HORMONES THAT INFLUENCE REABSORPTION OF WATER
Aldosterone is secreted by the adrenal cortex in re-sponse to a high blood potassium level, to a low blood sodium level, or to a decrease in blood pressure. When aldosterone stimulates the reabsorption of Na+ ions, water follows from the filtrate back to the blood. This helps maintain normal blood volume and blood pres-sure.
You may recall that the antagonist to aldosterone is atrial natriuretic peptide (ANP), which is secreted by the atria of the heart when the atrial walls are stretched by high blood pressure or greater blood vol-ume. ANP decreases the reabsorption of Na+ ions by the kidneys; these remain in the filtrate, as does water, and are excreted. By increasing the elimination of sodium and water, ANP lowers blood volume and blood pressure.
Antidiuretic hormone (ADH) is released by the posterior pituitary gland when the amount of water in the body decreases. Under the influence of ADH, the distal convoluted tubules and collecting tubules are able to reabsorb more water from the renal filtrate. This helps maintain normal blood volume and blood pressure, and also permits the kidneys to produce urine that is more concentrated than body fluids. Producing a concentrated urine is essential to prevent excessive water loss while still excreting all the sub-stances that must be eliminated.
If the amount of water in the body increases, how-ever, the secretion of ADH diminishes, and the kid-neys will reabsorb less water. Urine then becomes dilute, and water is eliminated until its concentration in the body returns to normal. This may occur follow-ing ingestion of excessive quantities of fluids.