How It Works: ADH and the Negative Feedback Loop

How It Works: ADH and the Negative Feedback Loop

Water regulation is controlled by a hormone called ADH (antidiuretic hormone), produced in the hypothalamus and released from the pituitary gland. The system operates as a classic negative feedback loop.

When blood water concentration falls — for example after exercise or on a hot day — osmoreceptors in the hypothalamus detect the change. The pituitary gland responds by releasing more ADH into the blood. ADH travels to the kidneys where it increases the permeability of the collecting duct, allowing more water to be reabsorbed back into the blood. ADH achieves this by causing aquaporin protein channels to be inserted into the walls of the collecting duct — more aquaporins mean more water can pass through by osmosis. The result is a smaller volume of concentrated (dark) urine. Blood water concentration rises back to normal, which switches off the ADH signal — negative feedback in action.

The reverse happens when you drink a lot of water: blood becomes too dilute, less ADH is released, the collecting duct becomes less permeable, less water is reabsorbed, and you produce a larger volume of dilute (pale) urine. The kidneys adjust continuously, keeping blood water concentration within a very narrow range at all times.