Digestive System Notes PDF

Digestive System Human Physiology General Course  The Gastrointestinal System The gastrointstinal tract: Mouth Oesophageus Stomach Small intestine Large intestine Associated organs: the salivary glands The liver The pancreas ...

Digestive System Human Physiology General Course  The Gastrointestinal System The gastrointstinal tract: Mouth Oesophageus Stomach Small intestine Large intestine Associated organs: the salivary glands The liver The pancreas  Functions of the Digestive System 1- Digestion: means breakdown of large molecules to small absorbable molecules. 2- Absorption: means passage of the digested food to the blood via crossing the mucosa of the GIT. 3- Secretion: means secretion of saliva and other GIT juices. 4- Motility: which helps transport of food from the mouth mouthdown downto all to parts of the all parts ofalimentary canal. the alimentary canal.  Control of the Gastrointestinal System Motility and secretion of the GIS is controlled by: Autonomic nervous system (ANS ). Gastrointestinal hormones.  The Autonomic Nervous System GIT have special ANS called the Enteric NS: The neurons of this system are found within the wall of the intestine. The activity of this system can be modified by the sympathetic and parasympathetic systems.  Gastrointestinal Hormones The walls of stomach and intestine secrete many hormones that control its own activity e.g.: Gastrin. Cholecystokinin. Secretin. Gastric inhibitory peptide (GIP). …..  Motility (movement of the stomach and intestine) Aim of motility: Mix the food with the digestive enzymes. Push the food down the GIT. 2 Types of motility: 1. Propulsive movement (Peristalsis) 2. Mixing-Movement (Segmentation-Movement).  Propulsive movement (Peristalsis) Carrying the food down the GIT. It is stimulated by distension. The part of the intestine behind the food contracts and the part in front of the food dilates.  Mixing Movement (Segmentation Movement): It mixes food with digestive enzymes.  Saliva While the food is in the mouth it is mixed with saliva. About 1.5 liters are secreted daily. Saliva is secreted mainly by 3 pairs of glands in the oral cavity: 1. Parotid glands: produce a serous, watery secretion(25%). 2. Submandibular glands: produce a mixed serous and mucous secretion(70%). 3. Sublingual glands: secrete mainly mucous secretion (5%).  Composition of Saliva Water. Electrolytes. Enzymes (amylase). Mucin. Saliva is rich in HCO3,calcium and phosphate. The alkaline pH, calcium and phosphate protect the teeth from demineralization.  Functions of Saliva 1-Lubrication and binding: the mucus in saliva binds masticated food into a slippery bolus that slides easily through the esophagus. 2-Coating the oral cavity and esophagus: protecting against solid food particles. 3-Dissolving dry food: to stimulate taste receptors, the molecules in food must be dissolved. 4-Oral hygiene: (Mechanical flushing. Lysozyme, an enzyme that lyses many bacteria. Antibodies (IgA)) 5-Initiates starch digestion: contains the enzyme amylase which starts digestion of dietary starch into maltose. 6-Helps in speech: facilitating movement of the lips, tongue and teeth.  Control of salivary secretion Secretion of saliva is under control of the autonomic nervous system: Parasympathetic stimulation produces a large volume of saliva that is rich in electrolytes. Sympathetic stimulation produces a small, thick secretion, that is rich in enzyme.  Salivary secretion has 2 phases: 1. Conditioned (Aquired) reflex: starts before food enters the mouth by sight, smell, or thinking of food. 2. Simple (Inborn)(Unconditioned) reflex: the presence of food or any object in the mouth stimulates salivary secretion.  Conditioned reflex: 1. Specific visual, auditory, olfactory or memory. Stimuli associated with food, consciously interpreted in cerebrum. 2. Impulses sent to salivary centre in medulla. 3. Saliva production initiated. Conditioned Reflex for Saliv. Secretion cerebral cortex visual.,olfactory,.. Conditioned salivary centre reflex in medulla autonomic nerves salivary glands salivary secretion  Simple reflex: 1. Food ingestion stimulates both mechanical and chemical receptors in the mouth. 2. Sensory impulses sent to salivary centre in medulla. 3. Autonomic impulses from medulla initiate saliva production.  Simple Reflex for Saliv. Secretion salivary centre in medulla autonomic nerves simpl e Mechano- & reflex chemoreceptors salivary glands in the mouth salivary secretion  Abnormalities of Salivary Secretion 1. Excessive salivation is a symptom of almost any lesion in the oral cavity. 2. Xerostomia: decreased salivary secretion, may lead to oral infections  The Stomach The main parts are: Fundus Body Antrum Pylorus  The food enters the stomach in a semi-solid form (bolus), is turned into a semi-liquid form by the grinding action of the stomach and the gastric juice. As food is liquefied in the stomach it passes through the pyloric canal into the small intestine.  Functions of the Stomach 1. It serves as a short-term storage reservoir. 2. Grinds and mixes food with gastric secretions, resulting in liquefaction of food. 3. Slowly releases food into the small intestine for further processing. 4. Starts digestion of some foods particularly proteins. 5. Releases iron from its compounds.  The wall of the stomach contains 4 major types of cells: 1. Mucous cells: secrete an alkaline mucus that protects the epithelium physically and chemically. 2. Parietal cells: secrete hydrochloric acid and intrinsic factor. 3. Chief cells: secrete pepsin, a proteolytic enzyme. 4. G cells: secrete the hormone gastrin.  Gastric Secretion About 2.5 liters are secreted into the lumen of the stomach daily (gastric juice). The gastric juice consists of: 1-Mucus: a bicarbonate-rich mucus that coats and lubricates the gastric surface to protect it from acid and other agents. 2-Acid: Hydrochloric acid is secreted from parietal cells into the lumen providing an acidic medium (ph:1-2). This acid is important for activation of pepsinogen and inactivation of ingested microorganisms such as bacteria. 3-Pepsinogen: an inactive protease is secreted into gastric juice from the chief cells. Once secreted, pepsinogen is activated by Hcl acid into the active protease pepsin. Pepsin initiates digestion of proteins. 4-Intrinsic factor: secreted by parietal cells that is necessary for intestinal absorption of vitamin B12. 5-A number of other enzymes are secreted by gastric epithelial cells eg. lipase. N.B. In addition to the gastric juice the stomach secretes hormones(into the blood): eg. gastrin, a peptide that is important in control of acid secretion and gastric motility.  Control of Gastric Secretion It is divided into 3 phases: 1-Cephalic phase: Seeing, smelling and anticipating food is perceived in the brain. The brain informs the stomach that it should prepare for reception of a meal. This communication is composed of parasympathetic stimuli transmitted thought the vagus nerve.  The vagus nerve stimulation releases ACH and stimulates gastrin and histamine secretion. ACH, gastrin and histamine stimulate secretion of gastric juice. 2-Gastric phase: When a meal enters the stomach, it causes distension and mucosal irritation. These stimulate further secretion of gastric juice, by increasing gastrin & histamine more. 3-Intestinal (inhibitory) phase : The wall of the small intestine has chemical and osmoreceptors. If the contents reaching the small intestine are highly acidic or hyperosmolar, the small intestine sends inhibitory impulses and inhibitory hormones, to the stomach. The impulses and hormones are conveyed to the stomach to inhibit its secretion.  The three phases of secretion of gastric juice. N.B. Two of the hormones {secretin and cholecystokinin (CCK)} are forming enterogastrone.  Abnormalities of Gastric Secretion 1-If there is excessive secretion or if the protective alkaline mucous barrier is damaged, this may result in peptic ulcer. The protective layer may be damaged by drugs eg. aspirin, by alcohol, by bacteria (Helicobacter pylori). 2-Acid secretion can be decreased by drugs like cimetidine, ranitidine and omeprazole. Cimetidine, Ranitidine are histamine antagonists.  The Small Intestine The small intestine is the longest section of the digestive tube extending from the pylorus to the large intestine. It consists of 3 segments: Duodenum Jejunum Ileum The small intestine is the site for digestion and absorption of almost all nutrients. The small intestine is essential for life.  Digestion in Small Intestine A. Once within the small intestine, the food particles are exposed to pancreatic enzymes and bile, which enables digestion of all food particles. B. The final stages of digestion occur on the surface of the small intestinal epithelium.(the brush border).  Absorption in Small Intestine The absorptive surface area of the small intestine (the brush border) is about 250 square meters. This very large surface area is because the wall of the small intestine has 3 modifications: a. Mucosal folds: the inner surface of the small intestine is not flat, but thrown into circular folds. b. Villi: the mucosa forms projections which protrude into the lumen. c. Microvilli: the villi is studded with densely-packed microvilli.  All nutrients from the diet are absorbed into blood across the mucosa of the small intestine. In addition, the intestine absorbs water and electrolytes  Absorption of Water and Electrolytes The small intestine absorbs massive quantities of water. A normal person ingest roughly 1 to 2 liters of dietary fluid every day. On top of that, another 6 to 7 liters of fluid is received by the small intestine daily as secretions from salivary glands, stomach, pancreas, liver and the small intestine itself.  By the time the contents enters the large intestine, approximately 80% of this fluid has been absorbed. The absorption of water is dependent on absorption of solutes, particularly sodium. Interference with water absorption may cause diarrhea.  The Pancreas The pancreas consists of 2 main parts: i. Exocrine pancreas: Secretes the pancreatic juice that is important for digestion. ii. Endocrine pancreas: Secretes hormones; insulin and glucagon that are important in glucose metabolism.  Pancreatic Juice About 1.5 liters are secreted daily into the duodenum by the exocrine pancreas. The pancreatic secretion and bile enter the duodenum through a common opening. The pancreatic juice is alkaline (rich in bicarbonate). Contains enzymes that are able to digest all types of nutrients e.g.: Amylase. Trypsin. Chymotrypsin. Lipase.  Functions of Pancreatic Juice 1- Na bicarbonate: - Neutralizes the acids which come from the stomach to the duodenum and this: Prevents development of duodenal ulcer. Adjusts the PH for the action of pancreatic enzymes. 2- Digestion of protein: - By trypsin, chymotrypsin, carboxypeptidase enzymes. 3- Digestion of lipids: - By pancreatic lipase which acts on triglycerides to form monoglycerides and fatty acids. - phospholipase A2. 4- Digestion of carbohydrate: By pancreatic amylase it converts starch, glycogen to disaccharides.  Control of Pancreatic Secretion Pancreatic secretion is stimulated by: a. Secretin: produced by the duodenum, stimulates HCO3 secretion. b. Cholecystokinin(CCK): produced by the duodenum stimulates enzyme secretion. c. Parasympathetic (Vagus): stimulates enzyme secretion.  Functions of the Liver 1. Carbohydrate metabolism. 2. Fat metabolism. Desaturation of fat, i.e. converts stored fat to a form in which it can be used by the tissues to provide energy. 3. Protein metabolism. Deamination of amino acids: removing nitrogenous portion from the amino acids to form urea which is excreted in the urine and uric acid which is excreted in the urine. Transamination: removes the nitrogenous portion of amino acids and attaches it to other carbohydrate molecules forming new non-essential amino acids Synthesis of plasma proteins and most of the blood clotting factors from the available amino acids occurs in the liver. 4. Breakdown of erythrocytes and defense against microbes. by phagocytic Kupffer cells (hepatic macrophages) in the sinusoids. 5. Detoxification of drugs and noxious substances. These include ethanol (alcohol) and toxins produced by microbes. 6. Inactivation of hormones. These include insulin, glucagon, cortisol, aldosterone, thyroid and sex hormones. 7. Synthesis of vitamin A from carotene. 8. Production of heat. It is the main heat-producing organ of the body. 9. Secretion of bile. Synthesize the constituents of bile from the mixed arterial and venous blood in the sinusoids. These include bile salts, bile pigments and cholesterol. 10. Storage: Fat-soluble vitamins: A, D, E, K. Iron and copper. Some water-soluble vitamins, e.g. riboflavine, niacin, pyridoxine, folic acid and vitamin B12.  Bile Bile is a yellow secretion formed by the liver cells. It is collected by the bile ducts and secreted into the duodenum at the time of meals. In between meals bile is stored in the gall bladder.  Composition of bile Bile contains bile salts and bile pigments. A. Bile salts are responsible for emulsification of fat. B. Bile pigments (bilirubin) is a waste product to be excreted in stools.  Control of Bile Secretion Bile secretion is stimulated by the vagus nerve and the hormone cholecystokinin. The secretion starts when a meal enters the duodenum. Obstruction in the bile ducts e.g. by stone causes jaundice due to increased bile pigments in blood.  Digestion And Absorption Of Carbohydrates Polysaccharides and disaccharides must be digested to monosaccharides prior to absorption. a) The main dietary carbohydrate is starch. Digestion of starch starts in the mouth by the salivary amylase. Pancreatic amylase continues the digestion of carbohydrates. Amylase breaks down starch into maltose. b) Disaccharides: The dietary disaccharides, lactose (milk sugar) , sucrose (table sugar), and maltose are broken down by enzymes on the wall of the small intestine (the brush border): Maltase cleaves maltose into two molecules of glucose. Lactase cleaves lactose into a glucose and a galactose. Sucrase cleaves sucrose into a glucose and a fructose. c) Monosaccharides: The monosacharides, glucose, galactose and fructose are absorbed into the intestinal cells and then into the blood. Deficiency of lactase may cause lactose intolerance (abdominal discomfort and diarrhea following intake of milk).  Digestion And Absorption Of Proteins They must be digested to amino acids first. Digestion of proteins starts in the stomach. The stomach secretes pepsinogen (activated to pepsin by the action of HCl). The pancreas secretes a group of potent proteases, e.g.: Trypsin and chymotrypsin. They are secreted as inactive and activated after reaching the lumen of the intestine.  Gastric and pancreatic proteases digest dietary proteins within the lumen of the small intestine into medium and small peptides. The surface of the small intestine has peptidase enzymes. The peptidases complete the breakdown of peptides, converting them to free amino acids. The free amino acids are absorbed into the blood.  Digestion And Absorption Of Lipids The main dietary lipid is triglyceride, composed of a glycerol with 3 fatty acids. Additionally, most foodstuffs contain cholesterol, fat-soluble vitamins. In order for the triglyceride to be absorbed, two processes must occur: 1. Emulsification: large aggregates of dietary triglyceride, which are insoluble in an aqueous environment, are broken down physically. 2. Digestion: Triglyceride molecules must be enzymatically digested to produce monoglyceride and fatty acids.  Emulsification is done by bile salts. Bile salts have water soluble and fat soluble poles. Lipids are dissolved by the fat soluble part (the inside). The combination of bile salts with various dietary lipids are called micelles.  Digestion of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by pancreatic lipase.  Micelles: Monoglycerides, fatty acids, bile salts and other lipids form structures called micelles. Micelles have a hydrophilic surface. Chylomicrons: After entry into the intestinal cells , lipids combine with proteins to form chylomicrons inside the cells. The chylomicrons enter the lymphatics first before reaching the blood. Short chain fatty acids may enter the blood directly.  Functions of the Large intestine, Rectum and Anal canal 1. Absorption Water until the feces is semisolid. Mineral salts, vitamins and some drugs are also absorbed. 2. Microbial activity Contain certain types of bacteria, which synthesize vitamin K and folic acid. Gases in the bowel consist of N2, O2, CO2, H2 and methane are produced by bacterial fermentation of unabsorbed nutrients, especially carbohydrate. Gases pass out of the bowel as flatus. 1. Mass movement About twice an hour does a wave of strong peristalsis sweep along the transverse colon forcing its contents into the descending and sigmoid colons. 2. Defecation When a mass movement forces the contents of the sigmoid colon into the rectum, the nerve endings in its walls are stimulated by stretch. The external anal sphincter is under conscious control through the pudendal nerve. Defecation 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 assist the process of defecation.  Feces Normal feces consist roughly of 75% water, and 25% solids. The bulk of fecal solids are bacteria, undigested organic matter, and undigested fibers, mainly cellulose in man. The characteristic brown color of feces is due to stercobilinogen produced by bacterial degradation of bilirubin. Fecal odor results from gases produced by bacterial metabolism, including skatole, mercaptans, and hydrogen sulfide.  Intestinal Gas A considerable amount of gas is normally present in the gastrointestinal tract. Complaints of excessive gastrointestinal gas production in people is common. Five major intestinal gases are: N2, O2, CO2, H2 and methane.