494
Chapter 14
decreases
P
GC
and thus GFR, whereas dilation of the afferent
arteriole increases
P
GC
and thus GFR (
Figure 14–9d
). Finally,
simultaneous constriction or dilation of both sets of arterioles
tends to leave
P
GC
unchanged because of the opposing effects.
In addition to the neuroendocrine input to the arteri-
oles, there is also neural and humoral input to the mesangial
cells that surround the glomerular capillaries. Contraction of
these cells reduces the surface area of the capillaries, which
causes a decrease in GFR at any given net fi ltration pressure.
It is possible to measure the total amount of any nonpro-
tein or nonprotein-bound substance fi ltered into Bowman’s
space by multiplying the GFR by the plasma concentration of
the substance. This amount is called the
ltered load
of the
substance. For example, if the GFR is 180 L/day and plasma
glucose concentration is 1 g/L, then the fi ltered load of glu-
cose is 180 L/day
×
1 g/L = 180 g/day.
Once the fi ltered load of the substance is known, it
can be compared to the amount of the substance excreted.
This indicates whether the substance undergoes net tubular
reabsorption or net secretion. Whenever the quantity of a
substance excreted in the urine is less than the fi ltered load,
tubular reabsorption must have occurred. Conversely, if the
amount excreted in the urine is greater than the fi ltered load,
tubular secretion must have occurred.
Tubular Reabsorption
Table 14–2
summarizes data for a few plasma components
that undergo fi ltration and reabsorption. It gives an idea of the
magnitude and importance of reabsorptive mechanisms. The
values in this table are typical for a healthy person on an aver-
age diet. There are at least three important conclusions we can
draw from this table: (1) The fi ltered loads are enormous, gen-
erally larger than the amounts of the substances in the body.
For example, the body contains about 40 L of water, but the
volume of water fi ltered each day is 180 L. (2) Reabsorption of
waste products is relatively incomplete (as in the case of urea),
so that large fractions of their fi ltered loads are excreted in the
urine. (3) Reabsorption of most useful plasma components,
such as water, inorganic ions, and organic nutrients, is rela-
tively complete so that the amounts excreted in the urine are
very small fractions of their fi ltered loads.
An important distinction should be made between reab-
sorptive processes that can be controlled physiologically and
those that cannot. The reabsorption rates of most organic
nutrients, such as glucose, are always very high and are not
physiologically regulated. Thus, the fi ltered loads of these
substances are normally completely reabsorbed, with none
appearing in the urine. For these substances, like substance Z
in Figure 14–7, it is as though the kidneys do not exist because
Dilate
AA
Blood
flow
P
GC
(a)
Dilate
EA
Constrict EA
Constrict AA
Blood
flow
(b)
(c)
(d)
P
GC
GFR
GFR
P
GC
P
GC
GFR
GFR
Blood
flow
Blood
flow
Increased GFR
Decreased GFR
Figure 14–9
Control of GFR by constriction or dilation of afferent (AA) or efferent (EA) arterioles. Constriction of the afferent arteriole (a) or dilation of
the efferent arteriole (c) reduces
P
GC
, thus decreasing GFR. Constriction of the efferent arteriole (b) or dilation of the afferent arteriole (d)
increases
P
GC
, thus increasing GFR.
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