Intestinal function

Intestinal Function

Digestive Processes in the Small Intestine

  • Chyme from stomach contains:

    • Partially digested carbohydrates

    • Partially digested proteins

    • Undigested fats

  • Duration: It takes 3-6 hours in the small intestine to absorb all nutrients and most H2O.

  • Source of enzymes and substances for digestion: These are imported from the liver and pancreas.

  • Brush border enzymes perform the final digestion of chyme.

Enzymatic Functions and Bicarbonate Supply from the Pancreas

  • Pancreas Supplies:

    • Enzymes and bicarbonate ions.

  • Enzymes for Protein Digestion:

    • Trypsin

    • Chymotrypsin

    • Carboxypolypeptidase

    • Note: These enzymes are secreted in inactive forms and activated after secretion into the intestinal tract.

  • Enzymes for Carbohydrate Digestion:

    • Pancreatic amylase

  • Enzymes for Fat Digestion:

    • Pancreatic lipase

    • Phospholipase

  • Bicarbonate Ions Function:

    • Serve to neutralize HCl emptied into the duodenum from the stomach.

Liver Function and Bile Production

  • Liver produces bile which is:

    • Yellow-green alkaline solution.

  • Components of Bile:

    • Bile Salts: Cholesterol derivatives that function in fat emulsification and absorption.

    • Bilirubin: Pigment formed from heme, broken down in the intestine by bacteria into stercobilin, contributing to the brown color of feces.

    • Other components include cholesterol, triglycerides, phospholipids, and electrolytes.

Digestion of Lipids in the Small Intestine

  1. Initial state: Large fat globules are not digested efficiently by lipase.

  2. Action of Bile Salts: Bile salts produced in the liver act as emulsifying agents.

    • Conversion of large fat globules into smaller droplets.

  3. Outcome of Emulsification: Small fat droplets result from emulsification, which then allows lipase to digest them into:

    • Monoglycerides

    • Free fatty acids

  4. Reabsorption of Bile Salts:

    • Reabsorbed bile salts travel via the hepatic portal vein back to the liver, where they are recycled. Note: Only 5% of bile salts are newly synthesized each time.

  5. Completion of Lipid Digestion: Lipase digests the small fat droplets, completing the digestion into monoglycerides and free fatty acids.

Secretion of Bile and Pancreatic Juices

  1. Hormonal Regulation:

    • CCK (Cholecystokinin) and Secretin are secreted by duodenal enteroendocrine cells.

    • CCK Release: Stimulated by proteins and fats in chyme.

    • Secretin Release: Stimulated by acidic chyme.

  2. Effects of CCK and Secretin:

    • Pancreatic Secretion:

      • CCK induces enzyme-rich pancreatic juice secretion by acinar cells.

      • Secretin stimulates HCO₃⁻-rich pancreatic juice secretion by duct cells.

    • Bile Secretion:

      • Bile salts returning from enterohepatic circulation are a powerful stimulus for bile secretion.

    • Gallbladder Contraction:

      • CCK causes contraction of the gallbladder, releasing bile into the duodenum.

    • Hepatopancreatic Sphincter Relaxation:

      • CCK causes the sphincter to relax, allowing bile and pancreatic juice to enter the duodenum.

Anatomy of the Large Intestine

  • Main Sections of the Large Intestine Include:

    • Cecum: First part connected to the ileum via the ileocecal valve.

    • Appendix: A small tube attached to the cecum.

    • Ascending Colon and Right Colic (Hepatic) Flexure

    • Transverse Colon

    • Left Colic (Splenic) Flexure

    • Descending Colon

    • Sigmoid Colon

    • Rectum

    • Anal Canal with External Anal Sphincter

    • Important structures include Haustra, Epiploic Appendages, Tenia Coli, and Transverse Mesocolon.

Ileocecal Valve and its Function

  • Functionality:

    • Facilitates absorption by preventing backflow from the colon; it closes forcefully when pressure builds up in the cecum.

  • Gastroileal Reflex: Occurs after meals, intensifying peristalsis in the ileum to empty into the cecum.

    • Ileocecal Sphincter: Typically slows emptying into the cecum, prolonging chyme retention in the ileum to maximize nutrient absorption.

Functions of the Colon

  • Principal Functions Include:

    • Absorption of water and electrolytes from chyme to form solid feces (proximal half of the colon).

    • Storage of fecal matter until it can be expelled (distal half of the colon).

  • Water Reabsorption: Occurs through aquaporins within the colon's epithelial cells.

Bacterial Flora of the Large Intestine

  • Composition: Consists of over 1000 different types of bacteria.

    • Functions include:

    • Fermentation of undigested carbohydrates.

    • Synthesis of vitamins.

    • Suppression of pathogenic bacteria.

  • Health Impact: Gut bacteria can significantly impact overall health.

Clinical Snapshot: Antibiotic-associated Diarrhea

  • Statistics: Accounts for approximately 14,000 deaths per year.

  • Causative Agent: Clostridium difficile infection (C. difficile).

    • Endospores: C. difficile contains endospores that can survive stomach acidity and reach the large intestine.

    • Symptoms Include: Fever, crampy abdominal pain, diarrhea.

    • The bacteria flourish within the colon, leading to the most common infectious cause of nosocomial diarrhea.

  • Impact of Antibiotics: Broad-spectrum antibiotics (notably clindamycin, cephalosporins, ampicillin, amoxicillin, and fluoroquinolones) alter normal gut flora, allowing C. difficile to proliferate.

  • Toxins A and B: Produced by C. difficile, causing mucosal damage leading to diarrhea and pseudomembranous colitis characterized by yellowish plaques over damaged epithelium.