|Functions of the Proximal Convoluted Tubule|
At the proximal tubule Na+/K+ active transport occurs at the basal and lateral surfaces of the tubular cells. This active transport reduces the intracellular sodium ion concentration and thus sodium enters the cell from the lumen by passive diffusion. Chloride ions enter the cell down an electrical gradient. Water follows the entry of sodium and chloride ions.
Approximately 65% of water, sodium and chloride ions are absorbed in the proximal tubule. This amount is called obligatory absorption as the amount of fluid absorbed is not related to the needs but to the amount filtered. Though 65% of the sodium chloride load is absorbed at the proximal tubule, the osmolality remains constant (300mOsm) - the same as plasma. The fact that a constant fraction and not a constant volume, is reabsorbed, ensures that any large changes in the GFR does not result in a drastic change in the quantity of fluid entering the loop of Henle or the distal tubule.