The Kidneys and Regulation of Water and Inorganic Ions
495
the kidneys do not eliminate these substances from the body
at all. Therefore, the kidneys do not regulate the plasma con-
centrations of these organic nutrients. Rather, the kidneys
merely maintain whatever plasma concentrations already exist.
Recall that a major function of the kidneys is to elimi-
nate soluble waste products. To do this, the blood is fi ltered in
the glomeruli. One consequence of this is that substances nec-
essary for normal body functions are fi ltered from the plasma
into the tubular fl uid. To prevent the loss of these important
nonwaste products, the kidney has powerful mechanisms to
reclaim useful substances from tubular fl uid while allowing
waste products to be excreted.
The reabsorptive rates for water and many ions, although
also very high, are under physiological control. For example, if
water intake is decreased, the kidney can increase water reab-
sorption to minimize water loss.
In contrast to glomerular fi ltration, the crucial steps in
tubular reabsorption—those that achieve movement of a sub-
stance from tubular lumen to interstitial fl
uid—do
not
occur
by bulk fl ow because there are inadequate pressure differences
across the tubule and inadequate permeability of the tubular
membranes. Instead, two other processes are involved: (1) The
reabsorption of some substances from the tubular lumen is
by diffusion, often across the tight junctions connecting the
tubular epithelial cells (
Figure 14–10
). (2) The reabsorption
of all other substances involves mediated transport, which
requires the participation of transport proteins in the plasma
membranes of tubular cells.
The fi
nal step in reabsorption is the movement of sub-
stances from the interstitial fl uid into peritubular capillaries
that occurs by a combination of diffusion and bulk fl ow. We
will assume that this fi nal process occurs automatically once
the substance reaches the interstitial fl
uid.
Reabsorption by Diffusion
The reabsorption of urea by the proximal tubule provides an
example of passive reabsorption by diffusion. An analysis of
urea concentrations in the proximal tubule will help clarify
the mechanism. Because the corpuscular membranes are freely
fi lterable to urea, the urea concentration in the fl uid within
Bowman’s space is the same as that in the peritubular capillary
plasma and the interstitial fl uid surrounding the tubule. Then,
as the fi ltered fl uid fl ows through the proximal tubule, water
reabsorption occurs (by mechanisms to be described later).
This removal of water increases the concentration of urea in
the tubular fl
uid so it is higher than in the interstitial fl
uid
and peritubular capillaries. Therefore, urea diffuses down this
concentration gradient from tubular lumen to peritubular cap-
illary. Urea reabsorption is thus dependent upon the reabsorp-
tion of water. Reabsorption by diffusion in this manner occurs
for a variety of lipid-soluble organic substances, both naturally
occurring and foreign (e.g., the pesticide DDT).
Reabsorption by Mediated Transport
Figure 14–10 demonstrates that a substance reabsorbed by
mediated transport must fi rst cross the
luminal membrane
separating the tubular lumen from the cell interior. Then, the
substance diffuses through the cytosol of the cell, and fi nally
crosses the
basolateral membrane,
which begins at the tight
junctions and constitutes the plasma membrane of the sides
and base of the cell. This route is termed
transcellular
epithe-
lial transport.
A substance need not be actively transported across
both
the luminal and basolateral membranes in order to be actively
transported across the overall epithelium, thus moving from
lumen to interstitial fl uid against its electrochemical gradi-
ent. For example, sodium moves “downhill” (passively) into
the cell across the luminal membrane either by diffusion or by
facilitated diffusion and then is actively transported “uphill”
Table 14–2
Average Values for Several Components
that Undergo Filtration and
Reabsorption
Substance
Amount
Filtered Per
Day
Amount
Excreted
Per Day
Percent
Reabsorbed
Water, L
180
1.8
99
Sodium, g
630
3.2
99.5
Glucose, g
180
0
100
Urea, g
54
30
44
Basolateral
membrane
Peritubular
capillary
Tubular
lumen
Tight junction
Luminal
membrane
Interstitial
fluid
Tubular
epithelial cell
Figure 14–10
Diagrammatic representation of tubular epithelium.
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