188
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
fluid exchange. W hen examining fluid flux
across capillaries within an organ, filtration
constant (KF) and surface area (A) are sub-
stituted for hydraulic conductivity of a sin-
gle capillary. With these substitutions, a new
expression relating net fluid flux is obtained
(Equation 8-2):
Eq. 8-2
J = KF
A (NDF)
Equation 8-2 and Figure 8.6 show that net
fluid movement (net fluid flux, J) is directly
related to the filtration constant (KF), the sur-
face area available for fluid exchange (A), and
the net driving force (NDF).
At a given NDF
(assuming that the NDF is not equal to zero),
the amount of fluid filtered or reabsorbed per
unit time is determined by the filtration con-
stant and surface area available for exchange.
The filtration constant is determined by the
physical properties of the barrier (i.e., size
and number of “pores” and the thickness of
the capillary barrier), and therefore, it rep-
resents the permeability of the capillaries to
fluid.
Fenestrated capillaries, for example,
have a much higher KF (i.e., permeability)
than
continuous
capillaries.
Furthermore,
paracrine
substances
such
as
histamine,
bradykinin, and leukotrienes can significantly
increase Kp The surface area (A) is primarily
related to the length, diameter, and number
of vessels (capillaries and postcapillary ven-
ules) available for exchange. The surface area
is dynamic in vascular beds such as skeletal
muscle. In that tissue, the number of perfused
capillaries
can increase severalfold
during
exercise. In experimental studies using whole
organs, KF and A, which cannot be indepen-
dently measured, are combined and called the
capillary filtration coefficient (CFC).
The direction of fluid movement (filtration
or reabsorption) in Equation 8-2 depends on
whether the NDF is positive (filtration) or
negative (reabsorption). If the NDF is zero,
no net fluid movement occurs even if K
and
F
A are very large.
As already mentioned, the NDF is deter-
mined by hydrostatic and
oncotic forces.
Two hydrostatic and two oncotic pressures
affect transcapillary fluid exchange: capillary
f
N D F
j
K F . A
J = K F . A (N D F )
■ FIGURE 8.6 Factors determ ining fluid movement. The rate of fluid movement (flux,
J)
across the capil-
lary endothelium, designated as water molecules in this figure, is determ ined by the net driving force
(NDF),
the capillary filtration constant
(KF),
and the capillary surface area (A) available for exchange.
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