The number of people waiting for organs is considerably greater than the number of donors. Water reabsorption occurs because of the countercurrent mechanism, in which an osmotic gradient builds up in the renal medulla, drawing water out of the filtrate. Most is reabsorbed at the proximal convoluted tubule, with the distal convoluted tubule and collecting duct absorbing the rest. The juxtaglomerular apparatus secretes renin when blood pressure is insufficient to drive filtration. I. This is later converted to angiotension II, which stimulates the adrenal cortex to release aldosterone.
Aldosterone promotes sodium reabsorption, increasing blood volume and pressure.
The heart secretes atrial natriuretic hormone when blood volume is too high. This inhibits the release of renin, and thus aldosterone. Diuretics increase urine output, decreasing blood volume and pressure. During respiration, the lungs remove CO2 from the blood. The sweat glands perspire, giving off salt, urea, and water.