passes from the hepatic duct along the cystic duct to the gall bladder where it is stored .Bile has a pH of 8 and between 500 and 1000 ml are secreted daily.
• What are the consists of bile?
• water
• mineral salts
• mucus
• bile salts
• bile pigments, mainly bilirubin
• cholesterol.
• What are the functions of the bile
• The bile salts, sodium taurocholate and sodium glycocholate, emulsify fats in the small intestine.
• The bile pigment, bilirubin, is a waste product of the breakdown of erythrocytes and is excreted in the bile rather than in the urine because of its low solubility in water. Bilirubin is altered by microbes in the large intestine. Some of the resultant urobilinogen, which is highly water soluble, is reabsorbed and then excreted in the urine, but most is converted to stercobilin and excreted in the faeces.
• Fatty acids are insoluble in water, which makes them very difficult to absorb through the intestinal wall. Bile salts make fatty acids soluble, enabling both these and fat-soluble vitamins (e.g. vitamin K) to be readily absorbed.
• Stercobilin colours and deodorises the faeces
• Release from the gall bladder
• When a meal has been eaten the hormone CCK is secreted by the duodenum during the intestinal phase of secretion of gastric juice .This stimulates contraction of the gall bladder and relaxation of the hepatopancreatic sphincter, enabling the bile and pancreatic juice to pass into the duodenum together. A more marked activity is noted if chyme entering the duodenum contains a high proportion of fat.
• Intestinal secretions
• The principal constituents of intestinal secretions are:
• water
• mucus
• mineral salts
• enzyme: enterokinase (enteropeptidase)
• Most of the digestive enzymes in the small intestine are contained in the enterocytes of the walls of the villi. Digestion of carbohydrate, protein and fat is completed by direct contact between these nutrients and the microvilli and within the enterocytes. The enzymes involved in completing the chemical digestion of food in the enterocytes of the villi are:
• peptidases
• lipase
• sucrase, maltase and lactase.
• Chemical digestion associated with enterocytes
• Alkaline intestinal juice (pH 7.8 to 8.0) assists in raising the pH of the intestinal contents to between 6.5 and 7.5.
• Enterokinase activates pancreatic peptidases such as trypsin which convert some polypeptides to amino acids and some to smaller peptides. The final stage of break down to amino acids of all peptides occurs inside the enterocytes.
• Lipase completes the digestion of emulsified fats to fatty acids and glycerol partly in the intestine and partly in the enterocytes.
• Sucrase, maltase and lactase complete the digestion of carbohydrates by converting disaccharides such as sucrose, maltose and lactose to monosaccharides inside the enterocytes
• Control of secretion
• Mechanical stimulation of the intestinal glands by chymeis believed to be the main stimulus for the secretion of intestinal juice, although the hormone secretin may also be involved
• Absorption of nutrients
• Absorption of nutrients occurs by two possible processes
• Diffusion
• Active transport
• Diffusion. Monosaccharides, amino acids, fatty acids and glycerol diffuse slowly down their concentration gradients into the enterocytes from the intestinal lumen.
• Active transport. Monosaccharides, amino acids, fatty acids and glycerol may be actively transported into the villi; this is faster than diffusion. Disaccharides, dipeptides and tripeptides are also actively transported into the enterocytes where their digestion is completed before transfer into the capillaries of the villi.
• Monosaccharides and amino acids pass into the capillaries in the villi and fatty acids and glycerol into the lacteals.
• Some proteins are absorbed unchanged, e.g. antibodies present in breast milk and oral vaccines, such as poliomyelitis vaccine. The extent of protein absorption is believed to be limited.
• Other nutrients such as vitamins, mineral salts and water are also absorbed from the small intestine into the lacteals along with fatty acids and glycerol. VitaminB12 combines with intrinsic factor in the stomach and is actively absorbed in the terminal ileum
• The surface area through which absorption takes place in the small intestine is greatly increased by the circular folds of mucous membrane and by the very large number of villi and microvilli present. It has been calculated that the surface area of the small intestine is about five times that of the whole body.
• Large amounts of fluid enter the alimentary tract each day .Of this, only about 500ml is not absorbed by the small intestine, and passes into the large intestine.
• The large intestine
• This is about 1.5 metres long, beginning at the caecum in the right iliac fossa and terminating at the rectum and anal canal deep in the pelvis. Its lumen is larger than that of the small intestine. It forms an arch round the coiled-up small intestine
• The parts of the colon
• caecum
• ascending colon
• transverse colon
• descending colon
• sigmoid colon,
• rectum
• anal canal
• The caecum
• This is the first part of the colon. It is a dilated region which has a blind end inferiorly and is continuous with the ascending colon superiorly. Just below the junction of the two the ileocaecal valve opens from the ileum. The vermiform appendix is a fine tube, closed at one end, which leads from the caecum. It is usual 13 cm long and has the same structure as the walls of the colon but contains more lymphoid tissue
• The ascending colon
• . This passes upwards from the caecum to the level of the liver where it curves acutely to the left at the hepatic flexure to become the transverse colon
• The transverse colon
• . This is a loop of colon which extends across the abdominal cavity in front of the duodenum and the stomach to the area of the spleen where it forms the splenic flexure and curves acutely down wards to become the descending colon.
• The descending colon
• . This passes down the left side of the abdominal cavity then curves towards the midline. After it enters the true pelvis it is known as the sigmoid colon
• The sigmoid colon
• . This part describes an S-shaped curve in the pelvis then continues downwards to become the rectum.
• The rectum
• . This is a slightly dilated section of the colon about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal.
• The anal canal
• . This is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior. Two sphincter muscles control the anus; the internal sphincter, consisting of smooth muscle fibres, is under the control of the autonomic nervous system and the external sphincter, formed by skeletal muscle, is under voluntary control
• Structure
• The four layers of tissue described in the basic structure of the gastrointestinal tract are present in the colon, the rectum and the anal canal. The arrangement of the longitudinal muscle fibres is modified in the colon
• They do not form a smooth continuous layer of tissue but are collected into three bands, called taeniae coli, situated at regular intervals round the colon. They stop at the junction of the sigmoid colon and the rectum. As these bands of muscle tissue are slightly shorter than the total length of the colon they give a sacculated or puckered appearance to the organ
• The longitudinal muscle fibres spread out as in the basic structure and completely surround the rectum and the anal canal. The anal sphincters are formed by thickening of the circular muscle layer
• In the submucosal layer there is more lymphoid tissue than in any other part of the alimentary tract, providing on-specific defence against invasion by resident and other microbes.
• In the mucosal lining of the colon and the upper region of the rectum are large numbers of goblet cells forming simple tubular glands, which secrete mucus. They are not present beyond the junction between the rectum and the anus.
• The lining membrane of the anus consists of stratified squamous epithelium continuous with the mucous membrane lining of the rectum above and which merges with the skin beyond the external anal sphincter. In the upper section of the anal canal the mucous membrane is arranged in 6 to 10 vertical folds, the anal columns. Each column contains a terminal branch of the superior rectal artery and vein.
• Blood supply
• Arterial supply is mainly by the superior and inferior mesenteric arteries
• The superior mesenteric artery supplies the caecum, ascending and most of the transverse colon.
• The inferior mesenteric artery supplies the remainder of the colon and the proximal part of the rectum.
• The distal section of the rectum and the anus are supplied by branches from the internal iliac arteries.
• Venous drainage is mainly by the superior and inferior mesenteric veins which drain blood from the parts supplied by arteries of the same names. These veins join the splenic and gastric veins to form the portal vein. Veins draining the distal part of the rectum and the anus join the internal iliac veins.
• Functions of the large intestine, rectum and
anal canal
• Absorption
• The contents of the ileum which pass through the ileocaecal valve into the caecum are fluid, even though some water has been absorbed in the small intestine. In the large intestine absorption of water continues until the familiar semisolid consistency of faeces is achieved. Mineral salts, vitamins and some drugs are also absorbed into the blood capillaries from the large intestine
• Microbial activity
• The large intestine is heavily colonised by certain types of bacteria, which synthesise vitamin K and folic acid. They include Escherichia coli, Enterobacter aerogenes, Streptococcus faecalis and Clostridium perfringens (welchii). These microbes are commensals in humans. They may become pathogenic if transferred to another part of the body, e.g. Escherichia coli may cause cystitis if it gains access to the urinary bladder.
Gases in the bowel consist of some of the constituents of air, mainly nitrogen, swallowed with food and drink and as a feature of some anxiety states. Hydrogen, carbondioxide and methane are produced by bacterial fermentation of unabsorbed nutrients, especially carbohydrate. Gases pass out of the bowel as flatus. Large numbers of microbes are present in the faeces.
• Mass movement
The large intestine does not exhibit peristaltic movement as it is seen in other parts of the digestive tract. Only at fairly long intervals (about twice an hour) does a wave of strong peristalsis sweep along the transverse colon forcing its contents into the descending and sigmoid colons.
This is known as mass movement and it is often precipitately the entry of food into the stomach. This combination stimulus and response is called the gastrocolic reflex
• Defaecation
Usually the rectum is empty, but when a mass movement forces the contents of the sigmoid colon into the rectum the nerve endings in its walls are stimulated by stretch. In the infant defaecation occurs by reflex (involuntary) action. However, sometime in the second or third year of life the ability to override the defaecation reflex is developed.
• In practical terms this acquired voluntary control means that the brain can inhibit the reflex until such time as it is convenient to defaecate. The external anal sphincteris under conscious control through the pudendal nerve. Thus defaecation involves involuntary contraction of the muscle of the rectum and relaxation of the internal anal sphincter.
• Contraction of the abdominal muscles and lowering of the diaphragm increase the intra-abdominal pressure (Valsalva's manoeuvre) and so assist the process of defaecation. When defaecation is voluntarily postponed the feeling of fullness and need to defaecate tends to fade until the next mass movement occurs and the reflex is initiated again. Repeated suppression of the reflex may lead to constipation.
• Constituents of faeces.
• The faeces consist of a semisolid brown mass. The brown colour is due to the presence of stercobilin .Even though absorption of water takes place in the large intestine, water still makes up about 60 to 70% of the weight of the faeces
• The remainder consists of:
• fibre (indigestible cellular plant and animal material)
• dead and live microbes
• epithelial cells from the walls of the tract
• fatty acids
• mucus secreted by the epithelial lining of the large intestine
• Mucus helps to lubricate the faeces and an adequate amount of roughage in the diet ensures that the contents of the colon are sufficiently bulky to stimulate defaecation.
• Tissues
• Tissues
• The tissue of the body consists of large numbers of cells and they are classified according to the size, shape and functions of these cells. They are four main types.
• Epithelial tissue or epithelium
• Connective tissue
• Muscle tissue
• Nervous tissue
• Epithelial tissue
• This group of tissue is found covering the body lining cavities, hollow organs tube. It is also found in glands. The structure of epithelium is closely related to its functions they are
• Protection
• secretion
• Absorptions
• filtration
• The cells are very closely packed and the intercellular substance, called mafrix is minimal. The cells usually lie on a basement membrane, which is an inter connective tissue made by the epithelial cells themselves.
• epithelial tissue
• simple epithelial -
a single layer of cells
• stratified epithelial -
several layer of cells
• simple epithelium
• simple epithelium consists of a single layer of identical cells and is divided into three main types. It is usually found on absorptive or secretary surfaces, and single layer enhances these process. These types are named according to the shape of the cells , which differs according to their functions.
• Simple squamous epithelium
• This is composed of a single layer flattened cells. Here faming a thing and very smooth membrane and diffusion very easily occurs. It lining of the following structures.
• Heart – known as endocardium
•
• B/d
• Lymp
• Alveola of the lungs.
• Collecting ducts of nephrons. In the kidney.
• Cuboidal epithelium
• This consist of cube- shaped cells fitting closely together on a basement membrane – it lining kidney tubules, and some glands cuboidal epithelium is actively involved in secretion, absorption, and excretion
• Columnar epithelium
• Columnar epithelium
• This is formed by a single layer of cells, rectrangulor in shape, on a basement membrane. It lines many organs and often has adaptations that make it well specific function. The lining of the stomach is formed from simple columnar epithelium without surface structure.
• The surface of the columnar epithelium lining the small intestine is covered with microvilli.Microivlli provide a very large surface area for absorptions of nutrients from the small intestine. In the trachea, columnar epithelium is ciliated and also contains goblet cells. That secret mucus this means that inhaled particles that stick to the mucus layer are moves towards the throat by cilia in the respiratory tract, in the uterine tube ova are propelled along by ciliary action towards the uterine.
• Stratified epithelium
• Stratified epithelium
• Consists of several layers of cells of various shapes. Continual cell division in the lower (basal) layers pushes cell above nearer and nearer to the surface. Basement membranes are usually absent.
• The main function of stratified epithelium is to protect underlying structures from mechanical wear and tear. Two main types, stratified squamous and transitional.
• Stratified squamous epithelium
• This is composed of a number of layers of cells. In the deepest layers the cells are mainly columnar and as they grow towards the surface, they become flattened and are then shed.
• Keratinized stratified epithelium
• This is found on dry surfaces subjected to wear and tear, in skin, hair and nails.
• The surface layer consists of dead epithelial cells that have lost their nuclei and contain the protein keratin.
• This forms a tough relatively waterproof protective layer that prevents drying of the live cells underneath.
• The surface layer of skin is rubbed of and is replaced from below.
• Non keratinized stratified epithelium
• This protects moist surfaces subjected to wear and tear, and prevents them from drying out Eg-: the conjunctiva of the eyes, the lining of the mouth, the pharynx, the eosophagus and the vagina.
• Transitional epithelium
• This is composed of several layers of pear- shaped cells, It is found lining the urinary bladder and allows for stretching as the bladder fills.
• Ciliated columnar epithelium.
• It special form of columnar epithelium. The free surface of each cell is surmounted by fine hair-like processes or cilia.
• The cilia bend rapidly to one side and then straighten again. ciliated epithelium is found in the respiratory system and lines the nasal cavities,
the trachea and bronchi from the deeper parts of the lungs towards the exterior. it is also found in the fallopian tube (uterine). It assists the ovum on it passage to the uterus
• Connective tissue
• Connective tissue
• The connective tissues are the most wide spread and abundant tissue in the body, exist in more varied forms than the other basic types.
• The connective tissues cells are more widely separated from each other than in epithelial tissues and intercellular substance(matrix) is present in considerably larger amounts. fibres present in the matrix.
• The fibres form a supporting network for the cells to attach to. Most types of connective tissue have a good blood supply. Major functions of connective tissue are,
– binding and structural support.
– protection .
– Transport.
– Insulation.
• Types of connective tissue
• Fibrous tissue.
• Areola tissue.
• Adipose tissue.
• Lymphoid tissue.
• Cartilage tissue.
• Bone tissue.
• Blood tissue.
• Cells in connective tissue
• Connective tissue, excluding blood is found in all organs supporting the specialize tissue.
• The different types of cell. Involved include fibroblast , fat cells , macrophages, leukocytes and mast cells.
• Fibrous tissue -
there are two types.
1. white fibrous tissue.
2. yellow elastic tissue.
• White fibrous tissue
• White fibrous tissue
• Does not stretch, consists of bundles of white fibers Containing a few cells, tendons and ligaments.
• Consist of this material also the dura mater.
• The outer layer of pericardium, the fascia and fibrous covering of organs.
• It also contains a substance called collagen. Gelatine can be extracted from this tissue.
• Yellow elastic fibrous
• Yellow elastic fibrous.
(secred by fibroblasts).
• Stretch and consist of yellow fibers. This is found in the walls of arteries, in the bronchi and alveoli of the lungs and in a few special ligaments in the spine.
• Areola connective tissue. (loose).
• Consist of various cells, elastic fibers and collagen (white fibers).
• Described as the general packing and supporting tissue of the body.
• It is found under the skin and mucous membranes and surrounding blood vessels and nerves.
• Adipose tissue.
• Adipose tissue consists of fat cells, containing large fat globules. In a matrix of areola tissue. Two types
• 1.white
• 2.brown
• white adipose tissue
• white adipose tissue
• 20% 25% of body weight in well nourished adults. It is found supporting the kidneys and the eyes, between muscle Fibers and under the skin. It act as a thermal insulator and energy store.
• Brown adipose tissue
• Brown adipose tissue
• This is present in the newborn. It has a more extensive capillary network than white adipose tissue
• . When brown tissue is metabolized, it produces less energy and considerably more heat than other fat, contributing to the maintenance of body temperatures. In some adult it is present in small amounts.
Lymphoid tissue
• This tissue also know as reticular tissue, has a semisolid matrix with fine branching retcular fibers.
• It contains reticular cells and white blood cells. (monocyte and lymphocytes
• lymphoid tissue is found in lymph nodes and all organs of the lymphatic system.
• CARTILAGE.
• Is firm flexible tissue found mainly in connection with the skeleton.
• The outer surface of cartilage is covered by a fibrous membrane, the perichondrium, which is supplied with blood vessels.
• No blood vessels how ever enter the cartilage itself ,which is nourished by tissue fluid.
• There are three types,hyaline cartilage, fibro-cartilage and elastic cartilage.
• HYALINE CARTILAGE.
• A bluish white tissue with a smooth,glassy surface
• .It is found covering the end of bones where they form joints .(articular cartilage)In the coastal cartilage trachea and larynx
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