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Electrolyte Absorption & Secretion (SODIUM Absorption (Nutrient…
Electrolyte Absorption & Secretion
SODIUM Absorption
Nutrient Coupled
w/ Glucose (SGLT1) or AA's. Jejunum > Ileum
Secondary Active Tx due to Basolateral Na/K/ATPase pump
Na/H Exchange
Exchange for H+ via NHE3r
Jejunum > Ileum > Colon
Secondary Active Tx as above
Enhanced by Intracellular Acidosis or Luminal Alkalosis
Na/H Exchange + Cl/HCO3 Exchange
As above but coupled w/ Cl-/HCO3- exchange
Ileum & Proximal Colon
Na & Cl enter cell, HCO3 and H enter the lumen
Electrogenic
Na+ specific, Aldosterone sensitive ion channels
Distal Colon
Na+ gradient established by basolateral pump (2nd Active Tx)
Cl - absorbed passively in paracellular fashion
POTASSIUM
ABSORPTION
PASSIVE
Small intestine, "solvent drag"
due to bulk water movement
ACTIVE
Distal Colon, ATP dependent via H+/K+ pump
SECRETION
PASSIVE
Colon (Distal > Proximal).
Paracellular, driven by negative transepithelial voltage
ACTIVE
Throughout Colon. 1) Na/K/ATPase creates low Intracellular Na
2) Low Intracellular Na drives NKCC1 pump on basolateral surface 3) K+ secreted through apical channels.
CHLORIDE
ABSORPTION
PASSIVE in Small Intestine and Distal Colon
Cl/HC03 exchange (electroneutral) in Ileum & Colon
Na/H + Cl/HCO3 (see Na Absorption, Ileum & Proximal Colon)
SECRETION
Both Small & Large Intestines, Cl- secreted from crypt cells. Main driving force for fluid secretion into the lumen. Regulated by 2nd Mesengers (cAMP, cGMP & Calcium)