190
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
(e.g., right ventricular failure, cirrhosis of the
liver, venous thrombosis) can lead to edema in
peripheral organs and tissues by significantly
increasing capillary hydrostatic pressure and
capillary fluid filtration. This relationship also
shows that arteriolar vasodilation or venous
constriction increases
capillary hydrostatic
pressure, which increases filtration.
TISSUE (INTERSTITIAL) HYDROSTATIC
PRESSURE
Tissue (interstitial) hydrostatic pressure (P.)
is the pressure within the tissue interstitium
that is exerted against the outside wall of the
capillary and therefore opposes the capillary
hydrostatic pressure. In many tissues under
normal states of hydration, tissue hydro-
static pressure is subatmospheric by a few
millimeters of mercury (mm Hg), whereas
in others it is slightly positive by a few mm
Hg. Increased tissue fluid volum e, as occurs
during states of enhanced capillary fluid
filtration or lymphatic blockage, increases
tissue
hydrostatic
pressure.
In
contrast,
dehydration reduces tissue fluid volume and
hydrostatic pressure.
The effect of changes in interstitial fluid
volume on interstitial pressure is determined
by interstitial compliance (C). This is defined
as the change in interstitial fluid volume (AV)
divided by the change in interstitial fluid pres-
sure (AP.). Rearranging this relationship gives
the following:
Therefore, an increase in interstitial fluid vol-
ume increases interstitial fluid pressure, and
the magnitude of the change varies inversely
with the compliance of the interstitium.
Figure 8.8 is a graphical representation
of the relationship between interstitial fluid
volume and pressure, and interstitial compli-
ance. The slope of the relationship between
interstitial volume and pressure is intersti-
tial compliance. Note that the compliance
decreases at higher interstitial volumes, which
causes the pressure to increase disproportion-
ately as volume increases. Some tissues and
■ FIGURE 8.8 Effects of interstitial compliance on
interstitial fluid volumes and pressures. Com pli-
ance (C) is the change in interstitial volume
(AV)
divided by the change in interstitial pressure (AP;),
which is the slope of the relationship between
volume and pressure. Low interstitial compliance
(e.g., brain tissue) causes large increases in inter-
stitial fluid pressure when interstitial fluid volume
increases, which can occur during cerebral edema
or hemorrhage w ithin the brain. In contrast, tissue
w ith high interstitial compliance (e.g., subcutane-
ous tissues), show relatively small increases in
interstitial pressure as interstitial volume increases.
organs, such as the brain and kidney, have a
low interstitial compliance. The reason is that
the tissue is surrounded by a rigid boney skull
or capsule, respectively. Therefore, relatively
small
increases
in
interstitial
volume
can
lead to large increases in interstitial pressure.
A large increase in pressure can be very dam-
aging to the tissues and lead to cellular dys-
function and death. In contrast, subcutaneous
tissues have a relatively high interstitial com-
pliance so that large increases in intersti-
tial volume can occur with relatively small
increases in interstitial pressure. Despite a rel-
atively high compliance at low interstitial fluid
volumes, subcutaneous interstitial pressures
can still increase to high values at very high
interstitial volumes during severe limb edema.
CAPILLARY PLASMA ONCOTIC PRESSURE
Capillary plasma oncotic pressure (nc) is the
osmotic pressure within the capillary that
is determined by the presence of proteins.
Because this is an osmotic force within the
plasma, it opposes filtration and promotes
reabsorption. Because the capillary barrier is
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