Water reabsorption

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  • Created on: 10-03-16 23:03
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Water reabsorption
Learning objective(s):
Explain, using water potential terminology, the control of water content of the
blood, with reference to the roles of the kidney, osmoreceptors in the
hypothalamus and the posterior pituitary gland
Reabsorption of water
The role of the loop of Henle is to create a low water potential in the tissue of the medulla. This
ensures that even more water can be reabsorbed from the fluid in the collecting duct.
The loop of Henle
The loop of Henle consists of a descending limb that descends into the medulla and an ascending
limb that ascends back out to the cortex . The arrangement of the loop of Henle allows salts
(sodium and chloride ions) to be transferred from the ascending limb to the descending limb . The
overall effect is to increase the concentration of salts in the tubule fluid and consequently they
diffuse out from the thinwalled ascending limb into the surrounding medulla tissue, giving the
tissue fluid in the medulla a very low water potential .
1. The ascending limb actively transports Na+ ions and Cl
ions out into the intercellular space, producing a
concentration gradient in the medulla. Water cannot
flow because the ascending limb is impermeable to
water.
2. Water moves out of the permeable descending limb via
osmosis , increasing the filtrate concentration .
3. Filtrate at the base of the loop is very concentrated .
When it enters the ascending limb, Na+ ions are
pumped out .
4. This makes the filtrate at the top of the limb dilute again, allowing more water diffusion in
the distal convoluted tubule .
5. The filtrate drains into the collecting duct which flows back through the concentrated
medulla.
6. The hormone ADH causes the walls to become permeable to water , meaning water moves
via osmosis out into the surrounding blood capillaries
The arrangement of the loop of Henle is known as a hairpin
countercurrent multiplier system , meaning that one part of the tubule
passes close to another part of the tubule with the fluid flowing in
opposite directions . This allows exchange between the contents and can
be used to create a very high concentration of solutes . The overall effect
is to increase the efficiency of salt transfer from the ascending limb to
the descending limb. This causes a buildup of salt concentration in the
surrounding tissue fluid .

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It is beneficial for mammals living in arid regions to have higher salt concentration in their
medullas and a longer loop of Henle because the greater the gradient from the top to the bottom
of the tubule , the more water can be reabsorbed , which is important when there is little water
available in the environment.
The collecting duct
The distal convoluted tubule uses active
transport to adjust the concentrations of
various salts. From here fluid flows into
the collecting duct .…read more

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The collecting duct must pass back through a region of low water potential so that the water can
diffuse out of the collecting duct to be reabsorbed back into the blood . Terrestrial mammals must
reabsorb as much water as possible because they lose water in other ways such as sweating and
through water vapour when exhaling.…read more

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