Fundamentals of Anatomy and Physiology

The Cardiovascular Circulatory System 331

Left atrium^
Sinoatrial node
(pacemaker)
Atrioventricular
node
Purkinje’s
fibers
Right atrium

Purkinje’s fibers

Atrioventricular

bundle

Right and left

bundle branches

Figure 14- 6 The conduction system of the heart.

A Cardiac Cycle

In a normal heartbeat, the two atria contract simultane-
ously while the two ventricles relax. Then, when the two
ventricles contract, the two atria relax. Systole (SIS-toh-
lee) is the term used to refer to a phase of contraction and
diastole (dye-ASS-toh-lee) is the term for a phase of
relaxation. A cardiac cycle or complete heartbeat, there-
fore, consists of the systole and diastole of both atria and
the systole and diastole of both ventricles. The pressure
developed in a heart chamber is related to the chamber size
and the volume of blood it contains. The greater the volume
of blood, the higher the pressure.
The average heart beats approximately 72 times per
minute. Therefore, we will assume that each -cardiac -cycle
requires about 0.8 second. During the first 0.1 -second, the
atria contract and the ventricles relax. The atrioventricular
valves (the bicuspid and tricuspid) are open and the
semilunar valves (aortic and pulmo-nary) are closed.
During the next 0.3 second, the atria are relaxing and the
ventricles are contracting. During the first part of this
period,- all valves are closed to help build pressure. During
the second part of this period, the semilunars are open. The
last 0.4 second of the cycle is the relaxation or quiescent
period. All chambers are in

(^) ®
InterventricularLearning
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diastole. So for a full one-half period of a cycle, the heart
muscle is resting-. For the first part of this period, all valves
are closed. -During the latter half, the tricuspid and
bicuspid valves open to allow blood to start draining into

The Perineum.

Media Link

Watch an animation about the

-conduction system of the heart on the

Student Companion Website.

Some Major Blood

Circulatory Routes

The systemic circulation (see Figure 14-5) route

includes all of the oxygenated blood that leaves the left

ventricle of the heart through the aortic semilunar valve and

goes to the aorta and the deoxygenated blood that returns to

the right atrium of the heart via the superior and inferior

venae cavae after traveling to all the organs of the body,

including the nutrient arteries to the lungs. There are