The Human Digestive System

Overview

• Complex system evolved over millions of years.
• Composed of several organs including:
  • Mouth
  • Esophagus
  • Stomach
  • Small Intestine
  • Large Intestine
  • Rectum
  • Pancreas
  • Liver and Gallbladder
  • Various Salivary Glands

Mouth and Esophagus

• Function: Initial breakdown of food.
• Mouth:
  • Teeth break up food.
  • Saliva from salivary glands mixes with food.
  • Saliva contains amylase, an enzyme that begins carbohydrate digestion.
  • Forms a bolus, a ball-like mixture of food and saliva.
• Esophagus:
  • Transports bolus to the stomach.
  • Flexible lumen to accommodate various bolus sizes.
  • Uses peristalsis to move bolus.

Stomach

• Function: Further digestion of food.
• Composed of six areas with multiple muscle layers:
  • Longitudinal, circular, oblique muscle layers.
• Contains rugae to expand when food is ingested.
• Gastric glands produce:
  • Mucus: Protects stomach wall from acid.
  • Gastric acid: Stimulated by food, spices, and stomach stretching.
  • Hydrochloric acid: Destroys bacteria, converts pepsinogen to pepsin.
  • Intrinsic factor: Essential for vitamin B12 absorption.
  • Gastric lipase: Breaks down fats.

Small Intestine

• Duodenum, Jejunum, Ileum
• Duodenum:
  • Mixes chyme with bile and pancreatic secretions via the ampulla of Vater.
  • Bicarbonate neutralizes stomach acid.
  • Contains enzymes for digestion.
• Pancreatic Enzymes:
  • Trypsinogen to trypsin (protein digestion).
  • Amylase (carbohydrate digestion).
  • Lipase (fat digestion).
• Absorption:
  • Villi and microvilli increase surface area.
  • Nutrients like glucose absorbed directly.
  • Fats require transport proteins (e.g., chylomicrons).
  • Ileum absorbs electrolytes, trace elements, vitamins, and bile acids.

Large Intestine

• Function: Water absorption and waste formation.
• Composed of ascending, transverse, and descending colon.
• Bacteria:
  • Produce vitamins.
  • Decompose fiber.
  • Part of the immune system.
• Water is reabsorbed and mucus added to chyme.
• Waste is stored in the rectum and excreted through the anus.