Regulation of Na+ and Water Reabsorption

Regulation of Na+ and Water Reabsorption

Na+ are dominant EC cation, thus they exert about (90-95%) osmotic pressure in ECF (in combination with Cl- anion). After Na+ freely filtered at glomerulus, its reabsorption which is active occur in all tubular segment except descending limb of loop of Henle, while HOH reabsorption is by diffusion and dependent on Na+ reabsorption.
In proximal tubules by the rate of Na+ move out of cell to fluid and achieved by Na+/K+ ATPase pumps in the basolateral membrane of cells.
This active transport of Na+ out of cell diminish intracellular Na+ concentration compared to luminal concentration. This in turn establish Na+ concentration gradient, thus Na+ diffuses from tubule through brush border to cell.
The continuous active transport of Na+ out of cell creates electrical potential (-70 mv) which is an additional factor favours Na+ diffusion from tubular lumen to cell.
Cl- will be cotransported with Na+ by difference in electrical charge when the concentration of solute decrease in lumen and increase in interstitium. This resulted in concentration difference which promote water osmosis in same direction.