Cambridge International A Level Biology Revision Guide

H 2 O
H 2 O
Na
5
Key
part of the ascending limb.
Chapter 14: Homeostasis
wall permeable to Na
permeable to water
a
1
Na + and Cl − are actively transported
out of the ascending limb.
b
wall permeable to Na
impermeable to wate
3
2
H 2 O
H 2 O
Na +
Cl −
1
2
3
This raises the concentration of Na +
and Cl – in the tissue fluid.
This in turn causes the loss of water
from the descending limb.
1
Cl − 2
H 2 O
H 2 O
H 2 O
H 2 O
4
H 2 O
H 2 O
Na +
5
4
5
Key
Cl − 2
The loss of water concentrates Na +
and Cl − in the descending limb.
Na + and Cl − ions diffuse out of this
concentrated solution in the lower
part of the ascending limb.
wall permeable to Na + ,
permeable to water
1
2
urine
The tissue in the deeper layers of the medulla
contains a very concentrated solution of Na + , Cl −
and urea.
As urine passes down the collecting duct, water
can pass out of it by osmosis. The reabsorbed
water is carried away by the blood in the capillaries.
wall permeable to Na + ,
impermeable to water
Figure 14.13 b How the loop of Henle allows the production of concentrated urine. a The counter-current mechanism in the loop of
Henle builds up high concentrations of sodium ions and chloride ions in the tissue fluid of the medulla. b Water can pass out of the
fluid in the collecting duct by osmosis, as the surrounding tissue fluid has a lower water potential.
Having the two limbs of the loop running H 2 O side by side
H 2 O
like this, with the fluid flowing down in one and up in the
other, enables the maximum concentration H 2 of O
H solutes to
1
2 O
be built up both inside and outside the tube at the bottom
of the loop. This mechanism is called urine a counter-current
multiplier.
But the 1 The story tissue not in the yet deeper complete. layers You of the have medulla seen that the
contains a very concentrated solution of Na
fluid flowing up the ascending limb of the loop of + , Cl
Henle
−
and urea.
loses sodium 2 As urine and chloride passes down ions the as collecting it goes, so duct, becoming water
more dilute can and pass having out of a it higher by osmosis. water The potential. reabsorbed The
cells of the water ascending is carried limb away of by the the loop blood of Henle in the capillaries. and the
cells lining the collecting ducts are permeable to urea,
which diffuses into the tissue fluid. As a result, urea is
also concentrated in the tissue fluid in the medulla. In
Figure 14.13b you can see that the fluid continues round
through the distal convoluted tubule into the collecting
duct, which runs down into the medulla again. It therefore
passes once again through the regions where the solute
concentration of the tissue fluid is very high and the water
potential very low. Water therefore can move out of the
collecting duct, by osmosis, until the water potential of
urine is the same as the water potential of the tissue fluid
in the medulla, which may be much greater than the water
potential of the blood. The degree to which this happens is
controlled by antidiuretic hormone (ADH).
The ability of some small mammals, such as rodents, to
produce a very concentrated urine is related to the relative
thickness of the medulla in their kidneys. The maximum
concentration of urine that we can produce is four times
that of our blood plasma. Desert rodents, such as gerbils
and kangaroo rats, can produce a urine that is about 20
times the concentration of their blood plasma. This is
possible because the medulla is relatively large and the
cells that line the ascending limb of their loops have deep
infolds with many Na + –K + pumps and cytoplasm filled
with many mitochondria, each with many cristae that
allow the production of much ATP to provide the energy
for the pumping of sodium ions into the tissue fluid.
Reabsorption in the distal convoluted
tubule and collecting duct
The first part of the distal convoluted tubule functions
in the same way as the ascending limb of the loop of
Henle. The second part functions in the same way as
the collecting duct, so the functions of this part of the
distal convoluted tubule and the collecting duct will be
described together.
In the distal convoluted tubule and collecting duct,
sodium ions are actively pumped from the fluid in the
tubule into the tissue fluid, from where they pass into the
blood. Potassium ions, however, are actively transported
into the tubule. The rate at which these two ions are
moved into and out of the fluid in the nephron can be
varied, and helps to regulate the concentration of these
ions in the blood.
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