What would happen if there were an absence of ADH?

In the absence of antidiuretic hormone (ADH), also known as vasopressin, the kidneys are unable to reabsorb water effectively. ADH typically promotes the insertion of water channels called aquaporins in the collecting ducts of the kidneys, allowing more water to be reabsorbed back into the bloodstream. Without ADH, these aquaporins are not present in sufficient quantities, leading to a significant reduction in water reabsorption.

As a result, the urine produced becomes very dilute. This occurs because, even though the kidneys still filter blood and produce urine, the lack of adequate water reabsorption means that excess water remains in the urine. Consequently, the final urine output is not only increased in volume but also has a low concentration of solutes, resulting in very dilute urine. This mechanism is crucial for the body's fluid balance and maintains hydration levels within appropriate limits.