100
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
increase.
If,
however,
cardiac
output
is
not changed when stroke volume changes
(e.g., if a decrease in heart rate accompa-
nies the increase in stroke volume), then
only
the
pulse
pressure
changes— the
mean
aortic
pressure
does
not
change
(Fig. 5.5, panel C).
No
single
value
for
aortic compliance
exists because the relationship between vol-
ume and pressure (compliance curve; red
line in Fig. 5.5) is not linear. At higher aortic
volumes and pressures, the slope of the rela-
tionship decreases and compliance decreases
(see Fig. 5.5, panel D). Therefore, at elevated
mean arterial pressures, the reduced compli-
ance results in an increase in pulse pressure at
a given stroke volume.
In summary, aortic pulse pressure is deter-
mined by ventricular
stroke
volume
and
aortic compliance (Fig. 5.6). Any factor that
t Preload
\
Afterload
t Inotropy
| Heart Rate
\
^
Stroke
I
Volume
1
Aortic Pulse
I
Pressure
t
Aortic
Compliance
t
Age
Arteriosclerosis
Hypertension
■ FIGURE 5.6 Factors affecting aortic pulse pres-
sure. Pulse pressure is increased by those factors
that increase stroke volume or decrease aortic
compliance.
changes
stroke
volume
(e.g.,
ventricular
preload, afterload, and inotropy; heart rate)
or aortic compliance (e.g., age, arteriosclero-
sis, hypertension) alters aortic pulse pressure.
Beat-to-beat changes in pulse pressure occur
owing to changes in stroke volume. In con-
trast, chronic, long-term increases in pulse
pressure
are
commonly due to
decreased
aortic compliance.
HEMODYNAMICS (PRESSURE,
FLOW, AND RESISTANCE)
The term hemodynamics describes the physi-
cal factors governing blood flow within the
circulatory system. Blood flow through an
organ is determined by the pressure gradient
(AP) driving the flow divided by the resist-
ance (R) to flow (Equation 5-5), which is a
rearrangement of Equation 5-1. The pres-
sure gradient (or perfusion pressure) driving
flow through an organ is the arterial minus
the venous pressure. For an individual blood
vessel, the pressure gradient is the pressure
difference between two defined points along
the vessel.
Eq. 5-5
F = AP
R
Blood flow through organs is determined
largely by changes
in
resistance
because
arterial and venous pressures are normally
maintained within a narrow range by vari-
ous feedback mechanisms. Therefore, it is
important to understand what determines
resistance in individual vessels and within
vascular networks.
Effects of Vessel Length,
Radius, and Blood Viscosity on
Resistance to Blood Flow
Three factors determine the resistance (R)
to blood flow within a single vessel: vessel
length (L), blood viscosity (n), and diameter
(or radius, r) of the vessel. These are described
by Equation 5-6 as follows:
Eq. 5-6
R « n t
r4
previous page 113 Cardiovascular Physiology Concepts  2nd Edition read online next page 115 Cardiovascular Physiology Concepts  2nd Edition read online Home Toggle text on/off