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TES connect - 18/01/10

Homeostasis - kidneys and water balance

Age range 14-16

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

Fill in the following flow diagrams to show how homeostasis involving ADH works to balance the water levels in the blood.

Quiz Print 1
Too much water in the blood
renal tubule reabsorbs less water
large volumes of dilute urine produced
blood concentration returns to normal
detected by receptors in the hypothalamus
pituitary releases less ADH
Quiz Print 2
Too little water in the blood
blood concentration returns to normal
renal tubule reabsorbs more water
small amounts of concentrated urine produced
detected by receptors in the hypothalamus
pituitary releases more ADH
Anti-diuretic hormone, the hormone which controls the reabsorption of water in the kidney tubules
Chemicals which are made in glands and carried around the body in the bloodstream to coordinate many body processes
The region of the brain which detects the concentration of water in the blood
Negative feedback
The process by which levels are maintained at constant level in the blood
A collection of tissues specialised to carry out a particular function in the body
The movement of water through a partially permeable membrane down a concentration gradient from a dilute solution ( where there is a high concentration of water) to a concentrated solution (where there is a relatively low concentration of water).
A recreational drug which stimulates mental and physical activity. Several short and longer term hazards are associated with taking this drug.