388
Chapter 12
It should be clear from this description that the sounds
heard during measurement of blood pressure are not the same
as the heart sounds described earlier, which are due to closing
of cardiac valves.
Arterioles
The arterioles play two major roles: (1) the arterioles in individ-
ual organs are responsible for determining the relative blood
fl ows to those organs at any given mean arterial pressure, and
(2) the arterioles, all together, are a major factor in determin-
ing mean arterial pressure itself. The fi rst function will be
described in this section, and the second in Section D.
Figure 12–33
illustrates the major principles of blood-
fl ow distribution in terms of a simple model, a fl
uid-fi lled tank
with a series of compressible outfl
ow tubes. What determines
the rate of fl ow through each exit tube? As stated in Section A
of this chapter,
F
=
P/R
Because the driving pressure (the height of the fl
uid column
in the tank) is identical for each tube, differences in fl ow are
completely determined by differences in the resistance to fl ow
offered by each tube. The lengths of the tubes are approxi-
mately the same, and the viscosity of the fl uid is constant; there-
fore, differences in resistance are due solely to differences in the
radii of the tubes. Obviously, the widest tubes have the greatest
ows. If the radius of each outfl ow tube can be independently
altered, we can obtain various combinations of fl ows.
This analysis can now be applied to the cardiovascular
system. The tank is analogous to the major arteries, which
serve as a pressure reservoir, but are so large that they contrib-
ute little resistance to fl ow. Therefore, all the large arteries of
the body can be considered a single pressure reservoir.
The arteries branch within each organ into progres-
sively smaller arteries, which then branch into arterioles. The
smallest arteries are narrow enough to offer signifi cant resis-
tance to fl ow, but the still narrower arterioles are the major
sites of resistance in the vascular tree and are therefore anal-
ogous to the outfl
ow tubes in the model. This explains the
large decrease in mean pressure—from about 90 mmHg to
35 mmHg—as blood fl ows through the arterioles (see Figure
12–29). Pulse pressure also diminishes to the point that fl ow
beyond the arterioles—that is, through capillaries, venules,
and veins—is much less pulsatile.
Like the model’s outfl ow tubes (see Figure 12–33), the
arteriolar radii in individual organs are subject to independent
adjustment. The blood fl ow (
F
) through any organ is repre-
sented by the following equation:
F
organ
= (MAP – venous pressure)/Resistance
organ
Venous pressure is normally close to zero, so we may write:
F
organ
= MAP/Resistance
organ
Pressure reservoir
(“arteries”)
Variable-resistance
outflow tubes
(“arterioles”)
Flow to “organs”
1, 2, 3, 4, and 5
123
45
P
123
45
P
(a)
(b)
Figure 12–33
Physical model of the relationship between arterial pressure, arteriolar radius in different organs, and blood-fl ow distribution. In (a), blood fl ow
is high through tube 2 and low through tube 3, whereas just the opposite is true for (b). This shift in blood fl ow was achieved by constricting
tube 2 and dilating tube 3.
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