What is the primary function of Antidiuretic Hormone (ADH)?

Antidiuretic Hormone (ADH), also known as vasopressin, primarily functions to regulate the concentration of urine. It achieves this by acting on the kidneys, specifically on the collecting ducts, where it increases the permeability of the duct walls to water. When ADH is present, more water is reabsorbed back into the bloodstream rather than being excreted as urine, leading to concentrated urine. Conversely, when levels of ADH are low, the body expels more dilute urine, as less water is reabsorbed.

This mechanism is crucial for maintaining homeostasis in the body, particularly in response to changes in fluid balance, blood volume, and blood pressure. By concentrating or diluting urine, ADH helps control the overall hydration status of the body, making it a vital hormone in fluid regulation.