Sodium Reabsorption in the Distal Nephron
Sections
Key function of the kidneys is to ensure sodium balance:
Sodium intake = sodium excretion.
Sodium Reabsorption by Segment:
- Proximal tubule = 67% of filtered load
- Thick ascending limb = 25%
- Early distal tubule = 5%
- Late distal tubule and collecting duct = 3%
Less than 1% of the filtered load is excreted in the urine
Load-dependent Sodium Reabsorption
In the distal segments, load-dependent sodium reabsorption ensures that reabsorption rate remains relatively constant despite changes in filtered load.
Thick ascending limb
Sodium reabsorption is linked to potassium and chloride reabsorption.
The sodium-potassium ATPase on the basolateral membrane pumps sodium out of the cell and pumps potassium into it.
This drives cotransport of sodium, potassium, and chloride into the cell.
In the presence of ADH, cotransporter activity is increased;
Loop diuretics block the chloride-binding site, cotransport ceases, and sodium is not reabsorbed.
Chloride and potassium diffuse out of the cell through the basolateral membrane;
some potassium "leaks" back into the lumen.
The thick ascending limb is impermeable to water.
Early distal tubule
Sodium and chloride reabsorption are linked.
Sodium-potassium ATPase creates electrochemical gradient that drives the cotransport of sodium and chloride from the lumen into the cell.
Thiazide diuretics block chloride binding site on the contransporter, so sodium is not reabsorbed.
Chloride exits via simple diffusion.
Early distal tubule is impermeable to water.
Late distal tubule and collecting duct
Principal cells link sodium reabsorption to potassium secretion.
Sodium-potassium ATPase creates electrochemical gradient that drives sodium diffusion via epithelial sodium channels.
Potassium is secreted into lumen.
Aldosterone increases sodium reabsorption and potassium secretion.
The late distal tubule and collecting duct are only permeable to water in presence of ADH, which increases aquaporin-2 water channels.
Water exits cell via aquaporin 3 and 4 water channels.
K-sparing diuretics act on late distal tubule and collecting ducts to reduce sodium reabsorption and potassium secretion.