Page 780 - Fundamentals of anatomy physiology

Chapter 21 Blood Vessels and Circulation 767
to tissue—from 16 mm Hg in the brain to 26 mm Hg in
subcutaneous tissues. A positive IHP opposes CHP, and the
tissue hydrostatic pressure must be overcome before fluid
can move out of a capillary. A negative IHP assists CHP,
and additional fluid will be pulled out of the capillary.
However, under normal circumstances the average IHP is
0 mm Hg, and we can assume that the net hydrostatic pres-
sure is equal to CHP. (For this reason, IHP is not included
in Figure 21–11.)
Plasma proteins in capillary blood create capillary colloid
osmotic pressure. The net capillary colloid osmotic pressure tends
to pull water and solutes into a capillary from the interstitial
fluid. The net colloid osmotic pressure is the difference between
1. the blood colloid osmotic pressure (BCOP), which is about
25 mm Hg, and
2. the interstitial fluid colloid osmotic pressure (ICOP). The ICOP
is as variable and low as the IHP, because the interstitial
fluid in most tissues contains negligible quantities of sus-
pended proteins. Reported values of ICOP are from 0 to
5 mm Hg, within the range of pressures recorded for the
IHP. It is thus safe to assume that under normal circum-
stances the net colloid osmotic pressure is equal to the BCOP.
(For this reason, ICOP is not included in Figure 21–11.)
The net filtration pressure (NFP) is the difference be-
tween the net hydrostatic pressure and the net osmotic pres-
sure. In terms of the factors just listed, this means that
net filtration = net hydrostatic − net colloid
pressure pressure osmotic pressure
NFP = (CHP − IHP) − (BCOP − ICOP)
At the arterial end of a capillary, the net filtration pressure
can be calculated as follows:
NFP = (35 − 0) − (25 − 0) = 35 − 25 = 10 mm Hg
Because this value is positive, it indicates that fluid will
tend to move out of the capillary and into the interstitial fluid.
At the venous end of the capillary, the net filtration pressure
will be
NFP = (18 − 0) − (25 − 0) = 18 − 25 = −7 mm Hg
The minus sign indicates that fluid tends to move into the capil-
lary; that is, reabsorption is occurring.
The transition between filtration and reabsorption occurs
where the CHP is 25 mm Hg, because at that point the hydro-
static and osmotic forces are equal—that is, the NFP is 0 mm Hg.
If the maximum filtration pressure at the arterial end of the
capillary were equal to the maximum reabsorption pressure at
the venous end, this transition point would lie midway along
the length of the capillary. Under these circumstances, filtration
would occur along the first half of the capillary, and an identical
Clinical Note
Edema Edema (e-DE. -muh) is an abnormal accumulation
of interstitial fluid. Edema has many causes, and we will
encounter specific examples in later chapters. The underly-
ing problem in all types of edema is a disturbance in the
normal balance between hydrostatic and osmotic forces at
the capillary level. For instance:
You usually have swelling at a bruise. When a capillary
is damaged, plasma proteins can cross the capillary wall
and enter the interstitial fluid. The resulting rise in the
interstitial fluid colloid osmotic pressure (ICOP) reduces
the rate of capillary reabsorption and produces a local-
ized edema.
In starvation, the liver cannot synthesize enough plasma
proteins to maintain normal concentrations in the blood.
Blood colloid osmotic pressure (BCOP) declines. Fluids
then begin moving from the blood into peripheral tis-
sues. In children, fluid builds up in the abdominopelvic
cavity, producing the swollen bellies typical of starvation
victims. A reduction in BCOP also takes place after severe
burns and in several types of liver and kidney diseases.
In the U.S. population, most serious cases of edema
result from increases in arterial blood pressure, venous
pressure, or total circulatory pressure. The increase
may be due to heart problems such as heart failure,
venous blood clots that elevate venous pressures, or 21
other c­ ardiovascular abnormalities. The net result is
an increase in capillary hydrostatic pressure (CHP) that
a­ ccelerates fluid movement into the tissues.
amount of reabsorption would occur along the second half.
However, the maximum filtration pressure is higher than the
maximum reabsorption pressure, so the transition point be-
tween filtration and reabsorption normally lies closer to the
venous end of the capillary than to the arterial end. As a result,
more filtration than reabsorption occurs along the capillary. Of
the nearly 24 liters of fluid that move out of the plasma and
into the interstitial fluid each day, 20.4 liters (85 percent) are
reabsorbed. The remainder (3.6 liters) flows through the tissues
and into lymphatic vessels, for eventual return to the venous
system (Figure 21–11).
Any condition that affects hydrostatic or osmotic pres-
sures in the blood or tissues will shift the balance between
hydrostatic and osmotic forces. We can then predict the effects
on the basis of an understanding of capillary dynamics. For
example,
If hemorrhaging occurs, both blood volume and blood
pressure decrease. This reduction in CHP lowers the NFP