Page 777 - Fundamentals of anatomy physiology

764 Unit 4 Fluids and Transport
For a systolic pressure of 120 mm Hg and a diastolic pres-
sure of 90 mm Hg, we calculate MAP as follows:
MAP = 90 + (120 - 90) = 90 + 10 = 100 mm Hg
3 shout. Each time an echo is produced, the reflecting surface
21 A normal range of systolic and diastolic pressures occurs in
healthy individuals. When pressures shift outside the normal
range, clinical problems develop. Abnormally high blood pres-
sure is termed hypertension. Abnormally low blood pressure
is hypotension. Hypertension is much more common. In fact,
many cases of hypotension result from overly aggressive drug
treatment for hypertension.
The usual criterion established by the American Heart As-
sociation for stage 1 hypertension in adults is a systolic blood
pressure range of 140–159 and a diastolic range of 90–99.
Values from 120–139 for systolic pressure and 80–89 for di-
astolic pressure indicate pre-hypertension. Blood pressure less
than 120/80 is normal. Cardiologists often recommend some
combination of diet modification and drug therapy for people
whose blood pressures are consistently pre-hypertensive.
Hypertension significantly increases the workload on the
heart, and the left ventricle gradually enlarges. More muscle mass
means a greater demand for oxygen. When the coronary circula-
tion cannot keep pace, signs and symptoms of coronary ischemia
appear. p. 724 Increased arterial pressures also place a physical
stress on the walls of blood vessels throughout the body. This
stress promotes or accelerates the development of arteriosclerosis.
It also increases the risk of aneurysms, heart attacks, and strokes. venous pressures are low, veins offer comparatively little resis-
Elastic Rebound
As systolic pressure climbs, the arterial walls stretch, just as
an extra puff of air expands a partially inflated balloon. This
expansion allows the arterial system to accommodate some of
the blood provided by ventricular systole. When diastole begins
and blood pressures fall, the arteries recoil to their original di-
mensions. This phenomenon is called elastic rebound. Some
blood is forced back toward the left ventricle, closing the aortic
valve and helping to drive additional blood into the coronary
arteries. However, most of the push from elastic rebound forces
blood toward the capillaries. This maintains blood flow along
the arterial network while the left ventricle is in diastole.
Pressures in Small Arteries and Arterioles
The mean arterial pressure and the pulse pressure become
smaller as the distance from the heart increases (Figure 21–9):
The mean arterial pressure declines as the arterial branches
become smaller and more numerous. In essence, blood
pressure decreases as it overcomes friction and produces
blood flow.
The pulse pressure lessens due to the cumulative effects of
elastic rebound along the arterial system. The effect can be
likened to a series of ever-softer echoes following a loud
absorbs some of the sound energy. Eventually, the echo
disappears. The pressure surge accompanying ventricular
ejection is like the shout, and it is reflected by the wall of
the aorta, echoing down the arterial system until it finally
disappears at the level of the small arterioles. By the time
blood reaches a precapillary sphincter, no pressure fluctua-
tions remain, and the blood pressure is steady at approxi-
mately 35 mm Hg.
Venous Pressure and Venous Return
Venous pressure, although low, determines venous return—the
amount of blood arriving at the right atrium each minute.
Venous return has a direct impact on cardiac output. p. 741
Blood pressure at the start of the venous system is only about
one-tenth that at the start of the arterial system, but the blood
must still travel through a vascular network as complex as the
arterial system before returning to the heart.
Pressures at the entrance to the right atrium fluctuate,
but they average about 2 mm Hg. Thus, the effective pressure
in the venous system is roughly 16 mm Hg (from 18 mm Hg
in the venules to 2 mm Hg in the venae cavae). This pressure
compares with 65 mm Hg in the arterial system (from 100 mm
Hg at the aorta to 35 mm Hg at the capillaries). Yet, although
tance, so pressure declines very slowly as blood moves through
the venous system. As blood moves toward the heart, the veins
become larger, resistance drops, and the velocity of blood flow
increases (Figure 21–8).
When you stand, the venous blood returning from your
body inferior to the heart must overcome gravity as it travels up
the inferior vena cava. Two factors assist the low venous pressures
in propelling blood toward your heart: muscular compression of
peripheral veins and the respiratory pump during inhalation.
Muscular Compression. The contractions of skeletal muscles
near a vein compress it, helping to push blood toward the
heart. The valves in small and medium-sized veins ensure that
blood flows in one direction only (Figure 21–5). When you are
standing and walking, the cycles of contraction and relaxation
that accompany your normal movements assist venous return.
If you stand at attention, with knees locked and leg muscles
immobilized, that assistance is lost. The reduction in venous
return then leads to a fall in cardiac output, which reduces the
blood supply to the brain. This decline is sometimes enough to
cause fainting, a temporary loss of consciousness. You would
then collapse, but while you were in the horizontal position,
both venous return and cardiac output would return to normal.