The Kidneys and Regulation of Water and Inorganic Ions
493
protein causes the water concentration of the plasma to be
slightly less than that of the fl
uid in Bowman’s space, and this
difference in water concentration favors fl uid movement by bulk
fl ow from Bowman’s space into the glomerular capillaries—that
is, it opposes glomerular fi ltration.
Note that in Figure 14–8, the value given for this
osmotic force—29 mmHg—is slightly larger than the value—
28 mmHg—for the osmotic force given in Chapter 12 for
plasma in all arteries and nonrenal capillaries. The reason is
that, unlike the situation elsewhere in the body, enough water
fi lters out of the glomerular capillaries that the protein left
behind in the plasma becomes more concentrated than in arte-
rial plasma. In other capillaries, in contrast, little water fi lters
out and the capillary protein concentration remains essentially
unchanged from its value in arterial plasma. In other words,
unlike the situation in other capillaries, the plasma protein
concentration and, thus, the osmotic force, increases from the
beginning to the end of the glomerular capillaries. The value
given in Figure 14–8 for the osmotic force is the average value
along the length of the capillaries.
To summarize, the
net glomerular fi ltration pressure
is the sum of three relevant forces:
Net glomerular fi ltration pressure =
P
GC
P
BS
π
GC
Normally the net fi ltration pressure is always positive
because the glomerular capillary hydrostatic pressure (
P
GC
) is
larger than the sum of the hydrostatic pressure in Bowman’s
space (
P
BS
) and the osmotic force opposing fi ltration (
π
GC
).
The net glomerular fi ltration pressure initiates urine forma-
tion by forcing an essentially protein-free fi ltrate of plasma out
of the glomerulus and into Bowman’s space and then down
the tubule into the renal pelvis.
Rate of Glomerular Filtration
The volume of fl uid fi ltered from the glomeruli into Bowman’s
space per unit time is known as the
glomerular fi ltration
rate (GFR).
GFR is determined not only by the net fi ltra-
tion pressure but also by the permeability of the corpuscu-
lar membranes and the surface area available for fi ltration. In
other words, at any given net fi ltration pressure, the GFR will
be directly proportional to the membrane permeability and
the surface area. The glomerular capillaries are much more
permeable to fl
uid than most other capillaries. Therefore, the
net glomerular fi ltration pressure causes massive fi ltration of
fl uid into Bowman’s space. In a 70-kg person, the GFR aver-
ages 180 L/day (125 ml/min)! This is much higher than the
combined net fi ltration of 4 L/day of fl uid across all the other
capillaries in the body, as described in Chapter 12.
When we recall that the total volume of plasma in the
cardiovascular system is approximately 3 L, it follows that the
kidneys fi lter the entire plasma volume about 60 times a day.
This opportunity to process such huge volumes of plasma
enables the kidneys to regulate the constituents of the internal
environment rapidly and to excrete large quantities of waste
products.
GFR is not a fi xed value but is subject to physiological
regulation. This is achieved mainly by neural and hormonal
input to the afferent and efferent arterioles, which causes
changes in net glomerular fi ltration pressure (
Figure 14–9
).
The glomerular capillaries are unique in that they are situ-
ated between two sets of arterioles—the afferent and effer-
ent arterioles. Constriction of the afferent arterioles decreases
hydrostatic pressure in the glomerular capillaries (
P
GC
). This
is similar to arteriolar constriction in other organs and is due
to a greater loss of pressure between arteries and capillaries
(
Figure 14–9a
).
In contrast, efferent arteriolar constriction alone has the
opposite effect on
P
GC
in that it
increases
it (
Figure 14–9b
).
This occurs because the efferent arteriole lies beyond the
g
lome
ru
lu
s
,
so
tha
t
e
f
fe
ren
t
a
r
te
r
io
la
r
con
s
t
r
ic
t
ion
tend
s
to “dam back” the blood in the glomerular capillaries, rais-
ing
P
GC
. Dilation of the efferent arteriole (
Figure 14–9c
)
Figure 14–8
Forces involved in glomerular fi ltration. The symbol
π
denotes the
osmotic force due to the presence of protein in glomerular capillary
plasma.
*The concentration of protein in Bowman’s space is so low that
π
BS
, a force that would favor
fi ltration, is considered zero.
Figure 14–8
physiological
inquiry
What would be the effect of an increase in plasma albumin (the
most abundant plasma protein) on glomerular fi ltration rate
(GFR)?
Answer can be found at end of chapter.
P
GC
Glomerular
capillary
P
BS
π
GC
Bowman’s
space
Forces
60
15
29
16
mmHg
Favoring filtration*:
Glomerular capillary blood pressure (
P
GC
)
Opposing filtration:
Fluid pressure in Bowman’s space (
P
BS
)
Osmotic force due to protein in plasma (
π
GC
)
Net glomerular filtration pressure
=
P
GC
P
BS
π
GC
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