Homeostasis - kidneys and water balance

Hormones are special chemicals which coordinate many processes in your body. Hormones are made in glands which then release them into your bloodstream. They are carried around the body in your blood to their target organs. Hormones regulate the functions of many of your organs and cells, including the amount of water which is reabsorbed by your kidneys. You can read more about hormones here.

The amount of water in the blood must be kept more or less the same all the time to avoid cell damage as a result of osmosis (see page 4). There has to be a balance between the amount of water gained (from your diet though drinks and food and the water produced by cellular respiration) and the amount of water lost by the body (in sweating, evaporation, faeces and urine).

This is achieved by the action of the hormone ADH (anti-diuretic hormone). How does it work?

Perhaps you have not drunk anything for a while or you have been sweating a lot. Part of the brain, the hypothalamus, detects that there is not enough water in the blood. The hypothalamus sends a message to the pituitary gland which releases ADH. This travels in the blood to your kidneys and affects the tubules so more water is reabsorbed into your blood. As a result you make a smaller volume of more concentrated urine. The level of water in your blood increases until it is back to normal.

Sometimes the level of water in your blood goes up because, for example, it is cold and you have not been losing any water through sweating or because you have had a lot to drink. The hypothalamus detects the change and sends a message to the pituitary. The release of ADH into the blood is slowed down or even stopped. Without ADH the kidneys will not save as much water and you produce large volumes of dilute urine. The level of water in the blood falls back to the normal level.

This is an example of negative feedback. As the level of water in the blood falls, negative feedback ensures that the amount of ADH rises. As the level of water in the blood rises negative feedback ensures that the amount of ADH falls.