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the blood in a slightly different manner. That is, the two
atria contract simultaneously, while the two ventricles
relax. Then the two ventricles contract simultaneously,
while the two atria relax. Then all chambers rest before the
cycle begins again.
Refer to Figures 14-4A and 14-5 as we discuss blood
flow. Deoxygenated blood (blood high in carbon di-oxide
gas) returns from the upper portions of the body through
the superior or anterior vena cava and from the lower
portions of the body through the inferior or pos-terior vena
cava to the right atrium of the heart. The blood is then
squeezed by contraction of the right atrium through the
tricuspid valve into the right ventricle. As the right
ventricle contracts, it pumps the blood through the
pulmonary semilunar valve into the pulmonary trunk,
which branches into the right pulmonary artery that goes to
the right lung and the left pulmonary artery that goes
(^) ®
Learning
Cengage ©^
Figure 14- 5 Schematic drawing of blood flow through
the heart showing pulmonary and systemic circulation.
Chapter 14
to the left lung. In the alveoli of the lungs surrounded by
capillaries, the blood loses the carbon dioxide gas and picks
up oxygen. Deoxygenated blood looks dark; hence, veins
are usually depicted in textbooks as blue. Oxygen-ated
blood looks bright red; hence, arteries are usually shown in
red in textbooks. The oxygenated blood returns to the left
atrium of the heart through four pulmonary veins. When the
left atrium contracts, it squeezes the blood through the
bicuspid or mitral valve into the left ventricle. As this
ventricle with its thick muscular walls contracts, it pushes
the blood through the aortic semilu-nar valve into the
ascending aorta. The ascending aorta distributes the
oxygenated blood to all parts of the body.
The Conduction System
of the Heart
The heart is innervated by the autonomic nervous sys-tem.
However, it does not initiate a contraction but only
increases or decreases the time it takes to complete a
cardiac cycle. This is made possible because the heart has
its own intrinsic regulating system called the conduc-tion
system (Figure 14-6) that generates and distributes
electrical impulses over the heart to stimulate cardiac
muscle fibers or cells to contract.
The system begins at the sinoatrial (sigh-no-AY-tree-
al) node, known as the SA node or pacemaker, which
initiates each cardiac cycle and sets the pace for the heart
rate. It is located in the superior wall of the right atrium. It
can be modified by nerve impulses from the autonomic
nervous system; sympathetic impulses will speed it up, and
parasympathetic impulses will restore or slow it down.
Thyroid hormone and epinephrine car-ried by the blood
will also affect the pacemaker. Once an impulse is initiated
by the SA node, the impulse spreads out over both atria,
causing them to contract simultane-ously. At the same time,
it depolarizes the atrioventricular- (AY-tree-oh-vin-
TRIK-yoo-lar) (AV) node. It is located in the lower
portion of the right atrium.
From the AV node, a tract of conducting fibers called the
atrioventricular bundle or bundle of His (BUN-dull of
HIZ) runs through the cardiac mass to the top of the in-
terventricular septum. It then branches and continues down
both sides of the septum as the right and left bun-dle
branches. Thus, the bundle of His distributes the elec-
trical charge over the medial surfaces of the ventricles. The
actual contraction of the ventricles is stimulated by the
Purkinje’s (pur-KIN-jeez) fibers (also known as the
conduction myofibers) that emerge from the bundle
branches and pass into the cells of the myocardium of the
ventricles.