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CHAPTER FIFTEEN
Cardiovascular System
Central Venous Pressure
All veins, except those returning to the heart from the lungs,
drain into the right atrium. Therefore, the pressure in this
heart chamber is called
central venous pressure.
It affects the
pressure in the peripheral veins. For example, if the heart is
beating weakly, the central venous pressure increases, and
blood backs up in the venous network, raising its pressure too.
However, if the heart is beating forcefully, the central venous
pressure and the pressure in the venous network decrease.
Blood or tissue f
uid accumulating in the pericardial cavity increases
pressure, causing a condition called
acute cardiac tamponade.
It can
be liFe threatening. As the pressure around the heart increases, it may
compress the heart, interFere with the f
ow oF blood into its cham-
bers, and prevent pumping action. An early symptom oF acute cardiac
tamponade may be increased central venous pressure, with vis-
ible engorgement oF the veins in the neck. Other symptoms include
anxiety, rapid or di±
culty breathing, light-headedness, palpitations,
pallor, and chest pain. Acute cardiac tamponade has several causes,
including bacterial or viral inFection, injury, acute myocardial inFarc-
tion, advanced lung cancer, and dissecting aortic aneurysm.
Increase in blood volume or widespread venoconstric-
tion also increase blood fl
ow into the right atrium, elevating
the central venous pressure. An increase in central venous
the direct result of heart action and depends on other factors,
such as skeletal muscle contraction, breathing movements,
and vasoconstriction of veins. For example, contracting skel-
etal muscles press on veins, moving blood from one valve
section to another. This massaging action of contracting
skeletal muscles helps push the blood through the venous
system toward the heart
(f
g. 15.39)
.
Respiratory movements also move venous blood. During
inspiration, the pressure in the thoracic cavity is reduced
as the diaphragm contracts and the rib cage moves upward
and outward. At the same time, the pressure in the abdomi-
nal cavity is increased as the diaphragm presses downward
on the abdominal viscera. Consequently, blood is squeezed
out of the abdominal veins and forced into thoracic veins.
During exercise, these respiratory movements act with skel-
etal muscle contractions to increase return of venous blood
to the heart.
Venoconstriction also returns venous blood to the heart.
When venous pressure is low, sympathetic refl exes stimu-
late smooth muscles in the walls of veins to contract. The
veins also provide a blood reservoir that can adapt its capac-
ity to changes in blood volume (see ± g. 15.32). If some blood
is lost and blood pressure falls, venoconstriction can force
blood out of this reservoir, returning venous blood to the
heart. By maintaining venous return, venoconstriction helps
to maintain blood pressure.
To heart
To heart
Vein
Vein
Relaxed skeletal
muscle
Contracted
skeletal muscle
Valve open
Valve closed
FIGURE 15.39
The massaging action oF skeletal
muscles helps move blood through the venous system
toward the heart.
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