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U N I T 5
Circulatory Function
Disorders of Arterial Blood
Pressure
The arterial blood pressure reflects the rhythmic ejection
of blood from the left ventricle into the aorta.
26
It rises
as the left ventricle contracts and falls as it relaxes. In
healthy adults, the highest pressure, called the
systolic
pressure,
is ideally less than 120 mm Hg and the lowest
pressure, called the
diastolic pressure
, is less than 80 mm
Hg (Fig. 18-12). The difference between the systolic and
diastolic pressure is called the
pulse pressure
(approxi-
mately 40 mm Hg). The pulse pressure reflects the pulsa-
tile nature of arterial blood flow. It rises when the stroke
volume is increased and falls when the resistance to out-
flow is decreased. The
mean arterial pressure
represents
the average pressure (approximately 90 to 100 mm Hg)
in the arterial system during ventricular contraction and
relaxation and is a good indicator of tissue perfusion.
Mean arterial pressure can be estimated using the fol-
lowing equation: mean arterial pressure = 1/3 systolic
+ 2/3 diastolic pressures.
The systolic and diastolic components of blood pres-
sure are determined by cardiac output and total periph-
eral vascular resistance and can be expressed as the
product of the two (blood pressure = cardiac output ×
total peripheral resistance). The cardiac output is the
product of the stroke volume (amount of blood ejected
from the heart with each beat) and the heart rate: cardiac
ouput = stroke volume × heart rate. The total peripheral
resistance reflects changes in the radius of the arterioles
as well as the viscosity or thickness of the blood. The
arterioles often are referred to as the
resistance vessels
because they can selectively constrict or relax to control
the resistance to outflow of blood into the capillaries.
The body maintains its blood pressure by adjusting the
cardiac output to compensate for changes in peripheral
vascular resistance, and it changes the peripheral vas-
cular resistance to compensate for changes in cardiac
output.
In hypertension and disease conditions that affect
blood pressure, changes in blood pressure usually are
described in terms of the systolic and diastolic pressures,
pulse pressure, and mean arterial pressure. These pres-
sures are influenced by the stroke volume, the rapidity
with which blood is ejected from the heart, the elastic
properties of the aorta and large arteries and their abil-
ity to accept various amounts of blood as it is ejected
from the heart, and the properties of the resistance
blood vessels that control the runoff of blood into the
smaller vessels and capillaries that connect the arterial
and venous circulations.
Mechanisms of Blood Pressure
Regulation
An adequate systemic arterial pressure is perhaps the
single most important requirement for proper function-
ing of the cardiovascular system. Although different tis-
sues in the body are able to regulate their own blood
flow, without sufficient arterial pressure the brain and
the heart do not receive adequate blood flow, no matter
what adjustments are made in their vascular resistance
by local control mechanisms. The mechanisms used to
regulate the arterial pressure depend on whether short-
term or long-term adaptation is needed
29
(Fig. 18-13).
Short-Term Regulation
The mechanisms for short-term regulation of blood
pressure, those acting over seconds or minutes, are
intended to correct temporary imbalances in blood pres-
sure, such as occur during physical exercise and changes
in body position. These mechanisms also are respon-
sible for maintenance of blood pressure at survival lev-
els during life-threatening situations such as during an
acute hemorrhagic incident. The short-term regulation
of blood pressure relies mainly on neural and humoral
mechanisms, the most rapid of which are the neural
mechanisms.
Neural Mechanisms.
The neural control of blood pres-
sure is vested in centers that are located in the reticu-
lar formation of the medulla and lower third of the
pons, where integration and modulation of autonomic
nervous system (ANS) responses occur.
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This area of
the brain contains the vasomotor and cardiac control
centers and is often collectively referred to as the
car-
diovascular center.
The cardiovascular center transmits
parasympathetic impulses to the heart through the vagus
nerve and sympathetic impulses to the heart and blood
vessels through the spinal cord and peripheral sympa-
thetic nerves. Parasympathetic stimulation of the heart
produces a slowing of heart rate, whereas sympathetic
Systolic pressure (peak)
(mm sec)
Mean
arterial
pressure
Dicrotic notch
Pulse
pressure
Diastolic
pressure
(minimum)
120
0
40
80
FIGURE 18-12.
Intra-arterial pressure tracing made from
the brachial artery. Pulse pressure is the difference between
systolic and diastolic pressures.The darker area represents
the mean arterial pressure, which can be calculated using the
formula of mean arterial pressure = diastolic pressure + pulse
pressure/3.
