How do kidneys regulate water balance in the body?

Kidneys regulate water balance in the body by filtering blood and adjusting the reabsorption of water and electrolytes.

The kidneys are vital organs that perform several functions, one of which is the regulation of water balance in the body. This is achieved through a complex process known as filtration and reabsorption. The kidneys filter the blood, removing waste products and excess substances, including water. This filtered fluid then passes through the tubules of the nephron, the functional unit of the kidney.

In the tubules, a process of selective reabsorption occurs. This is where the body reclaims the substances it needs, including water and electrolytes such as sodium and potassium. The amount of water reabsorbed is influenced by the body's hydration status. If the body is dehydrated, the kidneys will reabsorb more water to conserve it, resulting in concentrated, dark urine. Conversely, if the body is overhydrated, the kidneys will reabsorb less water, leading to dilute, light-coloured urine.

The regulation of water balance by the kidneys is also closely linked to the regulation of electrolyte balance. This is because the reabsorption of water is influenced by the concentration of electrolytes in the blood. For example, the hormone aldosterone increases the reabsorption of sodium, which in turn increases the reabsorption of water, helping to increase blood volume and pressure.

Furthermore, the kidneys respond to the hormone vasopressin, also known as antidiuretic hormone (ADH). When the body is dehydrated, the pituitary gland releases ADH, which signals the kidneys to reabsorb more water, thereby reducing urine output and helping to restore water balance.

In summary, the kidneys play a crucial role in maintaining water balance in the body. They do this by filtering the blood and selectively reabsorbing water and electrolytes, with the amount reabsorbed adjusted according to the body's needs. This process is regulated by hormones such as aldosterone and vasopressin, which respond to changes in the body's hydration status and electrolyte balance.