Small Intestine

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  • 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