Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
308
The holes in the capillary endothelium and the gaps
between the podocytes are quite large, and make it easy for
substances dissolved in the blood plasma to get through
from the blood into the capsule. However, the basement
membrane stops large protein molecules from getting
through. Any protein molecule with a relative molecular
mass of around 69 000 or more cannot pass through the
basement membrane, and so cannot escape from the
glomerular capillaries. This basement membrane therefore
acts as a filter. Blood cells, both red and white, are also
too large to pass through this barrier, and so remain in
the blood. Table 14.1 shows the relative concentrations of
substances in the blood and in the glomerular filtrate. You
will see that glomerular filtrate is identical to blood plasma
except that there are almost no plasma proteins in it.
Substance
Concentration
in blood
plasma / g dm −3
Concentration
in glomerular
filtrate / g dm 3
water 900 900
proteins 80.0 0.05
amino acids 0.5 0.5
glucose 1.0 1.0
urea 0.3 0.3
uric acid 0.04 0.04
creatinine 0.01 0.01
inorganic ions
(mainly Na + , K +
and Cl − )
7.2 7.2
Table 14.1 Concentrations of substances in the blood and in
the glomerular filtrate.
Factors affecting glomerular filtration rate
The rate at which the fluid filters from the blood in
the glomerular capillaries into the Bowman’s capsule
is called the glomerular filtration rate. In a human,
for all the glomeruli in both kidneys, the rate is about
125 cm 3 min −1 .
What makes the fluid filter through so quickly? This is
determined by the differences in water potential between
the plasma in glomerular capillaries and the filtrate in the
Bowman’s capsule. You will remember that water moves
from a region of higher water potential to a region of
lower water potential, down a water potential gradient
(page 83). Water potential is lowered by the presence of
solutes, and raised by high pressures.
Inside the capillaries in the glomerulus, the blood
pressure is relatively high, because the diameter of the
afferent arteriole is wider than that of the efferent arteriole,
causing a head of pressure inside the glomerulus. This
tends to raise the water potential of the blood plasma
above the water potential of the contents of the Bowman’s
capsule (Figure 14.9).
However, the concentration of solutes in the blood
plasma in the capillaries is higher than the concentration
of solutes in the filtrate in the Bowman’s capsule. This
is because, while most of the contents of the blood
plasma filter through the basement membrane and into
the capsule, the plasma protein molecules are too big to
get through, and so stay in the blood. This difference in
solute concentration tends to make the water potential in
the blood capillaries lower than that of the filtrate in the
Bowman’s capsule.
Overall, though, the effect of differences in pressure
outweighs the effect of the differences in solute
concentration. Overall, the water potential of the blood
plasma in the glomerulus is higher than the water potential
of the filtrate in the capsule. So water continues to move
down the water potential gradient from the blood into
the capsule.
Reabsorption in the proximal
convoluted tubule
Many of the substances in the glomerular filtrate need to
be kept in the body, so they are reabsorbed into the blood
as the fluid passes along the nephron. As only certain
substances are reabsorbed, the process is called selective
reabsorption.
Most of the reabsorption takes place in the proximal
convoluted tubule. The lining of this part of the nephron
is made of a single layer of cuboidal epithelial cells.
These cells are adapted for their function of reabsorption
by having:
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microvilli to increase the surface area of the inner
surface facing the lumen
tight junctions that hold adjacent cells together so that
fluid cannot pass between the cells (all substances that
are reabsorbed must go through the cells)
many mitochondria to provide energy for sodium–
potassium (Na + –K + ) pump proteins in the outer
membranes of the cells
co-transporter proteins in the membrane facing
the lumen.