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CHAPTER TWENTY
Urinary System
than that of other capillaries. Finally, the relatively high rate
of glomerular ±
ltration has increased the protein concentra-
tion and, thus, the colloid osmotic pressure of the peritubu-
lar capillary plasma. These factors enhance the rate of fl
uid
reabsorption from the renal tubule.
Tubular reabsorption occurs throughout the renal tubule.
However, most of it is in the proximal convoluted portion.
The epithelial cells in this portion have many
microvilli
that
form a “brush border” on their free surfaces facing the tubular
lumen. These tiny extensions greatly increase the surface area
exposed to the glomerular ± ltrate and enhance reabsorption.
Segments of the renal tubule are adapted to reabsorb spe-
ci± c substances, using particular modes of transport. Glucose
reabsorption, for example, takes place through the walls of the
proximal convoluted tubule by active transport. Water also is
rapidly reabsorbed through the epithelium of the proximal con-
voluted tubule by osmosis; however, portions of the distal con-
voluted tubule and collecting duct may be almost impermeable
to water. This characteristic of the distal convoluted tubule and
collecting duct is important in the regulation of urine concen-
tration and volume, as described in a subsequent section.
Recall that active transport requires carrier proteins in a
cell membrane. The molecule to be transported binds to the
carrier; the carrier changes shape, releases the transported
molecule on the other side of the cell membrane, and then
returns to its original shape and repeats the process. Such a
mechanism has a
limited transport capacity;
it can transport
only a certain number of molecules in a given time because
the number of carriers is limited.
are in the glomerular ±
ltrate. Such changes in fl
uid composi-
tion are largely the result of
tubular reabsorption
, the pro-
cess by which substances are transported out of the tubular
fl uid, through the epithelium of the renal tubule, and into
the interstitial fl uid. These substances then diffuse into the
peritubular capillaries
(f
g. 20.21)
.
Tubular reabsorption returns substances to the internal
environment. The term
tubular
is used because this process
is controlled by the epithelial cells that make up the renal
tubules and collecting ducts. In tubular reabsorption, sub-
stances must ± rst cross the cell membrane facing the inside
of the tubule (mucosal surface) and then the cell membrane
facing the interstitial fl
uid (serosal surface).
The basic rules for movements across cell membranes
apply to tubular reabsorption. Substances moving down a
concentration gradient must either be lipid soluble or there
must be a carrier or channel for that substance. Active trans-
port, requiring ATP, may move substances uphill against a
concentration gradient. If active transport is involved at any
step of the way, the process is considered active tubular reab-
sorption. In all other cases, the process is considered passive.
RECONNECT
To Chapter 3, Movements Into and Out oF the Cell,
pages 90–99.
Peritubular capillary blood is under relatively low pres-
sure because it has already passed through two arterioles.
Also, the wall of the peritubular capillary is more permeable
FIGURE 20.21
Two processes in addition to glomerular f
ltration help to Form urine. (
a
) Tubular reabsorption transports substances From the
glomerular f
ltrate into the blood in the peritubular capillary. (
b
) Tubular secretion transports substances From the blood in the peritubular capillary
into the renal tubule.
Blood
flow
Blood
flow
Efferent
arteriole
Afferent
arteriole
Glomerular
capsule
Glomerulus
Glomerular
filtrate
Renal
tubule
Tubular
reabsorption
(a)
(b)
Peritubular
capillary
Efferent
arteriole
Afferent
arteriole
Glomerular
capsule
Blood flow
Blood flow
Glomerulus
Glomerular
filtrate
Renal
tubule
Tubular
secretion
Peritubular
capillary
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