Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
The kidney makes urine in a two-stage process.
The first stage, ultrafiltration, involves filtering small
molecules, including urea, out of the blood and into
the Bowman’s capsule. From the Bowman’s capsule the
molecules flow along the nephron towards the ureter. The
second stage, selective reabsorption, involves taking back
any useful molecules from the fluid in the nephron as it
flows along.
Ultrafiltration
Figure 14.10 shows a section through part of a glomerulus
and Bowman’s capsule. The blood in the glomerular
capillaries is separated from the lumen of the Bowman’s
capsule by two cell layers and a basement membrane.
The first cell layer is the lining, or endothelium, of the
capillary. Like the endothelium of most capillaries, this
has gaps in it, but there are far more gaps than in other
capillaries: each endothelial cell has thousands of tiny
holes in it. Next comes the basement membrane, which
is made up of a network of collagen and glycoproteins.
The second cell layer is formed from epithelial cells,
which make up the inner lining of the Bowman’s
capsule. These cells have many tiny finger-like projections
with gaps in between them, and are called podocytes
(Figure 14.11).
Figure 14.11 A false-colour scanning electron micrograph of
podocytes (× 3900). The podocytes are the blue-green cells with
their extensions wrapped around the blood capillary, which is
purple.
307
efferent
arteriole
Bowman’s capsule
epithelium
parts of
podocyte cell
pressure gradient
solute concentration gradient
afferent
arteriole
basement
membrane
glomerular
filtrate
red blood cell
blood plasma
circular hole in
capillary endothelium
basement
membrane
podocyte cell of
capsule wall
glomerular filtrate in
lumen of capsule
Figure 14.10 Detail of the endothelium of a glomerular capillary and Bowman’s capsule. The arrows show how the net effect of
higher pressure in the capillary and lower solute concentration in the Bowman’s capsule is that fluid moves out of the capillary
and into the lumen of the capsule. The basement membrane acts as a molecular filter.