Small intestine absorption | Anatomy2Medicine
Small intestine absorption,NEET PG

Small intestine absorption

Intestinal Fluid and Electrolyte Transport

 

  • Together, the small and large intestines absorb approximately 9 L of fluid daily (MCQ)

 

    • What is the source of this large volume of fluid that is absorbed?
      • there is slightly more than 9 L of fluid in the lumen of the gastrointestinal tract, volume of liquid in the diet (2 L) (MCQ)
      • combined volume of salivary, gastric, pancreatic, biliary, and intestinal secretions (7 L). (MCQ)
      • 100 to 200 mL  is excreted in feces
    • The tight junctions in the small intestine are “leaky” (have low resistance) and permit significant paracellular movement (MCQ)

 

  • tight junctions in the colon are “tight” (have a high resistance) and do not permit paracellular movement. (MCQ)

 

Intestinal absorption

  • Intestinal epithelial cells lining the villi absorb large volumes of fluid.
  • The fluid absorbed is always isosmotic, (MCQ)
  • Jejunum
    • The jejunum is the major site for Na+ absorption in the small intestine).Na+ enters the epithelial cells of the jejunum via several different Na+-dependent coupled transporters. (MCQ)
    • The apical membrane contains Na+- monosaccharide cotransporters (Na+-glucose and Na+-galactose), Na+–amino acid cotransporters, and Na+-H+ exchange.
    • After Na+ enters the cell on the coupled transporters, it is extruded across the basolateral membrane via the Na+-K+ ATPase.
    • Note that the source of H+ for Na+-H+ exchange is intracellular CO2 and H2O, which are converted to H+ and HCO3 in the presence of carbonic anhydrase.
    • The H+ is secreted into the lumen on the Na+-H+ exchanger, and the HCO3 is absorbed into blood.
  • Ileum
    • The ileum contains the same transport mechanisms as the jejunum plus a Cl-HCO3 exchange mechanism in the apical membrane and a Cl transporter, instead of an HCO3 transporter, in the basolateral membrane (MCQ)
    • Thus, when H+ and HCO3 are generated inside the epithelial cells in the ileum, the H+ is secreted into the lumen via the Na+-H+ exchanger, and the HCO3 also is secreted into the lumen via the Cl-HCO3 exchanger (rather than being absorbed into blood, as in the jejunum).
    • The result of the combined Na+-H+ exchange and Cl-HCO3 exchange in the apical membrane is net movement of NaCl into the cell, which then is absorbed.
    • Thus, in the ileum, there is net absorption of NaCl, whereas in the jejunum there is net absorption of NaHCO3. (MCQ)
    • Colon
      • The cellular mechanisms in the colon are similar to those in the principal cells of the late distal tubule and collecting ducts of the kidney (MCQ)

 

  • The apical membrane contains Na+ and K+ channels, which are responsible for Na+ absorption and K+ secretion. (MCQ)

 

    • Like the renal principal cells, synthesis of the Na+ channels is induced by aldosterone, which leads to increases in Na+ absorption and, secondarily, to increases in K+ secretion.