Digestive System Summary

Digestive System

Main Functions

  • Ingestion: Taking food into the mouth.
  • Propulsion: Movement of food through the tract.
    • Peristalsis.
  • Digestion: Breaking food down into absorbable nutrients.
    • Mechanical: Chewing.
    • Chemical:
      • Carbohydrates → simple sugars
      • Proteins → amino acids
      • Lipids → fatty acids
  • Secretion: Approx. 5L of digestive secretions produced each day.
  • Absorption: Bringing nutrients into the digestive system and blood.
  • Defecation: Removal of indigestible substances and wastes.

Digestive Organs

  • Gastrointestinal (GI) Tract (alimentary canal)
    • Continuous tube winding through the body; all hollow.
    • Includes: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.
  • Accessory Organs
    • Lie outside of GI tract: Teeth, tongue, salivary glands, liver, gall bladder, pancreas.

Histology of Alimentary Canal

  • Mucosa: Epithelium (simple columnar or stratified squamous).
  • Submucosa: Areolar and dense irregular connective tissue; contains blood and lymphatic vessels, autonomic nerve fibers.
  • Muscularis externa: Smooth muscle; performs peristalsis.
  • Serosa (visceral peritoneum): Lines portions of GI tract.

Oral Cavity, Pharynx, and Esophagus

  • Salivary glands: Produce saliva (1L/day).
    • Salivary amylase: Digests starch.
    • Lysozyme: Antibacterial action.
    • Mucin: With water forms mucus for lubrication.
  • Chewing and saliva form a bolus.
  • Pharynx
    • Epiglottis prevents food from entering the trachea.
  • Esophagus
    • Transports bolus from pharynx to stomach via peristalsis.

Stomach

  • Storage and mixing chamber with rugae for extensibility (1.5-4 liters).
  • Secretions (1.5 L/day):
    • Mucus.
    • Hydrochloric acid (HCl).
    • Pepsinogen → Pepsin: Digests protein into amino acids.
  • Movement:
    • Mixing waves and peristalsis.
    • Bolus becomes chyme, slowly released by the pyloric sphincter.

Small Intestine

  • Duodenum, jejunum, ileum.
  • Function: Majority of digestion and absorption.
  • Epithelial cells produce digestive enzymes for all food classes.
  • Intestinal juice: Water, mucus, and enzymes (about 2L/day).
  • Secretions from liver and pancreas via the hepatopancreatic ampulla.

Pancreas

  • Produces 1L/day of juice containing pancreatic enzymes for digesting major food classes (22 enzymes and bicarbonate).
  • Empties into the small intestine via the pancreatic duct.

Liver

  • Secretes bile into the gall bladder, which empties into the small intestine via the bile duct.
  • Bile emulsifies fats.
  • Performs over 500 functions, including:
    • Storing glycogen, lipids, vitamins, iron.
    • Synthesizing and interconverting nutrients.
    • Producing and releasing cholesterol.
    • Detoxifying toxins, poisons, drugs.
    • Phagocytosis of old RBCs and excretion of bilirubin.
    • Producing blood proteins and clotting factors.

Hepatic Portal System

  • Picks up digested nutrients from the stomach and intestines, delivering them directly to the liver for processing.
  • Blood enters via the hepatic portal vein and leaves via the hepatic vein.
  • Primary organ that regulates blood composition.

Large Intestine

  • Ileocecal sphincter.
  • Ascending colon, transverse colon, descending colon, sigmoid colon.
  • Absorption of water and salts.
  • Undigested material converted to feces.
  • E. coli synthesize vitamin K.
  • Solid wastes (feces) stored in rectum, released through anus (defecation).
  • Anal sphincter.