How does antidiuretic hormone regulate the reabsorption of water in the kidney?

Antidiuretic hormone (ADH) regulates water reabsorption in the kidneys by increasing the permeability of the collecting ducts.

ADH, also known as vasopressin, is a hormone produced in the hypothalamus and stored in the posterior pituitary gland. It plays a crucial role in maintaining the body's water balance. When the body is dehydrated, or when the blood concentration is too high, the hypothalamus stimulates the release of ADH. This hormone then travels in the bloodstream to the kidneys.

In the kidneys, ADH acts on the cells of the collecting ducts, which are the final part of the nephron where urine is collected before being excreted. The hormone binds to receptors on the cells, triggering a cascade of events inside the cell. This results in the insertion of water channels, known as aquaporins, into the cell membrane. These aquaporins increase the permeability of the cell membrane to water.

As a result, more water is reabsorbed from the urine back into the bloodstream. This process dilutes the blood, reducing its concentration. The concentrated urine, containing more waste products, is then excreted from the body. This mechanism allows the body to conserve water when it is needed, such as in situations of dehydration or high blood concentration.

In contrast, when the body is well-hydrated and the blood concentration is low, less ADH is released. This results in fewer aquaporins in the collecting duct cells, reducing water reabsorption. The urine is then more dilute, and more water is excreted from the body.

In summary, ADH plays a vital role in regulating the body's water balance. By controlling the permeability of the collecting ducts in the kidneys, it adjusts the amount of water reabsorbed from the urine and returned to the bloodstream, thereby maintaining the appropriate concentration of the blood.