The
Microcirculation
207
Pressure gradient in the Poiseuille's law is simply APM Poiseuille
resistance to steady flow is therefore:
8771
R,
=-
nr4
(7.3.3)
Steady flow was assumed because of the belief that small peripheral
vessels are resistance vessels, preventing pulsations occurring.
As
mentioned above, the largest mean pressure drop occurs in small
arterioles. Referring back to Chapter
2
regarding the structure of the
vascular walls, we see that this is also where smooth muscle tends to
exert its influence. Thus, accompanying the smooth muscle (Somlyo and
Somlyo, 1968) activation, is a change in vessel lumen radius. Since flow
varies by the fourth power of radius,
a
small change in radius can amount
to a large alteration in flow.
Thus, the peripheral resistance can alter
central arterial flow and hence, cardiac output.
It is now known that pulsatile ejection by the ventricle requires only
about 10% additional energy for the same stroke volume compared to
constant outflow. This minimal additional energy associated with
pulsatile ventricular ejection indicates the compliant properties of the
receiving arterial tree.
An appreciable fraction of the energy in the pressure and flow pulses
generated by the heart reaches the capillaries in pulsatile
form.
This has
been demonstrated experimentally by, for instance, Wiederhelm et al.
(1964), in frog's mesentery, Intaglietta et al. (1970,
1971) in cat
omentum, Zweifach (1974), Zweifach and Lipowsky (1977) and Smaje
et al.
(1
980). It has been postulated that pulsations are necessary to attain
optimal organ function. Steady perfusion could impair organ function
(Wilkins et al., 1967; Jacobs et al., 1969; Arnzelius, 1976).
Direct
recording
of
pressure obtained by Zweifach (1974) suggests that in the
terminal arteriole, the pulse pressure is still large, about
15
mmHg, with
a mean pressure of about 60 mmHg. Intaglietta et al. (1970) provided
pulsatile velocity data. Mean velocities in the microcirculation are in the
centimeters per second range, as measured by electro-optical methods.
