USMLE

Small Intestine

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Digestive
  1. Gastrin
  2. Pepsin
  3. Amylase
  4. Lipase
  5. Trypsin
  6. CCK
  7. Secretin
  8. GI System
  9. Ingestion
  10. Liver
  11. Bile
  12. Exocrine Pancreas
  13. Endocrine Pancreas
  14. Small Intestine
  15. Large Intestine
  16. GI Nervous Control
  • Small Intestine
    • Function
      • Breakdown (digestion) of macromolecules
        • Proteins broken down by enzymes (trypsin) into amino acids
        • Polysaccharides (starches) and complex sugars broken into simple sugars (e.g. amylase for starches, lactase for lactose, maltase for maltose, etc.)
        • Fats emulsified by bile, broken down by lipases into free fatty acids
        • Nucleotides broken down by nucleosidases into amino acids 
      • Absorption of nutrients/water
        • Once everything is broken down, they are absorbed by passive diffusion or active transport
          • Polar carbohydrates/proteins: secondary active transport
          • Nonpolar fats: passive diffusion 
        • Surface area is maximized for absorption through folds and villi
        • Most nutrients → portal blood → liver → bloodstream
        • Fat → lymph → bloodstream
        • Malabsorption = diarrhea
    • Structure (3 parts)
      • Duodenum
        • First section
        • Most chemical breakdown of food occurs (enzymes)
        • Neutralization of stomach acid by bicarbonate
      • Jejunum
        • Second (middle) section
        • Most absorption occurs here
      • Ileum
        • Third section
        • Some vitamins are absorbed here
    • Surface
      • Intestinal absorptive cells (enterocytes)
        • Contain villi (finger-like cell projections) to maximize surface area for absorption
        • Surface of intestine also called “brush-border” due to projections