A quick and perhaps over-simplified description of the system that keeps the water balance in humans.

1. Water balance
Another example of homeostasis

2. Kangaroo Rat
lives in hot desert
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3. Little water in environment. Does not drink water!
All water comes from breakdown of nutrients.
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4. Needs to conserve water. How?
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5. Behavioural - nocturnal (active at
night, sleeps during the day
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6. Physiological - miminum water lost from urination.
(5 times more concentrated that human urine)
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7. Also need to avoid having too much water.
Excess water leads to thinner blood.

8. Thinner blood leads to reduced concentration of red blood
cells (a.k.a. anemia). Not enough oxygen gets to the cells.
cc licensed flickr photo by kingdesmond1337: http://flickr.com/photos/kingdesmond/2872482711/

9. If blood is really dilute, the red blood cells can rupture
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10. Water balance is maintained homeostatically.
Sensor
Control centre
Effector

11. Sensors
Hypothalmus - senses
concentration of blood.
Blood vessels - sense
blood volume.
If concentration is too
cc licensed flickr photo by EUSKALANATO: http://
flickr.com/photos/17657816@N05/1971827663/ high or blood volume is
low, sends signal to the ...

12. Controller
Pituitary gland (right next to
hypothalmus)
If blood concentration is too
high, it secretes vasopressin
cc licensed flickr photo by EUSKALANATO: http://
flickr.com/photos/17657816@N05/1971827663/
(a.k.a. antidiuretic hormone
a.k.a. ADH), a hormone that
acts on the ...

13. Effector
Kidneys
ADH causes the kidney to
reduce urine production. Water
loss through kidneys is
reduced.
Brain
The hypothalmus activates parts
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flickr.com/photos/kaptainkobold/273001185/
of brain that cause thirst.

14. What if there’s too much water?
Hypothalmus
The hypothalmus stops sending signals to
thirst centres and to pituitary.
Pituitary decreases ADH secreted.
Kidneys release more water into urine