IN THE SMALL INTESTINE

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ABSORPTION

• IN THE SMALL INTESTINE

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ABSORPTION

• Takes place in the small intestine
• Energy is required for active absorption to take
  place
• Because of this, many mitochondria are present in
  the epithelial cells

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The Small Intestine

• Villi and microvilli increase the surface area
  for absorption.
• Glucose and amino acids are absorbed across the
  epithelium of the villi by diffusion and active
  transport.
• Fatty acids and glycerol are passed into the
  lacteal.
• Water and mineral salts are absorbed from the
  colon along with vitamins secreted by micro
  organisms in the colon.

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• Circular and longitudinal muscles contract and
  relax to carry out peristalsis.
• Lacteals absorb fats and fatty acids.
• The blood in vein X enters flows into the hepatic
  portal.
• Y is filled with lymph fluid.
• Brunners glands in the submucosa secrete alkaline
  mucus to neutralise stomach acid to keep the pH
  at the optimum for the enzymes present.

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Absorption of glucose

• Glucose is absorbed from the lumen by active
  transport.
• Uptake of glucose and amino acids is linked to
  the active transport of sodium ions.
• Both require specific carrier molecules in the
  cell membrane of the epithelial.
• Glucose diffuses down the concentration gradient
  (facilitated diffusion) into the capillary blood
  in the villus
• Na diffuses into the blood via sodium-potassium
  pumps (this requires ATP)
• Other monosaccharides are absorbed through
  facilitated diffusion.

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Absorption of amino acids

• Similar to that of glucose (active transport)
• However sodium-dependent amino acid transporters
  are required
• Absorption of amino acids is dependent on the
  gradient of the sodium ions

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Absorption of fats and glycerol
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• Fatty acids and glycerol are passed into the
  lacteal (blindly ending lymph capillary in the
  centre of each villus)
• Fatty acids and glycerol are transported in the
  lymphatic system which ultimately opens into the
  blood stream at the thoracic duct.

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Emulsification of fats

• Bile salts play a critical role in lipid
  assimilation by promoting emulsification
• Bile salts have hydrophobic domain that
  intercalate into the lipid.
• They also have a hydrophilic domain that remains
  at the surface
• This aids in the breakdown of large aggregates or
  droplets into smaller and smaller droplets

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Formation of micelles

• As monoglycerides and fatty acids are liberated
  through the action of lipase, they retain their
  association with bile salts and complex with
  other lipids to form structures called micelles.
  
• Micelles are essentially small aggregates of
  mixed lipids and bile salts suspended within the
  ingesta.
• As the ingesta is mixed, micelles bump into the
  brush border and the lipids, including
  monoglyceride and fatty acids, are absorbed.