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Science Reviewer: Endocrine System (Part 1)

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Here are some things that hormones do for the body. Remember, though, they are only the

"messengers" that start the cells into action. They go out into the body when told to by a

higher power (the gland) and it's the cell that actually does the work.

•  make people grow or stop growing,

• 

make people feel happy or sad,

• 

speed up or slow down the metabolism,

• 

starting puberty,•  starting menopause,

•  regulating your fighting, mating, and fleeing instincts.

 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.