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Water and Electrolyte Absorption [Small Intestine - SKF 839]

Overview

  • 8-10 L of fluid flows through the small intestine daily, sourced from dietary intake and various secretions (salivary, gastric, biliary, pancreatic, intestinal).
  • The small intestine is the primary site for water and electrolyte absorption, leaving <1 L to be absorbed by the colon.
  • Water absorption is regulated by electrolyte absorption and occurs through passive diffusion or osmotic pressure differences.

Mechanism of Water Absorption

  • Water follows electrolytes to maintain an isotonic environment between the intestinal lumen and tissue.
  • In the proximal small intestine, water moves passively via permeable tight junctions between enterocytes.
  • In the distal intestine, water absorption requires active transport coupled with electrolytes due to less permeable tight junctions.

Electrolyte Absorption

  • Electrolytes absorbed: Sodium (Na+), chloride (Cl-), bicarbonate (HCO3-), calcium (Ca2+), iron (Fe2+), potassium (K+), magnesium (Mg2+), phosphate (PO4^3-).

Sodium (Na+)

  • Sodium absorption via solute-coupling or electroneutral NaCl absorption.
  • Na+/K+-ATPase pump: Sodium exits into circulation via this pump, creating an electrochemical gradient.
  • Coupling with solutes: Glucose, amino acids, peptides, bile acids are absorbed along with Na+.

Chloride (Cl−)

  • Chloride follows sodium, diffusing across cells due to the electrochemical gradient created by sodium absorption.
  • Mostly absorbed in the duodenum and jejunum.

Bicarbonate (HCO3-)

  • Primarily absorbed in the duodenum and jejunum.
  • Bicarbonate comes from pancreatic secretions and bile.
  • Mechanism: H+ ions secreted in exchange for Na+, react with bicarbonate to form carbonic acid (H2CO3), which dissociates into CO2 (exhaled) and H2O (remains in the lumen).

Calcium (Ca2+)

  • Actively absorbed in the duodenum and jejunum.
  • Absorption depends on parathyroid hormone (PTH) and vitamin D.
  • Mechanism:
    • PTH converts vitamin D to its active form (1,25-dihydroxycholecalciferol).
    • Active vitamin D increases calcium-binding protein availability.
    • Calcium binds to this protein and is absorbed into the cell, then released into circulation by facilitated diffusion.

Iron (Fe2+)

  • Absorbed in the duodenum in the presence of bile.
  • Mechanism:
    • Bile contains apotransferrin, which binds with free iron to form transferrin.
    • Transferrin binds to intestinal epithelial receptors and is absorbed via pinocytosis.
    • Iron and transferrin pass into circulation as plasma transferrin.

Key Points

  • Water absorption is closely linked to electrolyte transport.
  • Disorders in water and electrolyte absorption can lead to significant disease states.
  • Further understanding of the molecular and genetic mechanisms of the small intestine is essential for improving clinical care.