Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
How ADH affects the kidneys
You have seen that water is reabsorbed by osmosis from
the fluid in the nephron as the fluid passes through the
collecting ducts. The cells of the collecting duct are the
target cells for ADH. This hormone acts on the cell surface
membranes of the collecting ducts cells, making them
more permeable to water than usual (Figure 14.17).
Low ADH
This change in permeability is brought about by
increasing the number of the water-permeable channels
known as aquaporins in the cell surface membrane of the
collecting duct cells (Figure 14.18). ADH molecules bind
to receptor proteins on the cell surface membranes, which
in turn activate enzymes inside the cells. The cells contain
ready-made vesicles that have many aquaporins in their
High ADH
increasing
concentration
of NaCl and
urea in tissue
fluid
collecting duct
wall impermeable
to water
H 2 O
H 2 O
H 2 O
H 2 O
collecting duct
wall permeable
to water – allows
osmotic absorption
of water by the
concentrated
tissue fluid in the
medulla
large volumes of dilute urine produced
small volumes of concentrated urine produced
Figure 14.17 The effects of ADH on water reabsorption from the collecting duct.
313
fluid from distal
convoluted tubule
tissue fluid
1
blood
with
ADH
2
lumen of
collecting duct
4
3
water
5
water
water
urine
1 ADH binds to receptors in the cell surface membrane of the cells
lining the collecting duct.
2 This activates a series of enzyme-controlled reactions, ending with
the production of an active phosphorylase enzyme.
3 The phosphorylase causes vesicles, surrounded by membrane
containing water-permeable channels (aquaporins), to move to
the cell surface membrane.
4 The vesicles fuse with the cell surface membrane.
5 Water can now move freely through the membrane, down
its water potential gradient, into the concentrated tissue
fluid and blood plasma in the medulla of the kidney.
Figure 14.18 How ADH increases water reabsorption in the collecting duct.