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]