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Transcription:
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.
This balance is
achieved by the action of the hormone ADH. When the body is losing too much
water, the hypothalamus detects that there is not enough water in the blood and
sends a message to the pituitary gland, which releases ADH. ADH travels in the
blood to the kidneys and affects the tubules so more water is reabsorbed into the
blood. As a result the body makes smaller volume of more concentrated urine.
The level of water in the blood increases until it is back to normal.
Sometimes the level
of water in the blood goes up because the body has not been losing any water,
or the body has been taking in a lot of water. 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 will produce large volumes of dilute urine. The level of water in the
blood falls back to the normal level.
The
secretion of ADH 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. ADH has a simple neurohormone pathway because the hypothalamus
detects the change and pituitary secrets ADH directly. ADH is composed of peptides.
As a result, it is water-soluble and cannot pass through the cell membrane.
Therefore, the receptor for ADH is located on cell membrane.
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