How do kidneys regulate the amount of water in the body?

The kidneys regulate body water content primarily through the nephron, the functional unit that filters blood and selectively reabsorbs water and solutes. This process is governed by a feedback mechanism involving antidiuretic hormone (ADH).123

The Role of ADH

When plasma osmotic pressure rises—due to dehydration or high salt intake—receptors in the brain trigger increased ADH secretion from the pituitary gland. ADH makes the collecting ducts of the kidney permeable to water, allowing it to be reabsorbed into the blood, which concentrates the urine. Conversely, when water intake is high, ADH secretion decreases, making the ducts impermeable to water; this results in the excretion of dilute urine to eliminate excess fluid.3

Countercurrent Multiplication

The kidney also utilizes the loop of Henle to create a high concentration of salt in the medullary fluid. This osmotic pressure gradient provides the necessary force to pull water out of the collecting ducts when ADH is present, ensuring the body maintains precise water and electrolyte equilibrium.31