Digestive System and the Digestive Tract

Role of the Digestive Tract

Oral Cavity

  • Digestion starts in the oral cavity.
  • Mechanical Digestion:
    • Chewing: teeth tear apart and flatten food, increasing surface area.
  • Saliva:
    • Contains salivary amylase.
      • Begins digestion of carbohydrates, especially branched ones.
    • Lingual lipase:
      • Produced by glands in the tongue, aids in initial digestion of lipids.
    • No protein breakdown occurs here, except mechanically.
    • Chewing food well is important, especially fibrous foods like meats.

Pharynx and Esophagus

  • Food passes through the pharynx and esophagus after the oral cavity.
  • Enzymes continue to act.

Stomach

  • The next major site of digestion.
  • Mechanical Digestion:
    • Three layers of muscle churn to mix food with enzymes.
  • Chemical Digestion:
    • Hydrochloric acid (HCl):
      • Starts to break down foods further.
      • Lowers pH, inactivating lingual lipase and salivary amylase.
      • Activates pepsinogen.
        • Pepsinogen (released by chief cells) converts to pepsin, which digests proteins.
    • Protein digestion starts here; carbohydrates and lipids begin earlier in the oral cavity.
    • No nutrient absorption occurs.
      • The stomach has a thick mucus layer and lacks transport mechanisms for absorption.
    • Digestion is not completed in the stomach.

Small Intestine

  • Chyme (food mixed with HCl) moves into the duodenum (first part of the small intestine).
    • Takes about 5 hours to pass through the entire small intestine.
  • Mechanical Digestion:
    • Stirring, mixing, and peristalsis.
  • Chemical Digestion:
    • Pancreas:
      • Secretes pancreatic enzymes and buffers to break down carbohydrates, lipids, and proteins.
    • Liver:
      • Produces bile, which is stored and concentrated in the gallbladder and released to break down lipids.
    • Brush Border Enzymes:
      • Located in the villi and microvilli; released just before absorption.
      • Break down small components of proteins, carbohydrates, and fats into absorbable units.
  • Absorption:
    • Carbohydrates, lipids, protein, water, vitamins, and ions are absorbed here.

Large Intestine

  • Mechanical Digestion:
    • Mixing, propulsion, and churning.
  • Microbiome:
    • Bacteria produce vitamin K, B5, and biotin, which can be absorbed.
  • Water Absorption:
    • Significant water absorption occurs.
    • About 1,5001,500 mL enters, and only about 200200 mL is lost in feces.