Digestive System 2nd Part Lecture Notes PDF

Digestive System nd 2 part Dr. Mai Adawi Lecturer Of Physiology FOM-SCU 2023-2024  PHASES OF GASTRIC SECRETION Gastric secretion occurs in three different phases: I. Cephalic phase II. II. Gastric phase III. III. Intestinal phase Each phase is regulate...

Digestive System nd 2 part Dr. Mai Adawi Lecturer Of Physiology FOM-SCU 2023-2024  PHASES OF GASTRIC SECRETION Gastric secretion occurs in three different phases: I. Cephalic phase II. II. Gastric phase III. III. Intestinal phase Each phase is regulated by neural mechanism or hormonal mechanism or both.  PHASES OF GASTRIC SECRETION Cephalic phase During this phase, gastric secretion occurs even without the presence of food in stomach. The quantity of the juice is less but it is rich in enzymes and HCL. Nervous mechanism regulates cephalic phase through reflex action. Two types of reflexes occur: 1. Unconditioned reflex 2. Conditioned reflex.  PHASES OF GASTRIC SECRETION Cephalic phase 1. Unconditioned Reflex It is the inborn reflex. When food is placed in the mouth, salivary secretion is induced. Simultaneously, gastric secretion also occurs. Stages of reflex action: i. Presence of food in the mouth stimulates the taste buds in the mouth ii. Sensory impulses from mouth pass via afferent nerve fibers of glossopharyngeal and facial nerves to amygdala and appetite center present in hypothalamus iii. The efferent impulses pass through vagal nerve fibers to the wall of the stomach iv. Vagal efferent nerve endings secrete acetylcholine, which stimulates gastric secretion.  PHASES OF GASTRIC SECRETION Cephalic phase 1. Conditioned Reflex Conditioned reflex is the reflex response acquired by previous experience. Presence of food in the mouth is not necessary to elicit this reflex. The sight, smell, hearing or thought of food, which induce salivary secretion and induce gastric secretion also. Stages of reflex action: i. Impulses from the special sensory organs (eye, ear and nose) pass through afferent fibers of neural circuits to the cerebral cortex. Thinking of food stimulates the cerebral cortex directly. ii. From cerebral cortex, the impulses pass through dorsal nucleus of vagus and vagal efferents and reach the stomach wall iii. Vagal nerve endings secrete acetylcholine, which stimulates the gastric secretion.  PHASES OF GASTRIC SECRETION Gastric phase Secretion of gastric juice when food enters the stomach is called gastric phase: 1. Distention of stomach 2. Mechanical stimulation of gastric mucosa by bulk of food 3. Chemical stimulation of gastric mucosa by the food contents. These events trigger nervous and hormonal sequences.  PHASES OF GASTRIC SECRETION Gastric phase 1. Nervous Mechanism Local myenteric reflex: These nerve fibers release acetylcholine, which stimulates the gastric glands to secrete a large quantity of gastric juice and stimulates G cells to secrete gastrin hormone.  PHASES OF GASTRIC SECRETION Gastric phase 1. Nervous Mechanism Vagovagal reflex: both afferent and efferent impulses pass through vagus n leading to secretion of gastric juice  PHASES OF GASTRIC SECRETION Gastric phase 2. Hormonal Mechanism – Gastrin Gastrin is a gastrointestinal hormone secreted by the G cells which are present in the pyloric glands of stomach. Gastrin stimulates the secretion of pepsinogen and hydrochloric acid by the gastric glands.  PHASES OF GASTRIC SECRETION Intestinal phase Intestinal phase is the secretion of gastric juice when chyme enters the intestine. It is regulated by nervous and hormonal control. Initial Stage of Intestinal Phase Chyme that enters the intestine stimulates the duodenal mucosa to release gastrin, which is transported to stomach by blood. There it increases gastric secretion.  PHASES OF GASTRIC SECRETION Intestinal phase Later Stage of Intestinal Phase A decrease or complete stoppage of gastric secretion by two factors: 1. Enterogastric reflex: Due to the distention of intestinal mucosa by chyme or chemical irritation by chemical substances in the chyme. It is mediated by myenteric nerve plexus and vagus.  PHASES OF GASTRIC SECRETION Intestinal phase 2- GI hormones which inhibit gastric secretion: i. Secretin: Secreted by the presence of acid chyme in the intestine ii. Cholecystokinin: Secreted by the presence of chyme containing fats and amino acids in intestine iii. Gastric inhibitory peptide (GIP): Secreted by the presence of chyme containing glucose and fats in the intestine iv. Vasoactive intestinal polypeptide (VIP): Secreted by the presence of acidic chyme in intestine v. Peptide YY: Secreted by the presence of fatty chyme in intestine PHASES OF GASTRIC SECRETION  Small Intestine INTESTINAL VILLI AND GLANDS OF SMALL INTESTINE: Mucous membrane of small intestine is covered by minute projections called villi.  Small Intestine Secretion from small intestine is called succus entericus. COMPOSITION OF SUCCUS ENTERICUS:  Small Intestine FUNCTIONS OF SUCCUS ENTERICUS: 1. DIGESTIVE FUNCTION Proteolytic Enzymes: peptidases, these convert peptides into amino acids. Amylolytic Enzymes: Lactase, sucrase and maltase convert the disaccharides (lactose, sucrose and maltose) into two molecules of monosaccharides Dextrinase converts dextrin, maltose and maltriose into glucose. Lipolytic Enzyme: Intestinal lipase acts on triglycerides and converts them into fatty acids.  Small Intestine FUNCTIONS OF SUCCUS ENTERICUS: 2. PROTECTIVE FUNCTION i. Mucus present in the succus entericus protects the intestinal wall from the acid chyme, which enters the intestine from stomach; thereby it prevents the intestinal ulcer. ii. Defensins secreted by paneth cells of intestinal glands are the antimicrobial peptides.  Small Intestine FUNCTIONS OF SUCCUS ENTERICUS: 3. ACTIVATOR FUNCTION Enterokinase present in intestinal juice activates trypsinogen into trypsin. Trypsin, in turn activates other enzymes. Functions of Small Intestine 1. MECHANICAL FUNCTION Mixing movements of small intestine help in mixing of chyme with the digestive juices like succus entericus, pancreatic juice and bile. „2. SECRETORY FUNCTION Small intestine secretes succus entericus, enterokinase and the GI hormones. Functions of Small Intestine 5- ABSORPTIVE FUNCTIONS Presence of villi and microvilli in small intestinal mucosa increases the surface area of mucosa. This facilitates the absorptive function of intestine. Digested products of foodstuffs, proteins, carbohydrates, fats and other nutritive substances such as vitamins, minerals and water are absorbed mostly in small intestine. From the lumen of intestine, these substances pass through lacteal of villi, cross the mucosa and enter the blood directly or through lymphatics Pancreas Pancreas is a dual organ having two functions, namely endocrine function and exocrine function. Endocrine function is concerned with the production of hormones Exocrine function is concerned with the secretion of digestive juice called pancreatic juice.  Exocrine Function Of The Pancreas FUNCTIONS OF PANCREATIC JUICE Pancreatic juice has digestive functions and neutralizing action. A- Neutralizing Function High bicarbonate content makes the pancreatic juice highly alkaline, so that it protects the intestinal mucosa from acid chyme by neutralizing it. B- DIGESTIVE FUNCTIONS Pancreatic juice plays an important role in the digestion of proteins and lipids. It also has mild digestive action on carbohydrates Pancreatic juice  Trypsin It is secreted as inactive trypsinogen, which is converted into active trypsin by enterokinase. Trypsin is the most powerful proteolytic enzyme. It breaks the interior bonds of the protein molecules and converts proteins into proteoses and polypeptides It activates the other enzymes of pancreatic juice, a. Chymotrypsinogen into chymotrypsin b. Procarboxypeptidases into carboxypeptidases c. Proelastase into elastase d. Procolipase into colipase  Pancreatic lipase Pancreatic lipase is a powerful lipolytic enzyme. It digests triglycerides into monoglycerides and fatty acids. Activity of pancreatic lipase is accelerated in the presence of bile. Digestion of fat by pancreatic lipase requires two main factors: i. Bile salts, which are responsible for the emulsification of fat, prior to their digestion ii. Colipase, which is a coenzyme necessary for the pancreatic lipase to digest the dietary lipids. Bile The liver produces and secretes 250 to 1,500 ml of bile per day. The major constituents of bile are: 1- bile pigment (bilirubin) 2- bile salts 3- phospholipids (mainly lecithin) 4- cholesterol 5- inorganic ions. Gallbladder Between meals, secreted bile is stored in the gallbladder, a small sac underneath the liver that branches from the common hepatic duct. The gallbladder concentrates the organic molecules in bile by absorbing some ions and water. During a meal, the smooth muscles in the gallbladder wall are stimulated to contract, causing a concentrated bile solution to be injected into the duodenum via the common bile duct  Bile Pigments Bile pigments are substances formed from the heme portion of hemoglobin when old or damaged erythrocytes are broken down in the spleen and liver. The predominant bile pigment is bilirubin, which is extracted from the blood by hepatocytes and actively secreted into the bile. Bilirubin is yellow and contributes to the color of bile. During their passage through the intestinal tract, some of the bile pigments are absorbed into the blood and are eventually excreted in the urine, giving urine its yellow color. After entering the intestinal tract, some bilirubin is modified by bacterial enzymes to form the brown pigments that give stools their characteristic color.  Function of bile salts 1. Emulsification of Fats Emulsification is the process by which the fat globules are broken down into minute droplets and made in the form of a milky fluid called emulsion in small intestine, by the action of bile salts. Lipolytic enzymes of GI tract cannot digest the fats directly because the fats are insoluble in water due to the surface tension. Bile salts emulsify the fats by reducing the surface tension due to their detergent action. Now the fats can be easily digested by lipolytic enzymes.  Function of bile salts 1. Absorption of Fats Bile salts help in the absorption of digested fats from intestine into blood. Bile salts combine with fats and make complexes of fats called micelles. The fats in the form of micelles can be absorbed easily.  Function of 3. Choleretic Action bile salts Bile salts stimulate the secretion of bile from liver. This action is called choleretic action. 4. Cholagogue Action Cholagogue is an agent which causes contraction of gallbladder and release of bile into the intestine. Bile salts act as cholagogues indirectly by stimulating the secretion of hormone cholecystokinin. This hormone causes contraction of gallbladder, resulting in release of bile. 5. Laxative Action Bile salts act as laxatives by stimulating peristaltic movements of the intestine. 6. Prevention of Gallstone Formation Bile salts prevent the formation of gallstone by keeping the cholesterol and lecithin in solution. In the absence of bile salts, cholesterol precipitates along with lecithin and forms gallstone Large Intestine PARTS OF LARGE INTESTINE Large intestine is made up of the following parts: 1. Cecum with appendix 2. Ascending colon 3. Transverse colon 4. Descending colon 5. Sigmoid colon or pelvic colon 6. Rectum 7. Anal canal Functions of Large Intestine 1- Neutralization of Acids Strong acids formed by bacterial action in large intestine are neutralized by the alkaline nature of large intestinal juice. The alkalinity of this juice is mainly due to the presence of large quantity of bicarbonate. 2- Lubrication Activity Mucin present in the secretion of large intestine lubricates the mucosa of large intestine and the bowel contents, so that, the movement of bowel is facilitated. Functions of Large Intestine 3- ABSORPTIVE FUNCTION Large intestine plays an important role in the absorption of various substances such as: i. Water ii. Electrolytes iii. Organic substances like glucose iv. Alcohol v. Drugs like anesthetic agents, sedatives and steroids. 4- FORMATION OF FECES After the absorption of nutrients, water and other substances, the unwanted substances in the large intestine form feces. This is excreted out Thank you