Antidiuretic hormone

This hormone is produced by nerve cells in the hypothalamus, part of the brain, and stored in the posterior lobe of the pituitary gland located on the under side of the brain. Antidiuretic hormone acts to decrease the amount of water the kidney releases in the urine. If a large meal is taken without water or if a large volume of fluid is lost as sweat, the blood becomes more concentrated. This increase in concentration of the blood is detected by receptors in the brain, and ADH is released from the pituitary. The hormone travels in the blood to the kidneys where it has an effect on the cells of the collecting ducts. These cells become more permeable to water in the presence of ADH, allowing water to flow from the collecting ducts into the interstitial fluid and hence back into the blood. This concentrates the urine and diverts water back into the bloodstream.

The well-known effect of drinking large amounts of water, alcohol, tea and coffee to raise urine flow is caused by the inhibition of ADH by these substances. As ADH is inhibited, the cells of the collecting ducts remain impermeable to water, and so a large volume of dilute urine is formed.