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.