The Kidneys and Regulation of Water and Inorganic Ions
501
evaporation from the skin by the production of sweat. Normal
gastrointestinal loss of water in feces is generally quite small,
but it can be severe in diarrhea. Gastrointestinal water loss can
also be signifi cant in vomiting.
Table 14–4
is a summary of total-body balance for
sodium chloride. The excretion of sodium and chloride
via the skin and gastrointestinal tract is normally small but
increases markedly during severe sweating, vomiting, or diar-
rhea. Hemorrhage can also result in the loss of large quanti-
ties of both salt and water.
Under normal conditions, as Tables 14–3 and 14–4
show, salt and water losses equal salt and water gains, and no
net change in body salt and water occurs. This matching of
losses and gains is primarily the result of the regulation of uri-
nary loss, which can be varied over an extremely wide range.
For example, urinary water excretion can vary from approxi-
mately 0.4 L/day to 25 L/day, depending upon whether one
is lost in the desert or participating in a beer-drinking contest.
Similarly, some individuals ingest 20 to 25 g of sodium chlo-
ride per day, whereas a person on a low-salt diet may ingest
only 0.05 g. Normal kidneys can readily alter the excretion of
salt over this range to balance loss with gain.
Basic Renal Processes for Sodium
and Water
Both sodium and water freely fi lter from the glomerular capil-
laries into Bowman’s space because they have low molecular
weights and circulate in the plasma in the free form (unbound
to protein). They both undergo considerable reabsorption—
normally more than 99 percent (see Table 14–2)—but no
secretion. Most renal energy utilization goes to accomplish
this enormous reabsorptive task. The bulk of sodium and
water reabsorption (about two-thirds) occurs in the proximal
tubule, but the major hormonal control of reabsorption is
exerted on the distal convoluted tubules and collecting ducts.
The mechanisms of sodium and water reabsorption can
be summarized in two generalizations: (1) sodium reabsorp-
tion is an active process occurring in all tubular segments
except the descending limb of the loop of Henle; and (2) water
reabsorption is by diffusion and is dependent upon sodium
reabsorption.
Primary Active Sodium Reabsorption
The essential feature underlying sodium reabsorption through-
out the tubule is the primary active transport of sodium out of
the cells and into the interstitial fl
uid, as illustrated for the prox-
imal tubule and cortical collecting duct in
Figure 14–14
. This
transport is achieved by Na
+
/K
+
-ATPase pumps in the basolat-
eral membrane of the cells. The active transport of sodium out
of the cell keeps the intracellular concentration of sodium low
compared to the tubular lumen, so sodium moves “downhill”
out of the lumen into the tubular epithelial cells.
The mechanism of the downhill sodium movement across
the luminal membrane into the cell varies from segment to
segment of the tubule depending on which channels and/or
transport proteins are present in their luminal membranes.
For example, the luminal entry step in the proximal tubule cell
occurs by cotransport with a variety of organic molecules such
as glucose, or by countertransport with hydrogen ions. In the
latter case, hydrogen ions move out of the cell to the lumen
as sodium moves into the cell (
Figure 14–14a
). Thus, in the
proximal tubule, sodium reabsorption drives the reabsorption
of the cotransported substances and secretion of hydrogen
ions. The luminal entry step for sodium in the cortical collect-
ing duct occurs primarily by diffusion through sodium chan-
nels (
Figure 14–14b
).
The movement of sodium downhill from lumen into cell
across the
luminal membrane
varies from one segment of the
tubule to another. By contrast, the
basolateral membrane
step
is the same in all sodium-reabsorbing tubular segments—the
primary active transport of sodium out of the cell is via Na
+
/
K
+
-ATPase pumps in this membrane. It is this transport pro-
cess that lowers intracellular sodium concentration and so
makes possible the downhill luminal entry step.
Coupling of Water Reabsorption to Sodium
Reabsorption
As sodium and chloride are reabsorbed, water follows passively by
osmosis (Chapter 4).
Figure 14–15
summarizes this coupling of
solute and water reabsorption. (1) Sodium is transported from
Table 14–3
Average Daily Water Gain and Loss
in Adults
Intake
In liquids
1200 ml
In food
1000 ml
Metabolically produced
350 ml
Total
2550 ml
Output
Insensible loss (skin and lungs)
900 ml
Sweat
50 ml
In feces
100 ml
Urine
1500 ml
Total
2550 ml
Table 14–4
Daily Sodium Chloride Intake and Loss
Intake
Food
10.50 g
Output
Sweat
0.25 g
Feces
0.25 g
Urine
10.00 g
Total
10.50 g
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