What is the action of antidiuretic hormone (ADH) on fluid balance?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in maintaining fluid balance in the body. Its primary action is to decrease urine production by promoting water reabsorption in the kidneys, specifically in the collecting ducts. When ADH is released into the bloodstream, it binds to receptors on the cells of the collecting ducts, leading to the insertion of aquaporin channels. These channels allow water to be reabsorbed back into the bloodstream rather than being excreted as urine, thereby concentrating the urine and reducing its volume.

This mechanism is vital for regulating hydration status and maintaining blood pressure. When the body is dehydrated or when blood osmolarity rises, ADH levels increase, resulting in more water being reabsorbed, which helps to dilute the blood and restore normal fluid balance. In contrast, lower levels of ADH are associated with increased urine output and less water reabsorption, which can lead to dehydration and lower blood volume. Hence, the action of ADH significantly impacts fluid balance by retaining water and decreasing urine volume.