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.