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Negative feedback

The process by which levels of substances in the blood, or other variables within the body, are maintained at a constant level.

Pituitary gland

An endocrine gland attached to the hypothalamus which has two lobes. The posterior lobe secretes several different hormones whereas the anterior lobe just stores and releases one particular hormone.

Hypothalamus

An area of the brain which regulates hormone release, temperature control, hunger, thirst and sleep.

Respiration

The biochemical process by which the cells in the body releases energy

Dehydration

The state of the body when it does not have enough water.

Ecstasy

A recreational drug which stimulates mental and physical activity. Several short and longer term hazards are associated with taking this drug.

Faeces

The waste material left at the end of the digestive process made up of undigested food, dead cells, bacteria and water

Brain

The main organ of the central nervous system made up mainly of grey matter

ADH and control of the water balance

What is a hormone?

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.

ADH and the water balance of the body

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 p4). 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.

Things Affecting ADH

Alcohol can decrease the amount of ADH being produced resulting in a greater volume of more dilute urine being produced. This can lead to dehydration.

Ecstasy has the opposite effect. It increases the amount of ADH which reduces the amount of urine produced.

Both alcohol and ecstasy interfere with the normal regulation of water in the body and may have harmful long-term effects on the body.

Question 5