SODIUM REABSORPTION (Na+)
Sodium ions (Na+) are the single most abundant cation in the filtrate, and about 80% of the energy is used for active transport is devoted to their reabsorption. Sodium reabsorption is almost always active and via the transcellular route.
In general, two basic processes that promote active Na+ reabsorption occur in each tubule segment. First, Na+ is actively transported out of the tubule cell by primary active transport-a Na+K+ ATPase pump present in the basolateral membrane. From there, Na+ is swept along by the bulk flow of the water into adjacent peritubular capillaries is rapid because the blood there has low hydrostatic pressure and high osmotic pressure (remember, most protein remain in the blood instead of being filtered out into the tubule).
Second, active pumping of Na+ from the tubule cells result in a strong electrochemical gradient that favors its passive entry at the luminal face via secondary active transport carries or via facilitated diffusion through channels. This occurs because (1)the pump maintains the intracellular Na+ concentrations at low levels (2)the K+ pumped into the tubule cells almost immediately diffuses out into the interstitial fluid via leakage channels, leaving the interior of the tubule cell with a net negative charge.
Because each tubule segment plays a slightly different role in reabsorption, the precise mechanism by which Na+ is reabsorbed at the luminal membrane varies.