400
Chapter 12
relatively small increase in internal pressure. Approximately 60
percent of the total blood volume is present in the systemic
veins at any given moment (
Figure 12–44
), but the venous
pressure averages less than 10 mmHg. (In contrast, the sys-
temic arteries contain less than 15 percent of the blood, at a
pressure of nearly 100 mmHg.)
The walls of the veins contain smooth muscle innervated
by sympathetic neurons. Stimulation of these neurons releases
norepinephrine, which causes contraction of the venous
smooth muscle, decreasing the diameter and compliance of
the vessels and raising the pressure within them. Increased
venous pressure then drives more blood out of the veins into
the right side of the heart. Note the different effect of venous
constriction compared to that of arterioles: When arterioles
constrict, it
reduces
forward fl ow through the systemic circuit,
whereas constriction of veins
increases
forward fl ow. Although
the sympathetic nerves are the most important input, venous
smooth muscle, like arteriolar smooth muscle, also responds
to hormonal and paracrine vasodilators and vasoconstrictors.
Two other mechanisms, in addition to contraction of
venous smooth muscle, can increase venous pressure and facili-
tate venous return. These mechanisms are the
skeletal muscle
pump
and the
respiratory pump.
During skeletal muscle con-
traction, the veins running through the muscle are partially
compressed, which reduces their diameter and forces more
blood back to the heart. Now we can describe a major function
of the peripheral-vein valves: When the skeletal muscle pump
raises venous pressure locally, the valves permit blood fl ow
only toward the heart and prevent fl ow back toward the tissues
(
Figure 12–45
).
The respiratory pump is somewhat more diffi cult to
visualize. As Chapter 13 will describe, at the base of the chest
cavity (thorax) is a large muscle called the diaphragm, which
separates the thorax from the abdomen. During inspiration
of air, the diaphragm descends, pushing on the abdominal
contents and increasing abdominal pressure. This pressure
increase is transmitted passively to the intraabdominal veins.
Simultaneously, the pressure in the thorax decreases, thereby
decreasing the pressure in the intrathoracic veins and right
atrium. The net effect of the pressure changes in the abdo-
men and thorax is to increase the pressure difference between
the peripheral veins and the heart. Thus, venous return is
enhanced during inspiration (expiration would reverse this
effect if not for the venous valves). The larger the inspiration,
the greater the effect. Thus, breathing deeply and frequently,
as in exercise, helps blood fl ow toward the heart.
You might get the (incorrect) impression from these
descriptions that venous return and cardiac output are
Heart — 9%
Systemic
vessels
Arteries — 11%
Arterioles
and
capillaries — 7%
Veins
Venules
61%
Pulmonary circulation — 12%
Figure 12–44
Distribution of the total blood volume in different parts of the
cardiovascular system.
Adapted from Guyton.
Figure 12–45
The skeletal muscle pump. During muscle contraction, venous
diameter decreases, and venous pressure rises. The increase in
pressure forces the fl ow only toward the heart because backward
pressure forces the valves in the veins to close.
Vein
Blood
flows
only
toward
heart
Valve
open
Contracted
skeletal
muscles
Valve
closed
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