Chapter 25
The Urinary System
969
25
created by Na
1
-K
1
pumping at the basolateral membrane) in-
clude glucose, amino acids, some ions, and vitamins. In nearly
all these cases, an apical carrier moves Na
1
down its concentra-
tion gradient as it cotransports (symports) another solute (Fig-
ure 25.14
3
). Cotransported solutes move across the basola-
teral membrane by facilitated diffusion via other transport pro-
teins (not shown) before moving into the peritubular capillaries.
Passive Tubular Reabsorption of Water
Te movement of
Na
1
and other solutes establishes a strong osmotic gradient, and
water moves by osmosis into the peritubular capillaries. ±rans-
membrane proteins called
aquaporins
aid this process by acting
as water channels across cell membranes (Figure 25.14
4
).
In continuously water-permeable regions of the renal tu-
bules, such as the PC±, aquaporins are always present in the
tubule cell membranes. Teir presence “obliges” the body to
absorb water in the proximal nephron regardless of its state of
over- or underhydration. Tis water flow is referred to as
oblig-
atory water reabsorption
.
Aquaporins are virtually absent in the apical membranes
of the collecting duct unless antidiuretic hormone (ADH) is
present. Water reabsorption that depends on ADH is called
fac-
ultative water reabsorption
.
Passive Tubular Reabsorption of Solutes
As water leaves the
tubules, the concentration of solutes in the filtrate increases and,
if able, they too follow their concentration gradients into the
peritubular capillaries. Tis phenomenon—solutes following
solvent—explains the passive reabsorption of a number of sol-
utes present in the filtrate, such as lipid-soluble substances, cer-
tain ions, and some urea (Figure 25.14
5
,
6
). It also explains in
Sodium Transport Across the Basolateral Membrane
Na
1
is actively transported out of the tubule cell by
primary active
transport
—a Na
1
-K
1
A±Pase pump in the basolateral mem-
brane (
Figure 25.14
1
). From there, the bulk flow of water
sweeps Na
1
into adjacent peritubular capillaries. Tis bulk flow
of water and solutes into the peritubular capillaries is rapid be-
cause the blood there has low hydrostatic pressure and high os-
motic pressure (remember, most proteins remain in the blood
instead of filtering out into the tubule).
Sodium Transport Across the Apical Membrane
Active
pumping of Na
1
from the tubule cells results in a strong elec-
trochemical gradient that favors its entry at the apical face
via
secondary active transport
(symport or antiport) carriers
(Figure 25.14
2
,
3
) or via facilitated diffusion through chan-
nels (not illustrated). Tis occurs because (1) the pump main-
tains the intracellular Na
1
concentration at low levels, and
(2) the K
1
pumped into the tubule cells almost immediately dif-
fuses out into the interstitial fluid via leakage channels, leaving
the interior of the tubule cell with a net negative charge.
Because each tubule segment plays a slightly different role
in reabsorption, the precise mechanism by which Na
1
is reab-
sorbed at the apical membrane varies.
Tubular Reabsorption of Nutrients, Water, and Ions
Te reabsorption of Na
1
by primary active transport provides
the energy and the means for reabsorbing almost every other
substance, including water.
Secondary Active Transport
Substances reabsorbed by
sec-
ondary active transport
(the “push” comes from the gradient
Filtrate
in tubule
lumen
Transcellular route
Paracellular route
Tight
junction
Lateral
intercellular
space
Apical
membrane
Capillary
endothelial
cell
H
2
O and
solutes
H
2
O and
solutes
Tubule cell
Basolateral
membranes
Interstitial
fluid
Peri-
tubular
capillary
1
Transport across the apical
membrane.
2
Diffusion through the
cytosol.
4
Movement through the
interstitial fluid and into the
capillary.
3
Transport across the
basolateral membrane. (Often
involves the lateral intercellular
spaces because membrane
transporters transport ions into
these spaces.)
The transcellular route
involves:
The paracellular route involves:
Movement through leaky
tight junctions, particularly in
the PCT.
Movement through the
interstitial fluid and into the
capillary.
4
3
1
2
4
3
Figure 25.13
Transcellular and paracellular routes of tubular reabsorption.
Generally,
water and solutes move into the peritubular capillaries through intercellular clefts. For simplicity,
transporters, ion channels, intercellular clefts, and aquaporins are not depicted.
previous page 1003 Human Anatomy and Physiology (9th ed ) 2012 read online next page 1005 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off