Renal handling of sodium, chloride, and water
Basics
- Resorption of sodium is mainly active, transcelluar process, driven by Na-K-ATPase pump in basolateral membrane
- Resorption of chloride is both passive (via paracellular diffusion) and [secondarily] active (transcellular). Ultimately dependent on Na-K-ATPase pump.
- Resorption of water is by osmosis, secondary to reabsorption of sodium and other solutes that depends on sodium gradient for resorption.
Iso-osmotic volume reabsorption
As sodium and associated solutes are reabsorbed
--> Water reabsorption by simple passive diffusion
In proximal tubule, water permeability is high
Thus,
Diffusion of water is fast enough to keep up with solute reabsorption
--> Osmolality stays unchanged
NB:
As filtrate moves down the proximal tubules,
- Amino acids, glucose, lactate concentration decrease the fastest
- Then followed by HCO3-
- Then followed by Pi (inorganic P)
- Then Na+ and osmolarity (concentration unchanged)
- Then Cl- (concentration increased)
[cf: WG21:p715]