digestive system part III

Pancreas

  • Location and Structure
    • Retroperitoneal organ, lies posterior to the stomach.
    • Extends from the duodenum to the spleen.
    • Regions:
    • Head: Broad part within the loop of the duodenum.
    • Body: Slender portion extending toward the spleen.
    • Tail: Short, rounded end of the pancreas.
  • Pancreatic Duct:
    • Delivers pancreatic secretions to the duodenum (initial segment of the small intestine).

Histology of the Pancreas

  • Structure:
    • Surrounded by a thin connective tissue capsule.
    • Lobules separated by connective tissue partitions.
    • Pancreatic Islets: Clusters of endocrine cells.
    • Ducts branch repeatedly in each lobule ending in pancreatic acini:
    • Acini lined with simple cuboidal epithelium.
    • Acinar cells and epithelial cells of the ducts secrete pancreatic juice into the duodenum.

Pancreatic Juice

  • Composition:
    • Alkaline (pH 7.5-8.8): Contains digestive buffers, enzymes, water, and ions.
  • Pancreatic Enzymes:
    • Pancreatic Alpha-Amylase: A carbohydrase breaking down starch.
    • Pancreatic Lipase: Breaks down complex lipids.
    • Nucleases: Breakdown of DNA and RNA.
    • Proteolytic Enzymes:
    • Proteases: Break down large protein complexes.
    • Peptidases: Break down small peptide chains into amino acids.
  • Activation:
    • Secreted as inactive proenzymes, activated in the small intestine.
    • Examples:
    • Trypsinogen ➔ Trypsin in the duodenum.
    • Chymotrypsinogen ➔ Chymotrypsin (pancreatic enzyme that breaks down proteins) by trypsin.
    • Proelastase ➔ Elastase by trypsin.
  • Pancreatitis: Inflammation of the pancreas due to duct blockage, infection, ischemia, or drug reactions.

Liver

  • Location & Structure:
    • Largest organ located in the right hypochondriac and epigastric regions.
    • Wrapped in a tough fibrous capsule, covered with visceral peritoneum.
    • Major Lobes: Left and Right.
  • Blood Supply:
    • Blood vessels converge at the porta hepatis:
    • Hepatic Artery Proper: Brings arterial blood.
    • Hepatic Portal Vein: Carries venous blood from digestive viscera.

Histology of the Liver

  • Each lobe divided by interlobular septa into lobules.
  • Lobule: Basic functional unit of the liver, hexagonal in cross-section.
    • Hepatocytes: Liver cells that adjust nutrient levels through selective absorption and secretion.
    • Form irregular plates arranged like wheel spokes.
  • Portal Triad: At each corner of a lobule containing:
    • Interlobular Vein: A branch of the hepatic portal vein.
    • Interlobular Artery: A branch of the hepatic artery proper.
    • Interlobular Bile Duct: Collects bile.
  • Blood Flow:
    • Blood from artery and vein flow through sinusoids between hepatocytes, emptying into a central vein.
    • Central veins merge to form hepatic veins.

Bile Duct System

  • Function: Hepatocytes secrete bile into bile canaliculi.
  • Pathway:
    • Bile canaliculi ➔ interlobular bile ducts ➔ right and left hepatic ducts ➔ common hepatic duct.
    • From common hepatic duct:
    • To bile duct (empties into duodenum) or cystic duct (leads to gallbladder).

Liver Functions

  1. Metabolic Regulation:
    • Blood from the digestive tract flows into the liver through the hepatic portal system.
    • Roles of Hepatocytes:
      • Extract nutrients and toxins from blood.
      • Store nutrients and mobilize reserves as needed.
      • Carbohydrate Metabolism: Regulates glucose levels through gluconeogenesis and glycogenesis.
      • Lipid Metabolism: Manages triglycerides, fatty acids, and cholesterol levels.
      • Amino Acid Metabolism: Controls amino acid levels.
      • Waste Removal: Converts ammonia to urea, removes other wastes, drugs, and toxins.
      • Vitamin & Mineral Storage: Stores fat-soluble vitamins, minerals like iron as ferritin, breaks down drugs.
  2. Production of Bile:
    • Synthesized by the liver, stored in gallbladder or secreted into duodenum.
    • Contains water, ions, bilirubin, cholesterol, and bile salts:
      • Bile salts emulsify fats for better digestion by bile.
    • Enterohepatic Circulation:
      • 90% bile salts reabsorbed in the small intestine and recycled via hepatic portal system.

Gallbladder

  • Structure:
    • Hollow, pear-shaped sac that stores and concentrates bile.
    • Located on the posterior surface of the liver's right lobe.
    • Regions:
    • Fundus: Bulging portion.
    • Body: Main segment.
    • Neck: Tapering part that connects to the cystic duct.
  • Functionality:
    • Bile composition changes within the gallbladder: Water absorbed, bile salts concentrated.
    • Hepatopancreatic Sphincter: Muscular control around bile duct; regulates bile flow based on digestive activity (e.g., triggered by arrival of chyme).

Small Intestine

  • Structure:
    • Long muscular tube; 90% of nutrient absorption occurs here.
    • Segments:
    • Duodenum: Mixing bowl receiving chyme and secretions.
    • Jejunum: Middle segment; primary site of digestion and absorption.
    • Ileum: Longest segment; connects to the cecum via ileocecal valve.
  • Histology:
    • Circular folds (permanent transverse folds).
    • Intestinal Villi: Finger-like projections, increase surface area for absorption.
    • Covered by simple columnar epithelium, has microvilli forming the brush border.

Chemical Digestion in the Small Intestine

  • Enzymes:
    • Brush Border Enzymes: Integral proteins on microvilli; break down materials in contact with the brush border.
    • Enzymes from the pancreas aid in the most digestion.

Regulation of Secretion and Absorption

  • Neural Regulation:
    • Parasympathetic Innervation: Increases secretion and motility.
    • Sympathetic Innervation: Inhibits secretion and motility.
    • Long Reflexes: Coordinate movement along the digestive tract.
  • Hormonal Regulation:
    • Gastrin: Promotes stomach activity when proteins are detected in the duodenum.
    • Secretin: Increases pancreatic secretion and bile release when chyme enters the duodenum.
    • Gastric Inhibitory Peptide (GIP): Inhibits gastric activity and promotes insulin release when fats and carbohydrates are present.
    • Cholecystokinin (CCK): Accelerates digestive enzyme secretion, relaxes sphincter for bile presence, inhibits gastric activity.
    • Vasoactive Intestinal Peptide (VIP): Stimulates intestinal gland secretion, dilates capillaries, inhibiting acid production in the stomach.

Large Intestine

  • Structure:
    • Extends from ileum to anus; frames small intestine.
    • Segments:
    • Cecum: Pouchlike initial segment; receives materials from ileum.
      • Appendix: Small structure dominated by lymphoid tissue; inflammation leads to appendicitis.
    • Colon: Largest segment with specialized features (Haustra, Omental appendices).
    • Rectum: Last part of digestive tract; stores feces temporarily.
  • Functions:
    • Absorption of water, nutrients (less than 10%), bile salts, organic wastes, vitamins.
    • Compaction of contents into feces.
    • Temporary fecal material storage prior to defecation.
  • Microbiome:
    • Microorganisms living in the large intestine that assist in breaking down undigested nutrients, producing essential vitamins.