CHAPTER 8 • EXCHANGE FUNCTION OF THE MICROCIRCULATION
187
Filtration = Reabsorption + Lymph Flow
■ FIGURE 8.5 Capillary filtration, reabsorption, and lym ph flow. Fluid filters out of the arteriolar end of
the capillary and into the interstitium . Most of this fluid is reabsorbed at the venular end of the capillary,
w ith the rest of the fluid entering terminal lym phatics to be carried away from the tissue and eventually
returned to the blood. Fluid exchange is in balance (i.e., at a steady state) when filtration equals reabsorp-
tion plus lymph flow.
the increased pressure stretches the vessel and
induces a myogenic contraction. Sympathetic
nerves can modulate this vasomotion. Lym-
phatic vessels contain one-way valves that
direct lymph away from the tissue and eventu-
ally back into the systemic circulation via the
thoracic duct and subclavian veins. Approxi-
mately 2 to 4 L/d of lymph are returned to the
circulation by this manner.
In the steady state, the rate of fluid enter-
ing the tissue interstitium by filtration is the
same as that of the fluid leaving the tissue by
capillary reabsorption and lymph flow. That
is, filtration equals reabsorption plus lymph
flow. W hen this balance is altered, the volume
and pressure of fluid within the interstitium
change. For example, if net filtration tran-
siently increases and lymph flow does not
increase to the same extent, interstitial volume
and pressure will increase, causing edema.
Factors that cause edema are discussed in the
last section of this chapter.
Physical Mechanisms Governing
Fluid Exchange
The movement of fluid across a capillary is
determined by several physical factors: the
hydrostatic pressure, oncotic pressure, and
physical nature of the barrier (i.e., the per-
meability of the capillary wall) separating the
fluid in the blood from the fluid within the
interstitium. As described earlier, the tran-
scapillary movement of fluid can be described
by
Poiseuille
equation
for
hydrodynamic
flow (see Equation 5-7), or in more simpli-
fied terms, it can be described by the general
hydrodynamic equation (Equation 5-5) that
relates flow (F), driving pressure (AP), and
resistance (R) (i.e., F = AP/R). In single cap-
illaries, a more common way to express this
hydrodynamic
equation
for
transcapillary
fluid movement (fluid flux, J) is to substitute
hydraulic conductivity (Lp) for resistance,
which are reciprocally related. Hydraulic con-
ductivity is related to the ease by which fluid
passes across the capillary wall. Fluid flux is
the number of molecules of water (or volume)
per unit time that moves across the exchange
barrier; therefore, fluid flux can be expressed
in similar units as flow. For a single capil-
lary, fluid flux equals the product of capillary
hydraulic conductivity and the net driving
force (i.e., J = Lp ■
NDF). The NDF combines
both those hydrostatic and oncotic pressures
that drive fluid movement across the capillary
wall.
In an organ, fluid is moving across many
capillaries, and therefore the net fluid flux is
related not only to the hydraulic conductiv-
ity of single capillaries and to the net driving
force, but also to the surface area available for
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