398
Chapter 12
the osmotic force due to interstitial fl uid protein concentration,
π
IF
(favoring fl uid movement out of the capillary). Thus:
NFP =
P
c
+
π
IF
P
IF
π
c
Note that we have arbitrarily assigned a positive value to the
forces directed out of the capillary and negative values to the
inward-directed forces. The four factors that determine net fi ltra-
tion pressure are termed the
Starling forces
(because Starling,
the same physiologist who helped elucidate the Frank-Starling
mechanism of the heart, was the fi rst to develop the ideas).
We may now consider this movement quantitatively in
the systemic circulation (
Figure 12–42b
). Much of the arte-
rial blood pressure has already dissipated as the blood fl ows
through the arterioles, so that hydrostatic pressure tending to
push fl uid out of the arterial end of a typical capillary is only
about 35 mmHg. The interstitial fl uid protein concentration
at this end of the capillary would produce a fl ow of fl uid out of
the capillary equivalent to a hydrostatic pressure of 3 mmHg.
Because the interstitial fl
uid hydrostatic pressure is virtually
zero, the only inward-directed pressure at this end of the capil-
lary is the osmotic pressure due to plasma proteins, with a value
of 28 mmHg. Thus, at the arterial end of the capillary, the net
outward pressure exceeds the inward pressure by 10 mmHg,
so bulk fi ltration of fl
uid will occur.
The only substantial difference in the Starling forces at
the venous end of the capillary is that the hydrostatic blood
pressure (
P
c
) has decreased from 35 to approximately 15
mmHg due to resistance as blood fl owed along the capillary
wall. The other three forces are essentially the same as at the
arterial end, so the net inward pressure is 10 mmHg greater
than the outward pressure, and bulk absorption of fl
uid will
occur. Thus, net movement of fl uid from the plasma into the
interstitial space at the arterial end of capillaries tends to be
balanced by fl uid fl ow in the opposite direction at the venous
end of the capillaries. For the aggregate of capillaries in the
body, however, there is a small net fi ltration amounting to
approximately 4 L/day (this number does not include the cap-
illaries in the kidneys). The fate of this fl uid will be described
in the section on the lymphatic system.
In our example, we have assumed a typical capillary
hydrostatic pressure varying from 35 down to 15 mmHg. In
p
Capillary
hydrostatic
pressure
(
P
C
)
Osmotic force
due to plasma
protein concentration
(
P
IF
)
Interstitial fluid
hydrostatic
pressure
(
IF
)
Osmotic force
due to interstitial fluid
protein concentration
Venous end of capillary
Net filtration pressure =
15 + 3 – 0 – 28 = –10 mmHg
10 mmHg favoring absorption
Net filtration pressure =
35 + 3 – 0 – 28 = 10 mmHg
10 mmHg favoring filtration
P
IF
= 0
P
C
= 35
= 3
= 28
P
C
= 15
P
IF
= 0
(a)
(b)
Arterial end of capillary
π
IF
π
= 3
IF
π
(
C
)
π
C
π
= 28
C
π
Net filtration pressure =
P
C
+
π
IF
P
IF
π
C
Figure 12–42
(a) The four factors determining fl uid movement across capillaries. (b) Quantitation of forces causing fi ltration at the arterial end of the
capillary and absorption at the venous end. Outward forces are arbitrarily assigned positive values, so a positive net fi ltration pressure favors
fi ltration, while a negative pressure indicates net absorption of fl uid will occur. Arrows in (b) denote magnitude of forces. No arrow is shown
for interstitial fl uid hydrostatic pressure (
P
IF
) in (b) because it is approximately zero.
Figure 12–42
physiological
inquiry
If an accident victim loses 1 L of blood, why would an intravenous injection of a liter of plasma be more effective for replacing the lost
volume than injecting a liter of an equally concentrated crystalloid solution?
Answer can be found at end of chapter.
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