GI Physiology PDF

GI Physiology GI Tract Mouth Esophagus & stomach Small & large intestine Rectum & anus Accessory organs: liver, gallbladder, exocrine pancreas Saliva Is composed of water with mucus, sodium, bicarbonate, chloride, potassium, and salivary α-amylase (carbohydrate digestion). Is controlled by ANS ○ Cho...

GI Physiology GI Tract Mouth Esophagus & stomach Small & large intestine Rectum & anus Accessory organs: liver, gallbladder, exocrine pancreas Saliva Is composed of water with mucus, sodium, bicarbonate, chloride, potassium, and salivary α-amylase (carbohydrate digestion). Is controlled by ANS ○ Cholinergic parasympathetic fibers stimulate salivary glands Clinical Example give anticholinergic agent like Atropine to inhibit salivation ○ Sympathetic Beta-adrenergic stimulation decreases salivary secretion but I teases protein concentration pH 7.4 which neutralizes bacterial acids aiding to prevent infection Contains immunoglobulin A (IgA) Esophagus Upper esophageal sphincter (cricopharyngeal muscle) prevents the entry of air into the esophagus during respiration. Lower esophageal sphincter (LES) was formerly called cardiac sphincter. ○ Prevents regurgitation from the stomach. ○ Increase sphincter tone: Cholinergic vagal stimulation and the digestive hormone gastrin. ○ Relax sphincter tone: Non-adrenergic, non-cholinergic vagal impulses and the hormones progesterone, secretin, and glucagon. ○ Mediation of relaxation during swallowing: Vagus nerve. Stomach Muscle layers ○ Longitudinal: Outer ○ Circular: Middle ○ Oblique: Inner Sphincters ○ Lower esophageal sphincter: chyme enters from esophagus to the stomach ○ Pyloric sphincter: Chyme exits the stomach into the duodenum of small intestine Blood supply via branch of celiac artery; drainage via splenic vein and tributaries Few substances absorbed in the stomach ○ Can absorb alcohol, aspirin, other non steroidal anti-inflammatory agents Gastric Motility Swallowing – causes the fundus to relax. Secretin and cholecystokinin – enhance relaxation of the stomach. Gastrin and motilin and low blood glucose – enhance peristalsis contractions of the stomach. Sympathetic activity, secretin, cholecystokinin – inhibit peristalsis contractions of the stomach. Vagus nerve – stimulates gastric secretion and motility Gastric emptying ○ Movement of gastric contents into the duodenum ○ Increase gastric emptying: Larger volumes of food ○ Delay gastric emptying: Solids, fats, and non isotonic solutions Fat digestion stimulates cholecystokinin: Hormone inhibits food intake, gastric motility, and decreases gastric emptying so that fats do not exceed the rate of bile and enzyme secretion Gastric Secretion Potassium levels higher in gastric secretions than in plasma Rate of secretion varies with time of day – lower in AM, higher in PM Stomach secretes large volumes of juices ○ Mucus: Protective barrier ○ Acid: Hydrochloric acid ○ Enzymes: Pepsinogen ○ Hormones: Gastrin ○ Intrinsic factor: Intestinal absorption of vitamin B12 ○ Gastroferrin: Facilitates small intestinal absorption of iron Gastric pits – common duct into which several gastric glands empty Gastric hydrochloric acid ○ Dissolves food fibers. ○ Acts as a bactericide against swallowed organisms. ○ Converts pepsinogen to pepsin. Pepsin ○ Enzyme that breaks down protein-forming polypeptides in the stomach ○ Strongest stimulation: Acetylcholine ○ Inactivation: Alkaline environment of duodenum Mucus ○ Prostaglandins and nitric oxide: Protect the mucosal barrier. ○ Aspirin or other non-steroidal anti-inflammatory drugs, Helicobacter pylori, ethanol, regurgitated bile, or ischemia from breaks: Cause inflammation and ulceration. Intrinsic factor (IF) ○ Required for the absorption of vitamin B12 in the ileum ○ Combines with vitamin B12 in the stomach. ○ Atrophic gastritis and failure of the absorption mechanisms of vitamin B12 can lead to pernicious anemia. Gastric glands: primary secretory units ○ Parietal cells – Hydrochloric acid and intrinsic factor, gastroferrin ○ Chief cells – Pepsinogen which is then converted to pepsin ○ G cells – Gastrin ○ Enterochromaffin-like cells – Histamine ○ D cells – Somatostatin Stimulates Hydrochloric Acid – Acetylcholine, Caffeine, Calcium, Gastrin, Histamine Inhibits Hydrochloric Acid – Prostaglandins, Gastric inhibitory peptide, Somatostatin, Secretin Phases of Gastric Secretion #1 Cephalic phase ○ Anticipatory and sensory experiences of smelling, seeing, tasting, chewing, and swallowing food start secretion process ○ Mediated by the vagus nerve Acid, pepsinogen, and gastrin secreted Insulin secreted by pancreas to hyperglycemia and is strong stimulus to gastric secretion and again is mediated by vagus nerve #2 Gastric Phase ○ Begins with arrival of food to stomach ○ 2 major stimuli have secretory effect Distention of stomach And presence of digested protein ○ Vagus nerve stimulated by distention contributing to secretion stimulus Mediated by acetylcholine and can be blocked by atropine #3 Intestinal Phase ○ Movement of chyme from stomach to duodenum initiates this phase. ○ There is decelerated gastric secretion ○ When chyme enters the higher pH environment of the duodenum inhibitory vagal reflexes decrease gastric motility ○ Secretin and cholecystokinin stimulate pancreatic secretions and inhibit gastric secretions Small Intestine Three segments: ○ Duodenum – begins at the pylorus and ends where it joins the jejunum called the Treitz ligament Duodenum receives chyme from the stomach through the pyloric valve; absorbs vitamins, minerals, fats, and sugars Presence of chyme stimulates the liver and gallbladder to deliver bile and the pancreas to deliver digestive enzymes and alkaline secretions ○ Bile and enzymes flow through an opening guarded by the sphincter of Oddi ○ Bile is produced by the liver and is necessary for the fat digestion and absorption ○ Bile’s alkalinity helps neutralize chyme, creating a pH that enables the pancreatic enzymes to digest proteins, carbohydrates, and sugars ○ Jejunum Jejunum absorbs carbs, and proteins across intestinal mucosa by active transport into the villus capillaries ○ Ileum Ileocecal valve – controls the flow of digested material from the ileum into the large intestine and prevents reflux into the small intestine Ileum absorbs bile salts, vitamin B12, and chloride Peritoneum – serous membrane surrounding the organs of the abd and pelvic cavity ○ Visceral – lies over the organs ○ Parietal – lines the wall of the abd cavity ○ Inflammation of the peritoneum, peritonitis, occurs with perforation of the intestine or after surgery Peritoneal cavity – space between the two layers of the peritoneum Muscle layers: ○ Outer—longitudinal ○ Inner—circular ○ Mucosal folds (plica) within the small intestine slow the passage of food providing more time for digestion and absorption (more numerous in the jejunum and upper ileum Blood flow to the small intestine is primarily provided by the superior mesenteric artery Myenteric plexus and mucosal plexus – site of intrinsic motor innervation mediation ○ Parasympathetic – secretion, motility, pain sensation, and intestinal reflexes mediation ○ Sympathetic – inhibits motility and produces vasoconstriction Villi – cover the mucosal folds and are the functional units of the intestine ○ Absorption occurs through the villi ○ Villus composed of absorptive columnar cells (enterocytes) and mucus secreting goblet cells of the mucosal epithelium ○ Villi and microvilli increase surface area for absorption All nutrients from food intake are absorbed in the small intestines as well as 85% of the water and fluid intake Digestion and Absorption Carbohydrates ○ Complex carbohydrates (polysaccharides and oligo-saccharides) must be hydrolyzed to their simplest form. ○ Salivary and pancreatic amylases break down starches. ○ Sugars are primarily absorbed in the duodenum and in the upper jejunum. Proteins ○ Pancreatic enzymes accomplish major protein hydrolysis in the small intestine. Trypsin and chymotrypsin: Hydrolyze the interior bonds of the large molecules (endopeptidases). Carboxypeptidase: Breaks away the end amino acids (exopeptidase). Fats ○ Emulsification Emulsifying agents cover the small fat particles and prevent them from re-forming into fat droplets. Fats are then ready for lipolysis (lipid hydrolysis). Lipase breaks down triglycerides into diglycerides, monoglycerides, free fatty acids, and glycerol. Phospholipase cleaves fatty acids from phospholipids. Cholesterol esterase breaks cholesterol esters into fatty acids and glycerol. ○ Micelle formation ○ Fat absorption ○ Resynthesized triglycerides and phospholipids Minerals and vitamins ○ Most minerals are absorbed by passive diffusion or active transport or are actively transported with a carrier. ○ Most of the water-soluble vitamins are passively absorbed or by sodium-dependent active transport. Large Intestine Massages fecal mass; absorbs water and electrolytes Cecum – pouch that receives chyme from the ileum Appendix – attached to the cecum Colon – ascending, transverse, descending, sigmoid Rectum and anus There are mucus-secreting cells (goblet cells) in the mucosa of the large intestine but NO VILLI Ileocecal valve – admits chyme from the ileum to the cecum. O’Beirne sphincter – controls the movement of wastes from the sigmoid colon into the rectum. Internal anal sphincter – smooth muscle External anal sphincter – striated skeletal muscle Intestinal Motility Peristaltic movements created by the longitudinal muscles propel the chyme along the intestinal tract Contractions of the circular muscles, called segmentation, mixes the chyme and promotes digestion Ileogastric reflex inhibits gastric motility when the ileum is distended The intestinointestinal reflex inhibits intestinal motility when one intestinal segment is over distended Gastroileal reflex increases intestinal motility when gastric motility increases ○ Initiates propulsion in the entire colon, usually during or immediately after eating, when chyme enters from the ileum ○ Causes fecal mass to pass rapidly into the sigmoid colon and rectum, stimulating defecation ○ Stimulates the reflex: Gastrin and cholecystokinin ○ Inhibits the reflex: Epinephrine Increased intraabdominal pressure facilitates defecation Accessory Muscles of Digestion Liver, gallbladder and exocrine pancreas all secrete substances necessary for digestion of chyme Liver produces bile, with salts necessary for fat digestion and absorption Bile stored in the gallbladder in between meals Exocrine pancreas produces enzymes needed for complete digestion of carbohydrates, proteins, and fats ○ Also produces an alkaline fluid that neutralizes chyme and produces duodenal pH necessary for enzymatic function Liver Liver lobules consist of plates of hepatocytes, which are the functional cells of the liver Hepatocytes synthesize 700-1200mL of bile per day and secrete it into the bile canliculi ○ Bile canliculi are small channels between the hepatocytes ○ Can drain bile into the common bile duct and then into the duodenum through an opening called the sphincter of Oddi Lipocytes are star-shaped cells that store lipids, including vitamin A Kupffer cells (part of the mononuclear phagocyte system) are phagocytic cells and are central to innate immunity Stellate cells contain retinoids (vitamin A), are contractile in liver injury, regulate sinusoidal blood flow, and may proliferate into myofibroblasts, participate in liver fibrosis, and remove foreign substances from the blood and trap bacteria Pit cells are natural killer cells – important in tumor defense Liver Functions Bile secretion ○ Alkaline, bitter-tasting, yellowish green fluid that contains bile salts, cholesterol, bilirubin, electrolytes, and water Bile salts: Conjugated bile acids that are required for the intestinal emulsification and absorption of fats ○ Formed by hepatocytes and secreted into the bile canaliculi ○ Primary bile acids are synthesized from cholesterol by the hepatocytes and are required for intestinal emulsification and absorption of fats Primary acids are then conjugated to form bile salts ○ Secondary bile acids are the product of bile salts deconjugation by bacteria in the intestinal lumen Enterohepatic circulation ○ Recycling of bile salts ○ Also called choleresis Choleretic agent: Substance that stimulates the liver to secrete bile ○ Strong stimulus: High concentration of bile salts ○ Other choleretic agents: Secretin, cholecystokinin, vagal stimulation Vascular and hematologic functions ○ Stores blood. ○ Removes bacterial and foreign particles. ○ Synthesizes clotting factors. Metabolizes fats ○ Liver produces bile to absorb fat-soluble vitamins. ○ Hydrolyzed triglycerides produce metabolic energy (adenosine triphosphate [ATP]) or are released into the bloodstream bound to proteins (lipoproteins). Synthesizes phospholipids and cholesterol Metabolizes proteins ○ Deamination: The removal of ammonia (converted to urea and excreted in the kidneys) converts amino acids into carbohydrates. Metabolizes carbohydrates ○ Liver releases glucose during states of hypoglycemia and takes up glucose during states of hyperglycemia; it stores glucose as glycogen (glyconeogenesis) or converts it to fat. ○ When all glycogen stores have been used, amino acids and glycerol are converted to glucose. Metabolic detoxification ○ Is called biotransformation. ○ Makes toxic substances less harmful. Storage of minerals and vitamins Produces bile to absorb fat-soluble vitamins ○ Vitamin K (fat soluble vitamin) is essential for synthesis of clotting factors ○ Vitamin K absorption depends on adequate bile production ○ Impairment of vitamin K absorption diminishes production of clotting factors and increases risk of bleeding Bilirubin A pigment which is the by-product of the destruction of aged red blood cells in the liver and spleen ○ Macrophages in the liver and spleen break down the RBCs. ○ Kupffer cells in the liver separate the hemoglobin or heme component from the globin component ○ The heme is converted into iron to be stored in the liver ○ By product is converted to bilirubin Bilirubin gives bile a greenish black color and produces the yellow tinge of jaundice Unconjugated bilirubin is lipid soluble and can cross the cell membrane, present in the circulation, bound to plasma protein Conjugated bilirubin ○ Liver changes bilirubin to a water-soluble form that can now be excreted. ○ Primary method of bilirubin excretion is in the bile Urobilinogen ○ In the ileum and colon, conjugated bilirubin is deconjugated by bacteria and then converted. ○ Gives feces its brown color. Higher percentage of unconjugated bilirubin = heme breaking down, or inability of liver to conjugate Higher percentage of conjugated bilirubin = problem with secretion into bile Exocrine Pancreas Enzymes Functions Trypsinogen, chymotrypsinogen, and procarboxypeptidase Inactivates proteases & is activated by enterokinase Trypsin inhibitor Prevents the pancreas from eating itself Pancreatic a-amylase Digests carbohydrate Pancreatic lipase Digests fats Pancreatic protease Digests protein