■ FIGURE 5.2 Distribution of pressures and volumes in the systemic circulation. The greatest pressure drop
occurs across small arteries and arterioles; most of the blood volume is found w ithin the veins and venules.
Equation 5-1 is a hydrodynamic form of Ohm
law (AV = I . R), where the voltage difference
(AV; analogous to AP) is equal to the current
(I; analogous to F) times the resistance (R).
This equation generally applies in the body
when blood is flowing under nonturbulent,
laminar flow conditions.
The mean blood pressure does not fall
much as the blood flows down the aorta and
through large distributing arteries because
these vessels have a low resistance relative
to their flow, and therefore, little loss of pres-
sure energy (AP) occurs along their lengths.
In contrast, when the blood flows through
the small arteries and arterioles (the primary
resistance vessels), there is a large fall in mean
arterial blood pressure. The reason is that
these vessels, as a group, have a high resist-
ance relative to their flow, and therefore, AP
across this group of vessels is large. In fact,
approximately 50% to 70% of the pressure
drop within the vasculature occurs within the
resistance vessels. By the time blood reaches
the capillaries, the mean blood pressure may
be 25 to 30 mm Hg, depending on the organ. It
is important that the capillary pressure is rela-
tively low; otherwise, large amounts of fluid
would leak through the capillaries (and post-
capillary venules), causing tissue edema (see
Chapter 8). The pressure falls further as blood
travels through veins back to the heart; how-
ever, the pressure drop is small compared to
the pressure drop across the small arteries and
arterioles because the resistance of the veins
is very low compared to the arterial resistance
vessels. Pressure within the thoracic vena
cava near the right atrium is very close to zero
millimeters of mercury (mm Hg), although it
fluctuates by a few mm Hg during the cardiac
cycle and because of respiratory activity.
The greatest volume (60% to 80%) of blood
within the circulation resides within the venous
vasculature. This is why veins are referred to
as capacitance vessels. The relative volume of
blood between the arterial and venous sides of
the circulation can vary considerably depend-
ing on total blood volume, intravascular pres-
sures, and vascular compliance as described
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