GI Physiology Quiz

Gastric Secretion and Motility

• Gastric secretion is inhibited by decelerated gastric secretion, secretin, and cholecystokinin.
• Inhibitory vagal reflexes decrease gastric motility when chyme enters the duodenum.
• Vagus nerve stimulates gastric secretion and motility.

Small Intestine

• The small intestine has three segments: duodenum, jejunum, and ileum.
• Duodenum:
  • Begins at the pylorus and ends at the Treitz ligament.
  • 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:
  • Produced by the liver and necessary for fat digestion and absorption.
  • Alkalinity helps neutralize chyme, creating a pH that enables pancreatic enzymes to digest proteins, carbohydrates, and sugars.
• Jejunum:
  • Absorbs carbohydrates and proteins across the 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.
  • Absorbs bile salts, vitamin B12, and chloride.

Peritoneum and Muscle Layers

• Peritoneum:
  • Serous membrane surrounding the organs of the abdominal and pelvic cavity.
  • Visceral peritoneum lies over the organs.
  • Parietal peritoneum lines the wall of the abdominal 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.

Digestion and Absorption

• Carbohydrates:
  • Complex carbohydrates (polysaccharides and oligosaccharides) must be hydrolyzed to their simplest form.
• Bile:
  • 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 is water-soluble and can be excreted.
• Urobilinogen:
  • In the ileum and colon, conjugated bilirubin is deconjugated by bacteria and then converted.
  • Gives feces its brown color.

Exocrine Pancreas

• Enzymes functions:
  • Trypsinogen, chymotrypsinogen, and procarboxypeptidase: inactivate proteases and are activated by enterokinase.
  • Trypsin inhibitor: prevents the pancreas from eating itself.
  • Pancreatic α-amylase: digests carbohydrates.
  • Pancreatic lipase: digests fats.
  • Pancreatic protease: digests proteins.

Gastric Emptying

• Gastric emptying is the 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

• Fat digestion stimulates cholecystokinin, which inhibits food intake, gastric motility, and decreases gastric emptying so that fats do not exceed the rate of bile and enzyme secretion.

Gastric Secretion and Hormones

• Gastric secretion:
  • 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 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 and Ulceration

• 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.

Stomach and Blood Supply

• Stomach:
  • Muscle layers: longitudinal, circular, and oblique.
  • Sphincters: lower esophageal sphincter and pyloric sphincter.
  • Blood supply: via branch of celiac artery; drainage via splenic vein and tributaries.
  • Few substances are absorbed in the stomach, such as alcohol, aspirin, and other non-steroidal anti-inflammatory agents.

Large Intestine

• 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, and sigmoid.
  • Rectum and anus.
  • 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

• Intestinal motility:
  • Peristaltic movements created by the longitudinal muscles propel the chyme along the intestinal tract.
  • Contractions of the circular muscles, called segmentation, mix the chyme and promote digestion.
  • Ileogastric reflex inhibits gastric motility when the ileum is distended.
  • The intestinointestinal reflex inhibits intestinal motility when one intestinal segment is overdistended.
  • Gastroileal reflex increases intestinal motility when gastric motility increases.

Liver Functions

• Liver:
  • Produces bile, with salts necessary for fat digestion and absorption.
  • Bile is 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 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 canaliculi.
  • Bile canaliculi 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:
  • Store lipids, including vitamin A.
• Kupffer cells:
  • Part of the mononuclear phagocyte system.
  • Phagocytic cells and are central to innate immunity.
• Stellate cells:
  • Contain retinoids (vitamin A).
  • Are contractile in liver injury.
  • Regulate sinusoidal blood flow.
  • May proliferate into myofibroblasts, participate in liver fibrosis, and remove foreign substances from the blood and trap bacteria.
• Pit cells:
  • Natural killer cells.
  • Important in tumor defense.

Liver Functions (continued)

• 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.