The control of sodium excretion.

The kidneys of a normal man filter approximately 24,000 meq sodium/day, reabsorb about 23,900, and yet can make a 1--2 meq change in 24-h urinary sodium excretion. The control of urinary sodium excretion, therefore, depends, first, on ensuring that the bulk of the sodium is reabsorbed, a function which is carried out in the proximal tubule and ascending loop of Henle. Second, it depends on adjusting the reabsorption of the small quantity of sodium which is delivered into the collecting duct so that the amount excreted in the urine is that required to maintain sodium balance. The bulk reabsorptive mechanisms can be considered as buffers to prevent large fluctuations in the amount of sodium delivered to the collecting duct, thus facilitating the fine adjustments of reabsorption which are made at this site. In conditions other than extreme salt loading or deprivation, changes in sodium reabsorption in the proximal tubule and loop of Henle probably have little, if any, effect on urinary sodium excretion. Sodium reabsorption in the proximal tubule and the collecting duct appears to be influenced by unidentified circulating substances.