Biomedical Engineering Reference
In-Depth Information
Fig. 14.9
Osmotic pressure as a driving force for water reabsorption in the tubules (Oncotic
pressure is the pressure of proteins in the liquid)
takes place in the distal and collecting tubules of the nephrons under control of anti-
diuretic hormone (ADH), sometimes called
vasopressin
. This hormone is released
by the
posterior pituitary
under control of the
hypothalamus
in the mid-brain area.
The hypothalamus monitors the water content of the blood [
1
]. If the blood contains
too little water (indicating dehydration) then more ADH is released. If the blood
contains too much water (indicating over-hydration) then less ADH is released into
the blood stream [
1
].
Release of ADH from the Posterior Pituitary into the Bloodstream
ADH released from the pituitary travels in the blood stream to the peri-tubular
capillaries of the nephron. ADH binds to receptors on the distal and collecting
tubules of the nephrons which causes water channels to open in the tubule walls.
This allows water to diffuse through the tubule walls into the interstitial fl uid where
it is collected by the peri-tubular capillaries. The more ADH present, the more water
channels are open and the more water is reabsorbed (Fig.
14.10
).
Over 99 % of the fi ltrate produced each day can be reabsorbed [
1
-
3
]. The amount of
water reabsorbed from the fi ltrate back into the blood depends on the water situation
in the body. When the body is dehydrated, most of the fi ltrate is reabsorbed but note that
even in cases of extreme of water shortage, the kidneys will continue to produce around
500 ml of urine each day
in order to perform their excretory function.
