What is the primary role of Antidiuretic hormone (ADH)?

Antidiuretic hormone (ADH), also known as vasopressin, primarily functions to promote the retention of water by the kidneys. It achieves this by increasing the permeability of the kidney's collecting ducts to water, allowing more water to be reabsorbed back into the bloodstream rather than being excreted in urine. This mechanism is crucial for maintaining the body’s fluid balance and regulating blood pressure.

When the body is dehydrated or when blood osmolarity increases, the release of ADH from the posterior pituitary gland is stimulated. This hormone acts on the nephrons in the kidneys, facilitating the reabsorption of water, which concentrates the urine and reduces water loss. This process is vital in conditions where maintaining hydration is critical, and it helps to prevent dehydration by conserving water in the body.

Other options refer to different physiological functions that are not related to the primary role of ADH, further highlighting its specific mechanism in fluid regulation.