Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
membranes. Once the enzymes in each cell are activated
by the arrival of ADH, these vesicles move towards the
cell surface membrane and fuse with it, so increasing the
permeability of the membrane to water.
So, as the fluid flows down through the collecting
duct, water molecules move through the aquaporins
(Figure 14.19), out of the tubule and into the tissue fluid.
This happens because the tissue fluid in the medulla
has a very low water potential and the fluid in the
collecting ducts has a very high water potential. The
fluid in the collecting duct loses water and becomes
more concentrated. The secretion of ADH has caused the
increased reabsorption of water into the blood. The volume
of urine which flows from the kidneys into the bladder
will be smaller, and the urine will be more concentrated
(Figure 14.20).
What happens when you have more than enough water
in the body – for example, after enjoying a large volume
of your favourite drink? When there is an increase in the
water potential of the blood, the osmoreceptors in the
hypothalamus are no longer stimulated and the neurones
in the posterior pituitary gland stop secreting ADH.
This affects the cells that line the collecting ducts. The
aquaporins are moved out of the cell surface membrane
of the collecting duct cells, back into the cytoplasm
as part of the vesicles. This makes the collecting duct
cells impermeable to water. The fluid flows down the
collecting duct without losing any water, so a dilute
urine collects in the pelvis and flows down the ureter to
the bladder. Under these conditions, we tend to produce
large volumes of dilute urine, losing much of the water
we drank, in order to keep the water potential of the
blood constant.
The collecting duct cells do not respond immediately to
the reduction in ADH secretion by the posterior pituitary
gland. This is because it takes some time for the ADH
already in the blood to be broken down; approximately
half of it is destroyed every 15–20 minutes. However, once
ADH stops arriving at the collecting duct cells, it takes
only 10–15 minutes for aquaporins to be removed from the
cell surface membrane and taken back into the cytoplasm
until they are needed again.
314
1200
with ADH
Concentration of fluid / mmol kg –1
900
600
300
without ADH
Figure 14.19 Aquaporin protein channels allow water to
diffuse through membranes such as those in the cells that line
collecting ducts.
0
proximal
convoluted
tubule
loop
of
Henle
distal
convoluted
tubule
collecting
duct
QUESTION
14.7 a Use the example of blood water content to explain
the terms set point and homeostasis.
b Construct a flow diagram to show how the water
potential of the blood is controlled. In your
diagram identify the following: receptor, input,
effector and output. Indicate clearly how different
parts of the body are coordinated and show how
negative feedback is involved.
Figure 14.20 The concentration of fluid in different regions of
a nephron, with and without the presence of ADH.