12.2 Hormonal control of urine concentration

While the kidneys operate to maintain osmotic balance and blood pressure in the body, they also act in concert with hormones. Hormones are small molecules that act as messengers within the body. Hormones are typically secreted from one cell and travel in the bloodstream to affect a target cell in another portion of the body. Different regions of the nephron bear specialized cells that have receptors to respond to chemical messengers and hormones. In this section, you will learn about two hormones, aldosterone and antidiuretic hormone, that control urine concentration.

Aldosterone

Aldosterone is a hormone synthesized by the adrenal cortex that affects urine concentration by regulating sodium levels in the blood. Almost all of the sodium in the blood is reclaimed by the renal tubules under the influence of aldosterone. Because sodium is always reabsorbed by active transport and water follows sodium to maintain osmotic balance, aldosterone manages not only sodium levels but also the water levels in urine. Aldosterone favors the production of a concentrated urine by the water following the reabsorbed sodium ions. A decrease in the secretion of aldosterone means that less sodium gets reabsorbed in the renal tubules; therefore, more of it gets excreted in the urine. Patients who have Addison's disease have a failing adrenal cortex and cannot produce aldosterone. They lose sodium in their urine constantly, and if the supply is not replenished, the consequences can be fatal.

Antidiurectic hormone

Diuretics are drugs that can increase water loss by interfering with the recapture of solutes and water from the forming urine. They are often prescribed to lower blood pressure. Coffee, tea, and alcoholic beverages are familiar diuretics. Antidiuretic hormone or ADH, as the name suggests, helps the body conserve water when body fluid volume, especially that of blood, is low. It is formed by the hypothalamus and is stored and released from the posterior pituitary gland. It acts by inserting aquaporins, protein channels that allow water to leave, in the collecting ducts and promotes reabsorption of water. This action results in the formation of a concentrated urine. ADH also acts as a vasoconstrictor and increases blood pressure during hemorrhaging.

Section summary

Hormonal cues help the kidneys synchronize the osmotic needs of the body. Hormones like aldosterone and anti-diuretic hormone (ADH) help regulate the needs of the body as well as the communication between the different organ systems.