Human Digestive System

Overview

• Complex system evolved over millions of years
• Includes: rectum, large intestine, small intestine, pancreas, stomach, liver, gallbladder, esophagus, salivary glands

Initial Digestion

• Mouth: Food broken down by teeth and mixed with saliva
  • Saliva contains enzyme amylase, begins carbohydrate digestion
  • Food-saliva mix is called bolus
• Esophagus: Transports bolus to the stomach
  • Flexible esophageal lumen for different sizes of bolus
  • Contains outer muscle layers for peristalsis

Stomach

• Divided into six areas, similar structure to esophagus
• Contains: longitudinal muscle layer, circular muscle fibers, oblique muscle layer, mucosa
• Gastric Glands: Produce mucus, gastric acid (1-2 liters/day), enzyme pepsin, intrinsic factor, lipase
• Functions:
  • Protects stomach wall with mucus
  • Digests protein with pepsin
  • Absorbs Vitamin B12 with intrinsic factor
  • Digests fats with gastric lipase

Transition to Small Intestine

• Pyloric Sphincter: Controls chyme release to duodenum

Small Intestine

• Three sections: duodenum, jejunum, ileum
• Duodenum: Bile and pancreatic secretions mix with chyme
  • Pancreatic juice: proenzymes, enzymes, sodium hydrogen carbonate
  • Neutralizes stomach acid, optimal pH 7-8
  • Bile emulsifies fats

Absorption

• Villi and Microvilli: Increase surface area for nutrient absorption
  • Villi contain blood capillaries and lymphatic capillary (lacteal)
  • Transports:
    • Glucose freely
    • Iron via transferrin
    • Fats via chylomicrons
• Ileum: Absorbs electrolytes, trace elements, vitamins, remaining bile acid

Large Intestine

• Approximately 1 meter long, surrounds small intestine
• Bauhinz Valve: Connects small to large intestine
• Lacks villi; absorbs water, adds mucus for waste excretion
• Contains beneficial bacteria for vitamin production and fiber decomposition
• Peristalsis: Transports chyme from ascending to descending colon
• Excretion: Undigested substances are excreted through the anus