502
Chapter 14
the tubular lumen to the interstitial fl uid across the epithelial
cells. Other solutes, such as glucose, amino acids, and bicar-
bonate, whose reabsorption depends on sodium transport,
also contribute to osmosis. (2) The removal of solutes from the
tubular lumen lowers the local osmolarity of the tubular fl
uid
adjacent to the cell (i.e., the local water concentration increases).
At the same time, the appearance of solute in the interstitial
fl uid just outside the cell increases the local osmolarity (i.e.,
the local water concentration decreases). (3) The difference in
water concentration between lumen and interstitial fl uid causes
net diffusion of water from the lumen across the tubular cells’
plasma membranes and/or tight junctions into the interstitial
fl uid. (4) From there, water, sodium, and everything else dis-
solved in the interstitial fl
uid move together by bulk fl ow into
peritubular capillaries as the fi nal step in reabsorption.
Water movement across the tubular epithelium can only
occur if the epithelium is permeable to water. No matter how
large its concentration gradient, water cannot cross an epithe-
lium impermeable to it. Water permeability varies from tubu-
lar segment to segment and depends largely on the presence of
water channels, called
aquaporins,
in the plasma membranes.
The water permeability of the proximal tubule is always very
high, so this segment reabsorbs water molecules almost as rap-
idly as sodium ions. As a result, the proximal tubule reabsorbs
large amounts sodium and water in the same proportions.
We will describe the water permeability of the next
tubular segments—the loop of Henle and distal convoluted
tubule—later. Now for the really crucial point: the water per-
meability of the last portions of the tubules, the cortical and
medullary collecting ducts, can vary greatly due to physio-
logical control. These are the only tubular segments in which
water permeability is under such control.
The major determinant of this controlled permeability,
and therefore, of passive water reabsorption in the collecting
ducts, is a peptide hormone secreted by the posterior pituitary
Basolateral
membrane
Sodium
channel
Potassium
channel
Potassium
channel
(Diffusion)
Luminal
membrane
Tight junction
Tubular
lumen
Cortical collecting
duct cells
Interstitial fluid
Basolateral
membrane
Luminal
membrane
Tight junction
Tubular
lumen
Proximal
tubule cells
Interstitial fluid
(a)
(b)
Cotransport
Countertransport
Na
+
K
+
K
+
K
+
Na
+
Na
+
ADP
ATP
Na
+
x
H
+
H
+
K
+
K
+
K
+
x
Na
+
Na
+
ADP
ATP
Figure 14–14
Mechanism of sodium reabsorption in the (a) proximal tubule and
(b) cortical collecting duct. Figure 14–15 shows the movement
of the reabsorbed sodium from the interstitial fl
uid into the
peritubular capillaries. The sizes of the letters denote high and
low concentrations. x represents organic molecules such as glucose
and amino acids that are cotransported with Na
+
. The fate of
the potassium ions that the Na
+
/K
+
-ATPase pumps transport is
discussed in the later section dealing with renal potassium handling.
Figure 14–14
physiological
inquiry
What would be the effect of a drug that blocks the sodium
channels in the cortical collecting duct?
Answer can be found at end of chapter.
Figure 14–15
Coupling of water and sodium reabsorption. See text for explanation
of circled numbers. The reabsorption of solutes other than sodium—
for example, glucose, amino acids, and bicarbonate—also contributes
to the difference in osmolarity between lumen and interstitial fl
uid,
but the reabsorption of all these substances ultimately depends on
direct or indirect cotransport and countertransport with sodium (see
Figure 14–14a). Therefore, they are not shown in this fi
gure.
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