Digestive System - Unit II

UNIT –II Digestive System Digestive System Food is vital for our body because it is the source of chemical energy. However, most of the food we eat consists of molecules that are too large to be used by body cells. Therefore, food must be broken down into small molecules that are small enough to enter body cells, and this process is known as digestion. The organs involved in the breakdown of food collectively called the digestive system. Two group of the organ compose the digestive system. 1. Gastrointestinal tract or Alimentary canal: It is the continuous tube that extends from the mouth to anus. Organs of the gastrointestinal tract include the mouth, pharynx, esophagus, stomach, small intestine and large intestine. The length of the GI tract is about 5-7 meters. 2. Accessory digestive Organs: It includes the teeth, tongue, salivary glands, liver, gallbladder and pancreas. Functions and process of digestive system: Over all, the digestive system performs six basic functions. I. II. III. IV. Ingestion: Process of the eating food s and liquid into the mouth i.e. eating. Secretions: Every cell, with in the walls of the GI tract and accessory digestive organs secrets a total of about 7 liters of water, acid, buffers and enzymes into the lumens (interior space) of the tract. Mixing and Propulsion: Alternating contraction and relaxation of the smooth muscle in walls of the GI tract mix food and secretions as well as propel the food towards the anus. This capability of the GI tract is called motility. Digestion: This consist of :- a) Mechanical breakdown of food by teeth cutting (mastication or chewing). b) Chemical digestion of the food into small molecules by enzymes present in secretions produced by glands and accessory organs of the digestive system. V. Absorption: The entry of ingested, secreted fluids, ions and the products of digestions into epithelial cell of the GI tract are called absorption. VI. Defecation: Food substance that has been eaten cannot be digested and absorbed is excreted from the alimentary canal as feces by the process of defecation. Basic structure of the alimentary canal (Layers of the GI tract): The wall of the GI tract from the lower esophagus to the anal canal has the four layered arrangement of the tissues. The four layers of the tract from deep to superficial are the mucosa, sub mucosa, muscularis and serosa/ adventitia (outer covering). a. Adventitia/Serosa (Outer covering): This is the outer most layers. This is composed of areolar connective tissue and simple squamous epithelium. The serosa is also called the visceral peritoneum because it forms a portion of the peritoneum. The esophagus lacks a serosa. It only has a single layer of areolar connective tissue called the adventitia. b. Peritoneum: It is the largest serous membrane of the body. It is a closed sac, containing a small amount of serous fluid, with in the abdominal cavity. Peritoneum has blood and lymph vessel and lymph nodes. It provides the physical barrier to local spread of infection. The arrangement of the peritoneum is such that the organs are invaginated into closed sac. There are five major peritoneal folds:-i) The Greater omentum, ii) Falciform ligaments, iii) Lesser omentum , iv) Mesentery, v) Mesocolon. c. Muscularis or Muscle layer: The muscularis of the mouth, pharynx, superior and middle part of the esophagus contains skeletal muscle that produce voluntary swallowing. Skeletal muscle also present in the external anal sphincter, which control the voluntary movement of defecation. The rest of the tract contains the muscularis of the smooth muscle. Involuntary contractions of the smooth muscle help break down food mix it with digestive secretions and propel it along the tract. Between the layers of the muscularis blood vessels, lymph vessel and a plexus (network) of sympathetic and parasympathetic nerves called myenteric plexus (Auerbach’s plexus) is present. Contraction and relaxation of these muscle layers occurs in waves which push the contents of the tract inwards. This type of the contraction of the smooth muscle is called the peristalsis and is under the control of the sympathetic and parasympathetic nerves. d. Submucosa: It contains the areolar connective tissue that binds to the muscle layers of the mucosa. It contains many blood and lymphatic vessels that receive absorbed food molecules. The extensive network of neurons known as the submucosal plexus (Meissner’s plexus), which contain sympathetic and parasympathetic nerves. e. Mucosa: The mucosa or inner lining of the GI tract is a mucous membrane. It is composed of: i). Epithelium: This is the inner most layers and made up of columnar epithelium. ii) Lamina propia: It consists of loose connective tissue, which supports the blood vessels that nourish the inner epithelial layer and varying amount of lymphoid tissue that has a protective function. These lymphoid tissues called mucosa associated lymphatic tissue (MALT). iii) Muscularis mucosa: This is the thin outer layer of smooth muscle that provides involutions of the mucosa layers e.g.: gastric glands, villi. Neural innervations of the GI tract: The GI tract and its related accessory organs are supplied by nerves from both divisions of the autonomic nervous system. The parasympathetic supply: One pair of the cranial nerve, the vagus nerve supplies most of the alimentary canal and the accessory organs. Sacral nerve supply most distal part of the tract. After activation of the Parasympathetic nervous system: i. ii. Peristalsis increases due to activation of the myenteric plexus which is present in the muscularis layers. Glandular secretion also increases due to increased activity of meissner plexus which is present in the submucosal layers. The Sympathetic supply: This is provided by numerous nerves that emerge from the spinal cord in the thoracic and lumber regions and forms plexuses (ganglia), from which nerve pass to the organs of the alimentary tract. After activation of the Sympathetic nervous system: i. ii. It decreases the peristalsis. It decreases glandular secretions. Mouth: It referred to as the oral or the buccal cavity (bucca = cheeks), is formed by the cheeks, tongue, hard and soft palates. The oral cavity is lined throughout with mucous membrane containing stratified squamous epithelium having small mucus secreting glands. Vestibule is part between the gum and cheeks and the remaining part called oral cavity. The palate forms the roof of the mouth and is divided into the anterior hard palate and the posterior soft palate. The hard palate is formed by the maxilla and palatine bone. The soft palate is muscular curve downwards from the posterior end of the hard palate and associated with the walls of the pharynx. 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. Uvula forms the palatopharyngeal arch and in between the arches, lingual tonsils are situated at the base of the tongue. Tongue: The tongue is the accessory digestive organ composed of skeletal muscle covered with mucous membrane. It is attached with base hyoid bone and by a fold of the mucous membrane covering called the frenulum. The superior surface consists of stratified squamous epithelium with numerous papillae (little projections). Many of these contain sensory receptors for the sense of taste in the taste buds. There are several types of papilla and the main arterial supply to the tongue is by the lingual branch of the external carotid artery. The venous supply is by the lingual vein. Nerve supply: The nerves involved are: i. ii. iii. The hypoglossal nerve (12th cranial nerve), which supply to the voluntary muscle. The lingual branch of the mandibular nerve, which arise from the 5th cranial nerve and controls pain, temperature and touch. The facial and glossopharyngeal nerve (7th and 9th cranial nerve), these are the nerve of the taste. Function of the tongue: The tongue plays an important role in:    Chewing (Mastication) Swallowing (Deglutition) Speech Taste Salivary gland: A salivary gland is a gland releases a secretion called saliva into the oral cavity. When food enters the mouth the secretion of saliva increases and it lubricates, dissolves and begins the chemical breakdown of the food. The mucous membrane of the mouth and tongue contains many small salivary glands that open into the oral cavity. There are three main pairs of the salivary glands. i. ii. iii. Parotid gland: These are situated on each side of the face inferior and anterior to the ear. Each gland has the parotid ducts opening into the mouth at the level of the second upper molar tooth. Submandibular glands: These lies at the angle of the jaw. The two sub mandibular ducts open on the floor of the mouth. Sublingual glands: These gland lies under the mucous membrane of the floor of the mouth in front of the submandibular gland. They have numerous small ducts that open into the floor of the mouth. Structure of the salivary glands: The salivary glands are surrounded by fibrous capsule. They consist of a numerous of lobule made up of small acini lined with secretary cell. The secretions are poured into ductules that made up the large ducts leading into the mouth. Figure 1: The salivary glands. Composition of Saliva: Chemically, saliva is 99.5% water and 0.5% solutes. About 1.5 liters of saliva is produced daily. Not all salivary glands supply the same ingredients. The parotids glands secrets a watery (serous) liquid containing salivary amylase. The submandibular secrete amylase but are thickened with mucus. The sublingual glands contains mostly mucous cells, so they secrete a much thicker fluid that have only a small amount of salivary amylase. So, the composition of the saliva is:       Water Mineral salt Digestive enzyme (Salivary amylase) Mucus Lysozyme Immunoglobulin Blood clotting factors Function of the saliva: a. Chemical digestion of polysaccharides: Saliva contains the enzyme amylase, which breakdown the complex sugar (starches), into the disaccharide maltose. b. Lubrication of food: Mucus lubricates the food so it can be moved around easily in the mouth, formed into a ball and swallowed. c. Cleaning and lubricating the mouth: Saliva clean the mouth and to keep it soft, moist. Saliva helps to prevent damage to the mucous membrane by rough and abrasive food. d. Non- specific defense: Lysozyme immunoglobulin and clotting factors present in saliva kills bacteria. f. Taste: The taste buds are stimulated only by chemical substances in solution and therefore dry food only stimulated the sense of taste after thorough mixing with saliva. Salivation / Secretion of saliva: The secretion of saliva is controlled by autonomic nerve system. Normally, parasympathetic stimulation promotes secretion of saliva. Sympathetic stimulate which is dominates during stress, resulting in dryness of the mouth. If the body become dehydrated the salivary glands stops secreting saliva to conserve water and resulting dryness in the mouth contributes to the sensation of thirst. Reflex secretion occurs when there is food in the mouth and the reflex can easily become conditional i.e. sight, smell and even the thought of food stimulate the flow of saliva. Teeth: The teeth or dentes are accessory digestive organs present in sockets or alveoli of the mandible and maxillae. These sockets and alveoli covered with gingivae or gum. Human have two dentitions or sets of teeth. a. Deciduous/ Temporary/ Milk teeth/ baby teeth: Begin to erupt at about 6 months of age and approximately two teeth appear each month. These are 20 in number, 10 in each jaw. b. Permanent teeth: All the deciduous teeth are lost generally between age of 6 to 12 years. These are 32 in number is usually complete by the 21st year. Table 1: Molar Pre Molar Canine Deciduous Teeth Upper 2 1 Jaw Lower 2 1 Jaw Permanent Teeth Upper 3 2 1 Jaw Lower 3 2 1 Jaw Incisors Incisors Canine Premolar Molar 2 2 1 - 2 2 2 1 - 2 2 2 1 2 3 2 2 1 2 3 Structure of a tooth: A typical tooth has three major external regions. Crown: This part is visible above the level of the gums. Root: This embedded in the bone. Neck: This is the constricted junction of the crown and root near the gum line. In the centre of the tooth is the pulp cavity containing blood vessels, lymph vessels, and nerves and surrounding of pulp cavity is called dentine, which is a hard ivory like substance. Outside of the dentine of the crown is a thin layer of very hard substance called enamel. The root of the tooth on the other hand is covered with a substance resembling like bone called cement, which secures the tooth in its socket. Most arterial supply to the teeth is by branches of the maxillary arteries. The nerve supply to the upper teeth is by branches of the maxillary nerve and to the lower teeth by branches of the mandibular nerves. These both are branches of the trigeminal nerves (5th cranial nerves). Functions of the teeth: According to function teeth have different shapes. 1. Incisors and canine are cutting teeth and generally used for cutting and biting off pieces of food. 2. Premolar and molar have broad, flat surface and used for grinding or chewing of food. Figure 2: Structure of the tooth. Pharynx: When food is first swallowed, it passes from the mouth into pharynx (throat). Pharynx is a funnel, shaped tube that extends from the internal nares to the esophagus posteriorly and to the larynx anteriorly. The pharynx is made up of skeletal muscle and covered with mucous membrane and is divided into three parts:i. ii. Nasopharynx: It is only used for respiration. Oropharynx and laryngopharynx: They have both digestive as well as respiratory function. Swallowed food passes from the moth into the oropharynx and laryngopharynx, the muscular contractions of these areas help propel food into the esophagus and then into the stomach. Blood supply to the pharynx is by several branches of the facial arteries. Nerve supply is of the both sympathetic as well as parasympathetic nervous system. Parasympathetic nerve supply is by vagus and glossopharyngeal nerves and sympathetic by the cervical ganglia. Figure 3: Diagrammatic presentation of the pharynx. Oesophagus/ Esophagus: It is a muscular tube about 25 c.m. (10 inches) long. It lies posterior to the trachea. It begins at the inferior end of the laryngopharynx above and just below the diaphragm it joins the stomach. The upper and lower ends of the oesophagus are closed by sphincters. The upper oesophageal sphincter prevents air passing into the oesophagus during respiration and regulates the movement of food from pharynx into oesophagus. The cardiac and lower oesophageal sphincter prevents the reflux of acid gastric contents into the oesophagus and regulates the movement of food from the oesophagus into stomach. The mucous of the oesophagus consists of non keratinized stratified squamous epithelium, lamina propia (areolar connective tissue), and muscularis mucosa (smooth muscle). Near the stomach the mucosa of the oesophagus contains mucous gland. The superficial layers of the oesophagus are known as the adventitia. Function of the Oesophagus:  It secrets the mucus and transport food into the stomach.  It does not produce digestive enzyme and it does not carry on absorption. Figure 4: Diagrammatic presentation of the Oesophagus. Deglutition: The movement of the food from the mouth into the stomach is achieved by act of swallowing or deglutition. It is facilitated by the secretion of saliva and mucus, involves the mouth, pharynx and oesophagus. Deglutition occurs in the three stages after chewing is complete and the bolus is formed. It is initiated voluntarily and completed by a reflex (involuntary) action. 1. The voluntary muscle of the tongue and cheeks push the bolus backwards into the pharynx (oropharynx). 2. With the passage of the bolus into the oropharynx the involuntary pharyngeal stage of swallowing begins. 3. The bolus stimulates receptor in the oropharynx which sends impulse into the deglutition center in the medulla oblongata and lower pons of the brain stem. The returning impulse contracts the involuntary muscle of pharynx and propels the bolus down into the oesophagus. The presence of the bolus in the pharynx stimulates a wave of peristalsis that propel the bolus through the oesophagus to the stomach by relaxing the cardiac sphincters which is prevent at the entrance of the stomach. The wall of the oesophagus is lubricated by mucus which assists the passage of the bolus during the peristaltic contraction of the muscular wall. If the lower esophageal sphincter fails to close properly after food has entered the stomach and the stomach contents reflex back in to the inferior portion of the oesophagus. This condition is known as gastroesophageal reflux disease (GERD). Stomach: It is the J- shaped enlargement of the GI tract directly inferior to the diaphragm in the abdomen. The stomach connects the oesophagus to the duodenum (the first part of the small intestine). Anatomy/Structure of the stomach: The stomach is divided into four main regions:i. ii. iii. iv. Cardia: It surrounds the superior opening of the stomach. Fundus: The rounded portion superior and to the left of the cardia, is called the fundus. Body: Inferior to the fundus is the large central portion of the stomach, is the body. Pylorus: The pyloric part is divided into three main parts:a. Pyloric antrum: It connects to the body of the stomach. b. Pyloric canal: It is the canal like structure connects to the end part of the pylorus. c. Pylorus: This is connects to the duodenum. When the stomach is empty the mucosa changes in large folds or rugae (wrinkles). Pylorus connects to the duodenum of the small intestine via. smooth muscle sphincter called the pyloric sphincter. The concave medial border of the stomach is called the lesser curvature. The convex lateral border is called the greater curvature. Figure 5: Different part of the stomach. Histology of the stomach: a. Muscle layers: This consists of the smooth muscle fibres. i. An outer layer of longitudinal fibres ii. A middle layers of circular fibres. iii. An inner layers of oblique fibres. This arrangement allows for the churning motion which is the characteristic feature of gastric activity as well as peristaltic movement. b. Mucosa: When the stomach is empty the mucous membrane changes into some folds called rugae and when stomach is full the rugae are ironed out and the surface has a smooth, velvety appearance. Numerous gastric glands are presents below the surface in the mucous membrane and open on to it. The gastric glands contain the three types of the exocrine glands that secrete their products into the stomach. i. Mucous neck cell secrete mucus. ii. Chief cells secrete pepsinogen and gastric lipase. iii. Parietal cells produce intrinsic factor (needed for absorption of Vitamin B12) and hydrochloric acid. The secretions of the mucous, parietal and chief cells from gastric juice are of 2000-3000 ml/day. In addition, a gastric gland includes a type of entero-endocrine cell (G-cell) present in the pyloric antrum and secretes the hormone gastrin. Figure 6: Histology of the stomach. Mechanical and Chemical digestion in stomach: The food accumulates in the layers of the stomach, after the meal has been eaten. 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. The activity of gastric muscle consist 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. Gastric juice: About 2 liters of gastric juice are secreted daily by specialized secretory glands in the mucosa. It consists of:i. Water secreted by gastric glands ii. Mineral salts secreted by gastric glands. iii. Mucous secreted by mucous neck cells in the glands and surface mucous cell on the stomach surface. iv. Hydrochloric acid and intrinsic factor both are secreted by parietal cells in the gastric glands. v. Inactive enzyme precursors like pepsinogens secreted by chief cells in the glands. Functions of the gastric juice:  Water of the gastric juice further liquefies the food.  Hydrochloric acid acidifies the food and stops the action of salivary amylase. As well as kill ingested microbes and provides the acid environment needed for functioning of pepsin. Pepsinogen is activated by HCl into pepsins and begins the digestions of proteins.  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. Phases of gastric juice / Secretion of gastric juice: There is small quantity of gastric juice present in the stomach, even when it contains no food. This is known as fasting juice. A secretion reaches its maximum level about one hour a meal then declines to the fasting level after about 4 hours. There are three phase of secretions of gastric juice:1. Cephalic phase: The flow of the juice occurs before food reaches the stomach and is due to reflex stimulation of the vagus (parasympathetic) nerves initiated by the sight, smell or taste of food. When the vagus nerves have been cut (vagotomy), this phase of gastric secretion stops. Sympathetic stimulation eg:- during emotional states, also inhibits gastric activity. 2. Gastric phase: When stimulated by the presence of food the enteroendocrine cells in the pylorus and duodenum secrets gastrin, a hormone which passes directly into the circulating blood. Gastrin, circulating in the blood, which supplies the stomach, stimulates the gastric glands to produce more gastric juice. Gastrin secretion is suppressed when the pH in the pylorus falls to about 1.5. 3. Intestinal Phase: When the partially digested contents of the stomach reach the small intestine, two hormone secretin and cholecystokinin (CCK), are produced by endocrine cells in the intestinal mucosa. They slow down the secretion of gastric juice and reduce gastric motility. By slowing the emptying rate of the stomach the chyme in the duodenum becomes more thoroughly mixed with bile and pancreatic juice. This phase of gastric secretion is most marked following a meal with a high fat content. The rate at which the stomach empties depends largely on the type of food eaten. A carbohydrate meal leaves the stomach in 2 to 3 hours, a protein meal remains longer and fatty meal remains in the stomach longest. Figure 7: Phases of gastric juice secretion. Function of the stomach: 1. 2. 3. 4. 5. 6. Temporary storage of food. Chemical digestion: Pepsins converts proteins to polypeptides. Mechanical breakdown: It is done by three smooth muscle layers of stomach wall. Limited absorption of the water, alcohol and some lipid soluble drugs. Non-specific defence against microbes by HCl of gastric juice. Production and secretion of intrinsic factor needed for absorption of Vitamin B12 in the ileum. 7. Regulation of the passage of gastric contents into the duodenum. 8. Secretion of the hormone gastrin. Small intestine: Most digestion and absorption of nutrients occurs in a long tube called the small intestine. Its length provides a large surface area for digestion and absorption and that area is further increased by circular folds i.e. villi and microvilli. The small intestine begins at the pyloric sphincter of the stomach and opens into the large intestine. Anatomy of the small intestine: The small intestine is divided into three regions. 1. Duodenum: Shortest region and about 25 c.m. long. 2. Jejunum: It is the middle section of the small intestine and is about 2 meters long. 3. Ileum: It is the terminal section and about 3 meters long, have ileocaecal valve which controls the flow of material from the ileum to the caecum (first part of large intestine and prevents regurgitation). Histology of the small intestine: The wall of the small intestine is composed of four layer of tissue that make up most of the tract: i. ii. iii. iv. Mucosa Submucosa Muscularis Serosa A double layer of peritoneum called the mesentery attaches the jejunum and ileum to the posterior abdominal wall. The surface area of the small intestine mucosa is greatly increased by permanent circular folds called villi and microvilli. The mucosa is made up of a layer of epithelium. The epithelial layer of the small intestinal mucosa consists of simple columner epithelium that contains various types of cells. Absorptions cells of the epithelium digest and absorb nutrients in small intestine. Goblet cells secrets mucus. 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. The intestinal glands are simple tubular glands situated below the surface between the villi. The entire epithelium is replaced every 3 to 5 days. During migration the cells form digestive enzyme that lodge in the microvilli and together with intestinal juice, complete the chemical digestion of carbohydrate, protein and fat. Numerous lymph nodes are found in the mucosa at irregular intervals throughout the length of the small intestine. Small nodules are called solitary lymphatic follicles. Group of lymphatic nodules referred to as aggregated lymphatic follicles or Peyer’s patches. These lymphatic tissues, packed with defensive cells. Figure 8: Layer of the small intestine showing villi. Intestinal juice: The main constituents of intestinal secretion are water, mucus and mineral salts. Most of the digestive enzyme in the small intestine is present in the cells of the walls of the villi. The enzymes that complete the digestion of carbohydrate, protein and fat at the surface of the cells of villi are:i. ii. iii. Peptidase Lipase Sucrase, maltase and lactase About 1-2 liter of intestinal juice secrets daily by the gland of the small intestine. The pH of intestinal juice is usually between 7.8 and 8.0. Chemical digestion in the small intestine: When acid chyme passes in to the small intestine, it is mixed by pancreatic juice, bile and intestinal juice. In the small intestine digestion of all the nutrients is completed. i. ii. iii. Enterokinase activates pancreatic peptidase such as trypsin which convert some polypeptides to the amino acid Lipase completes the digestion of emulsified fats to fatty acid and glycerol Sucrase, maltase and lactase complete digestion of carbohydrate. Control of intestinal juice: Mechanical stimulation of the intestinal glands by chyme is believed to be the main stimulus for the secretion of intestinal juice. Function of the small intestine: 1. Onward movement of its contents by peristalsis. 2. Secretion of intestinal juice. 3. Completion of chemical digestion of carbohydrate protein and fats in the enterocytes of the villi. 4. Protection against infection by microbes. 5. Secretion of the hormone Cholecystokinin (CCK) and Secretin. 6. Absorption of nutrients. Absorption of the nutrients: Absorption of nutrients from the small intestine through the cell of intestine occurs by several processes including diffusion, osmosis, facilitated diffusion and active transport. Water moves by osmosis, small fat soluble substance like fatty acid and glycerol are able to diffuse through cell membrane. Other substances are generally transported inside the villi by other mechanisms. Fatty acids and glycerol enter into the lacteals where they are transported along lymphatic vessels. 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. Large intestine, rectum and anal canal: The large intestine is about 1.5 meters long and is divided into:i. ii. iii. iv. v. Caecum: It is first part of the large intestine. It is a dilated region which has blind end inferiorly and is continuous with the ascending colon superiorly. The vermiform appendix is a fine tube closed at one end, which lead from the caecum. Colon: It has four parts which have the same structure and function. a. The ascending colon: The pass on the right side of the abdomen reaches the inferior surface of the liver and turn to the left at the hepatic flexure to become the transverse colon. b. The transverse colon: This is a loop of the colon that extant 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 downwards to become the descending colon. c. The descending colon: This passes down the left side of the abdominal cavity then curves towards the midline. After entering in the pelvis is known as sigmoid colon. Sigmoid colon: This part describes an S-shaped curve in the pelvis that contains downwards to become the rectum. Rectum: This is a slightly dilated section of the large intestine about 13 c.m. long. It leads from the sigmoid colon and terminates in the anal canal. Anal Canal: This is a short passage about 3.8 c.m. long in the adult and leads from the rectum to the exterior. Two sphincter muscle control the anus:a. Internal sphincter b. External sphincter Histology of the large intestine: The wall of the large intestine contains the typical four layers found in the rest of the GI tract:a) b) c) d) Mucosa Sub mucosa Muscularis Serosa The mucosa consists of simple columnar epithelium. The epithelium contains mostly absorption and goblet cells. The absorptive cells function primarily in water absorption. The goblet cells secrets mucus that lubricates the passage of the colonic contents. The submucosa of the large intestine consists of areolar connective tissue. The muscularis consists of layer of smooth muscle. Unlike other parts of the GIT, portions of the longitudinal muscle are thickened, forming the conspicuous bands called the teniae coli that run most of the length of the large intestine. Function of the large intestine, rectum and anal canal: 1. Absorption: The content of the ileum which pass into the caecum are fluid. In the large intestine absorption of water by osmosis occurs until the semisolid consistency of faeces is achieved. Mineral salt, vitamins and some drugs are also absorbed into the blood capillaries from the large intestine. 2. Microbial activity: The large intestine is heavily colonized by certain type of bacteria, which synthesize vitamin K and folic acid. They include Escherichia coli, Enterobacter aerogenes, Streptococcus faecalis and Clostridium. These microbes are commensals i.e. normally harmless in humans. Gases in the bowel consist of some of the constituents of air mainly nitrogens, hydrogens, carbon dioxide and methane are produced by bacterial fermentation of unabsorbed nutrients, especially carbohydrate. 3. Mass movement: Mass peristaltic movement pushes fecal material from the sigmoid colon into the rectum. The resulting distension of the rectal wall stimulates stretch receptors, which initiates a defecation reflex that empties the rectum. 4. Defecation: Usually the rectum is empty but when a mass movement forces the contents of the sigmoid colon into the rectum the nerve ending in its walls are stimulated by stretch. In the infant, defecation occurs by Involuntary action. Constipation refers to infrequent or difficult defecation caused by decreased motility of the intestine. Because the feces remain in the colon for prolonged periods, excessive water absorption occurs and the feces become dry and hard. Diarrhoea is an increase in the frequency of volume and fluid content of the feces caused by increased motility of intestine and decreased absorption by the intestine. Figure 9: Large intestine, rectum and anal canal Pancreas: The pancreas is about 12-15 c.m. long, thick and present posterior to the greater curvature of the stomach. It consists of a broad head, a body and a narrow tail. The head is present in the curve of the duodenum, the body behind the stomach and the tail is present in front of the left kidney. The pancreas is both an exocrine and endocrine gland. a) The exocrine pancreas: This part consists of a large number of lobules and walls of which consist of secretary cells. Each lobule is drained by a small duct and these ducts unite to form the pancreatic duct or duct of Wirsung extends the whole length of the gland and open into the duodenum. Just, before entering the duodenum the pancreatic duct joins the common bile duct to form the hepatopancreatic ampulla or ampulla of vater. The passage of pancreatic juice and bile through the hepatopancreatic ampulla into the small intestine is regulated by a smooth muscle surrounding ampulla, known as the sphincter of oddi. The function of the exocrine pancreas is to produce pancreatic juice containing enzymes that digest carbohydrates, protein and fat. b) The endocrine pancreas: The endocrine glands are distributed throughout of the pancreas called pancreatic islets (islets of Langerhans). The islets have no ducts, so the hormones diffuse directly into the blood. Pancreas secretes two hormone insulin and glucagon, which mainly regulate blood glucose level. Pancreatic juice: It is secreted by the exocrine part of the pancreas and enters the duodenum at the duodenal papilla. It consists of following things: a) Water b) Mineral salts c) Eneymes like:i) Amylase ii) Lipase iii) Nucleases that digest DNA and RNA d) Inactive enzyme precursors including:i) Trypsinogen ii) Chymotrypsinogen Pancreatic juice is alkaline (pH=8) because it contains significant quantities of bicarbonate ions, which are alkaline in solution. When acidic stomach content enters the duodenum they are mixed with pancreatic juice and with the bile. The pH is raised between to 6 & 8 because at this pH pancreatic enzyme (amylase and lipase) act most effectively. Actions of the Pancreatic juice: a) Digestion of the protein: b) Digestion of Carbohydrates: c) Digestion of fats: Pancreatic lipase converts fat to fatty acid and glycerol. To increase the action of lipase, bile salts emulsify fats i.e. reduce the size of the globules and increase their surface area. Control of the secretion of Pancreatic juice: The secretion of the pancreatic juice is stimulated by secretin and CCK, which is produced by endocrine cells in the walls of the duodenum. Figure 10: Diagrammatic presentation of the pancreas. Liver: The liver is the heaviest gland of the body. The liver is inferior to the diaphragm. It is covered by visceral peritoneum. The liver is divided into two principal lobes: i. ii. a large right lobe a smaller left lobe Both lobes are separated by falciform ligament. Although the right lobe is considered to include an inferior quadrate lobe and a posterior caudate lobe. The portal fissure: This is the name given to the region on the posterior surface of the liver where various structures enter and leave the gland. The portal vein enters carrying blood from the stomach, spleen, pancreas and the small and large intestines. The hepatic artery enters carrying arterial blood. Histology of liver: a) Hepatocytes: The lobes of the liver are made up of tiny functional units called lobules. Liver lobules are hexagonal in outline and are formed by cubical shaped cells called the hepatocytes. Hepatocytes arranged in pairs of columns radiating from a central vein. b) Hepatic Sinusoids: Between pairs of the hepatic cells column are sinusoids (blood vessels with incomplete walls) containing a mixture of blood. Amongst the cells lining the sinusoids are hepatic macrophages (kupffer cells), whose function is to destroy foreign particles. Blood drains from the sinusoids into central vein. The liver receives oxygenated blood through the hepatic artery and nutrient rich deoxygenated blood through the hepatic portal vein. Figure 11: Hepatic Sinusoids. Figure 12: Liver Functions of the liver: The liver is the extremely active organs. 1. Carbohydrate metabolism: The liver is especially important in maintain a normal blood glucose level. When blood glucose is low, the liver can break down glycogen to glucose. When blood glucose is high as occurs just after eating a meal, the liver converts glucose to glycogen and triglycerides for storage. 2. Lipid metabolism: Hepatocytes store some triglyceride break down fatty acids to generate ATP. 3. Protein metabolism: Hepatocytes deaminates the amino acids and remove the nitrogenous portion from the amino acids so that the amino acids can be used for ATP production or converted to carbohydrate or fats. The resulting toxic ammonia is then converted into the much less toxic urea, which is excreted in urine. Hepatocytes also synthesize most plasma proteins. 4. Detoxification of drugs and toxic substance: The liver can detoxify substances such as alcohol and excrete drugs such as penicillin, erythromycin into bile. 5. Phagocytosis: This is carried out by phagocytic hepatic macrophages (kupffer cells) in the sinusoids. 6. Activation of Vitamine D: The skin, liver and kidney synthesize the active form of Vitamine D. 7. Excretion of bilirubin: Bilirubin, which is produced by the breakdown of R.B.C and absorbed by the liver and secreted into the bile. Most of the bilirubin in bile is metabolized in the small intestine by bacteria and eliminated in feces. 8. Synthesis of the bile salts: bile salts are used in the small intestine for the emulsification and absorption of lipids. 9. Storage: Liver can stores the glycogen, iron and copper, fat soluble vitamin A, and D, E, K and water soluble vitamin like B12. Gall Bladder: The gall bladder is a pear shaped sac present to the posterior surface of the liver. It has a fundus, body and neck which is continues with the cystic duct. Gall bladder is covered with the peritoneum, having additional layer muscle fiber. It has small rugae internally. Function: 1. It is the reservoir for bile. 2. Increase the concentration of bile by absorbing the water through the walls of gall bladder. 3. When the muscle wall of the gall bladder contract bile passes through the bile ducts. This contraction is stimulated by the hormone cholecystokinin (CCK), which is secreted by the duodenum as well as presence of the fat and acid chyme in the duodenum. Biliary tract: Bile: Bile is 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 around 8. Composition of the bile: 500- 1000 ml of bile are secreted by the liver daily. Bile consists of:Water, mineral salts, mucus, bile pigments, mainly bilirubin, bile salts, which are derived from the primary bile acids, cholic acid and chenodeoxycholic acid, and cholesterol. The bile acids, cholic and chenodeoxycholic acid are synthesized by liver cells from cholesterol and then secreted into bile as sodium or potassium salts. In the small intestine bile salt emulsify fats and in the terminal ileum most of the bile salts are reabsorbed and return to the liver in the portal vein. Bilirubin is one of the products of haemolysis of R.B.C by hepatic kupffer cells. Bilirubin is insoluble in the water and is carried in the blood. In liver it is combined with glucuronic acid and become water soluble and excreted in bile. Jaundice is yellow pigmentation of the tissue seen in the skin and eye, due to excess blood bilirubin. Biliary ducts: The right and left hepatic ducts join to form the common hepatic duct. Common hepatic duct when joined with the cystic duct of the gall bladder form common bile duct, which passes downwards behind the head of the pancreas. Common bile duct is joined by the main pancreatic duct at the hepatopancreatic ampulla into the duodenum; opening is guarded by sphincter of oddi. Function of the Bile: The bile salts emulsify fats in the small intestine. Fatty acids are insoluble in water, which makes them very difficult to absorb through the intestinal wall. Bile salt makes cholesterol and fatty acids soluble. The bile pigment, bilirubin is a waste product of the breakdown of R.B.C is excreted in the bile after entering into the liver. Control of secretion of bile: After a meal, the duodenum secretes the hormone secretin and CCK; they stimulate contraction of gall bladder and relaxation of the hepatopancreatic sphincter and pass the bile and pancreatic juice into the duodenum together. Secretion of bile is markedly increased when chime entering the duodenum contains a high proportion of fat. Figure13: Gall bladder with Biliary ducts. Digestion and absorption of the nutrients: a. Digestion and absorption of the carbohydrates: Dietary intake of carbohydrates is 250-850 gm/day, which represents 50 to 60 percent of the diet. Major carbohydrates in the human diet are present in the forms of polysaccharides (starch, glycogen, and cellulose), oligosaccharide (sucrose, lactose, and maltose), and monosaccharide (glucose, fructose, and pentose). Digestion of the carbohydrates begins in mouth, continues in stomach but occurs mainly in the small intestine. Carbohydrate is absorbed from the GIT in the form of monosaccharides from the mucosal surface of jejunum and upper ileum. Monosaccharides are absorbed by following mechanism: 1. Glucose and galactose are absorbed by a active transport system. 2. Fructose is absorbed by facilitated diffusion 3. Pentose by simple diffusion The total number of ATP molecules which may be generated from the complete breakdown of one molecule of glucose is 38 during presence of oxygen. In the absence of oxygen this number is greatly reduced. b. Digestion and absorption of the Protein: Proteins are digested by the proteolytic enzyme in to amino acids. Digestion of protein does not occur in the mouth because there is no proteolytic enzyme in the mouth. Digestion of proteins begins in the stomach and is completed in the small intestine. The levo amino acids are absorbed by the active transport. The dextro amino acids are absorbed by passive diffusion. Proteins in the form of the amino acids are main source of the energy when other source are low e.g. during starvation. c. Digestion and absorption of the fat: Fat is synthesized from excess dietary carbohydrate and protein and stored in the fat depots under the skin or around the kidney. Fats that have been digested and absorbed as fatty acids and glycerol into the lacteals and transported to the blood circulation. When body tissue required more glucose as occurs in the prolonged fasting, starvation, energy restricted diets or during exercise, the body used the alternative energy source, mainly stored fats, but in their energy production pathway of the fat the ketone bodies are formed . At high concentration ketone bodies are toxic mainly to the brain. Disorders of the GIT: Constipation Constipation means it is hard to have a bowel movement (or pass stools), they are infrequent (less than three times a week), or incomplete. Constipation is usually caused by inadequate "roughage" or fiber in the diet, or a disruption of the regular routine or diet. Constipation causes a person to strain during a bowel movement. It may cause small, hard stools and sometimes anal problems such as fissures and hemorrhoids. Constipation is rarely the sign of a more serious medical condition. You can treat your constipation by:  Increasing the amount of fiber you eat  Exercising regularly  Moving your bowels when you have the urge (resisting the urge causes constipation) If these treatment methods don't work, laxatives are a temporary solution. Note that the overuse of laxatives can actually make symptoms of constipation worse. Always follow the instructions on the laxative medicine, as well as the advice of your doctor. Irritable bowel syndrome (IBS): Irritable bowel syndrome (also called spastic colon, irritable colon, or nervous stomach) is a condition in which the colon muscle contracts more often than in people without IBS. Certain foods, medicines, and emotional stress are some factors that can trigger IBS. Symptoms of IBS include:  Abdominal pain and cramps  Excess gas  Bloating  Change in bowel habits such as harder, looser, or more urgent stools than normal  Alternating constipation and diarrhea Treatment includes:  Avoiding caffeine  Increasing fiber in the diet  Monitoring which foods trigger IBS (and avoiding these foods)  Minimizing stress.  Sometimes taking medicines as prescribed by your healthcare provider Formation & role of the ATP: Adenosine triphosphate, also known as ATP, is a molecule that carries energy within cells. It is the main energy currency of the cell, and it is an end product of the processes of photophosphorylation (adding a phosphate group to a molecule using energy from light), cellular respiration, and fermentation. All living things use ATP. In addition to being used as an energy source, it is also used in signal transduction pathways for cell communication and is incorporated into deoxyribonucleic acid (DNA) during DNA synthesis. Energy Source: ATP is the main carrier of energy that is used for all cellular activities. When ATP is hydrolyzed and converted to adenosine diphosphate (ADP), energy is released. The removal of one phosphate group releases 7.3 kilocalories per mole, or 30.6 kilojoules per mole, under standard conditions. This energy powers all reactions that take place inside the cell. ADP can also be converted back into ATP so that the energy is available for other cellular reactions. ATP is produced through several different methods. Photophosphorylation is a method specific to plants and cyanobacteria. It is the creation of ATP from ADP using energy from sunlight, and occurs during photosynthesis. ATP is also formed from the process of cellular respiration in the mitochondria of a cell. This can be through aerobic respiration, which requires oxygen, or anaerobic respiration, which does not. Aerobic respiration produces ATP (along with carbon dioxide and water) from glucose and oxygen. Anaerobic respiration uses chemicals other than oxygen, and this process is primarily used by archaea and bacteria that live in anaerobic environments. Fermentation is another way of producing ATP that does not require oxygen; it is different from anaerobic respiration because it does not use an electron transport chain. Yeast and bacteria are examples of organisms that use fermentation to generate ATP. Signal Transduction ATP is a signaling molecule used for cell communication. Kinases, which are enzymes that phosphorylate molecules, use ATP as a source of phosphate groups. Kinases are important for signal transduction, which is how a physical or chemical signal is transmitted from receptors on the outside of the cell to the inside of the cell. Once the signal is inside the cell, the cell can respond appropriately. Cells may be given signals to grow, metabolize, differentiate into specific types, or even die. DNA Synthesis The nucleobase adenine is part of adenosine, a molecule that is formed from ATP and put directly into RNA. The other nucleobases in RNA, cytosine, guanine, and uracil, are similarly formed from CTP, GTP, and UTP. Adenine is also found in DNA, and its incorporation is very similar, except ATP is converted into the form deoxyadenosine triphosphate (dATP) before becoming part of a DNA strand. Formation & role of the Creatinine Phosphate: Phosphocreatine, also known as creatine phosphate, is an energy source for muscle contraction naturally present in the skeletal muscle tissue of humans and other vertebrates. It enables the expression of explosive power in the muscles, lasting no longer than eight to 12 seconds. It is an organic compound of creatine and phosphoric acid. The Phosphocreatine Energy System The muscles use the phosphocreatine energy system, also known as the anaerobic alactic system, during the first 10 seconds of sustained muscle contraction. It is anaerobic because it doesn't require oxygen to function and alactic because it doesn't produce lactic acid. The energy system that powers muscle contraction uses adenosine triphosphate (ATP). ATP is produced from glucose through the glycolytic pathway. But little ATP is stored in the muscle and the body uses it up in the first few seconds of muscle contraction. It needs to be replenished using phosphocreatine to convert adenosine diphosphate (ADP) into ATP in the mitochondria of cells in the muscle tissue. To make ATP, phosphocreatine transfers a phosphate molecule to ADP. The newly-formed ATP is now available to be used for muscle contraction. The leftover portion of phosphocreatine is creatine, which is processed by the liver into creatinine and eliminated through the kidneys and urine. The body doesn't recycle this creatine into phosphocreatine. The Phosphocreatine Cycle: Muscle fatigue is noticeable by the end of the phosphocreatine cycle. After 10 seconds of muscle contraction, the muscles are able to use anaerobic glycolysis and aerobic glycolysis pathways to provide energy. Phosphocreatine is regenerated by the liver, pancreas, and kidneys. Fast-twitch skeletal muscles store more phosphocreatine to have available for short periods of intense muscle contraction. The slow-twitch skeletal muscles and heart store less. By middle age, people have lower phosphocreatine stores in their muscles. For sports and exercises that require explosive muscle contraction, there is a lot of interest in maximizing phosphocreatine in the muscles to boost performance. If you have more phosphocreatine in a muscle, you are able to endure a longer period of intense muscle contraction before the muscle is fatigued and loses power. Weight lifting, sprinting, basketball, football, ice hockey, cycling, tennis and field sports could benefit from a longer phosphocreatine phase. Track sprinters over 100 meters use predominantly the phosphocreatine/ATP system of energy production. Dietary creatine supplementation can increase the total creatine concentration in the muscles. Naturally-occurring creatine in red meat is one source, although it isn't concentrated enough to boost phosphocreatine levels in the muscles above normal. To elevate phosphocreatine levels, supplementation with creatine is used. Other nutrients can boost uptake of creatine by the muscles, especially carbohydrates such as glucose and simple sugars. For this reason, low carb diets may not be the best choice for athletes who want to maximize their explosive muscle power. Vigorous exercise also increases muscle uptake of dietary creatine. BMR: BMR stands for basal metabolic rate and represents the number of calories you would burn in a day if you were inactive and stayed in bed all day. The BMR calculator allows you to calculate not only the number of calories you burn when inactive, but also a daily calorie figure that takes into account your lifestyle activity level. These two figures together give you a representative figure for your daily calorie intake. It is important to bear in mind that BMR calculations do not take into account for lean body mass, which will obviously have a factor of its own. Very muscular people, for example, will receive a figure that probably under-estimates their calorie needs and very overweight people will likely get a calculation that over-estimates their calorie requirements. Role of BMR: Once we know our BMR, we can use it to calculate the calories you actually burn in a day. From there, you can determine how many calories you need to eat to gain muscle, lose fat, or maintain your weight. The overall number of calories your body uses on a daily basis is referred to as your "total daily energy expenditure" (TDEE). It's determined based on your BMR as well as your activity level throughout the day. This varies significantly based on your activity level, age, and sex. Generally, men have a higher TDEE than women because they have more muscle mass, and both TDEE and BMR tend to fall regardless of gender as you age. Factors that influence basal metabolic rate are:  Body size: Metabolic rate increases as weight, height, and surface area increase.  Body composition: Fat tissue has a lower metabolic activity than muscle tissue. As lean muscle mass increases, metabolic rate increases.  Gender: The basal metabolic rate (BMR) averages 5 to 10 percent lower in women than in men. This is largely because women generally possess more body fat and less muscle mass than men of similar size.  Age: A decrease in lean muscle mass during adulthood results in a slow, steady decline of roughly 0 3 percent per year in BMR after the age of about 30. This can be largely avoided by strength training throughout adulthood.  Climate and body temperature: The BMR of people in tropical climates is generally 5 to 20 percent higher than their counterparts living in more temperate areas because it takes energy to keep the body cool. Exercise performed in hot weather also imposes an additional metabolic load. Body fat content and effectiveness of clothing determine the magnitude of increase in energy metabolism in cold environments; it takes energy to keep the body warm if you work or exercise in very cold weather.  Hormonal levels: Thyroxine (T4), the key hormone released by the thyroid glands has a significant effect upon metabolic rate. Hypothyroidism is relatively common, especially in women near or after menopause. Everyone with a weight problem should have their thyroid function checked by their doctor and treated appropriately if it turns out to be low.  Health: Fever, illness, or injury may increase resting metabolic rate two-fold.