What is the role of antidiuretic hormone (ADH) in the kidneys?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating the body's water balance, and its primary function in the kidneys is to increase water reabsorption. When ADH is released, it acts on the kidneys, specifically targeting the collecting ducts. It binds to receptors on the cells in these ducts, facilitating the insertion of aquaporin proteins into the cell membranes. These aquaporins are water channels that allow more water to be reabsorbed back into the bloodstream, effectively concentrating the urine and reducing water loss.

This mechanism is critical for maintaining homeostasis, particularly in conditions where the body needs to conserve water, such as dehydration or high osmolarity in the blood. By increasing water reabsorption, ADH helps to dilute the blood plasma and restore fluid balance. This process supports blood pressure regulation indirectly as well, but its primary role is in fluid retention rather than directly affecting sodium reabsorption, blood pressure, or inhibiting urination.

Therefore, the understanding of ADH's function in promoting water reabsorption is fundamental in appreciating how the kidneys regulate urine concentration and overall body fluid levels.