The Heart 279Ventriculardiastole0 .5secAtrial s(^0) .1 seystole
c
Most atrial blood
flows passively
into ventricles
Remainder
of atrial blood
is pumped into
ventricles
AV va
lves
open
AV
val
ve
s c
los
e
Semilu
narvalv
esopen
Sem
ilu
na
r val
ve
s
close
Ventricula
r s
ysto
le 0.3 sec
Atrial diastole 0.7 sec
Ventricular blood
is pumped into
arteries
Figure 12–5. The cardiac cycle
depicted in one heartbeat (pulse: 75). The
outer circle represents the ventricles, the
middle circle the atria, and the inner circle
the movement of blood and its effect on
the heart valves. See text for description.
QUESTION:What makes the AV valves
close and the semilunar valves open?
The important distinction here is that most blood
flows passively from atria to ventricles, but allblood
to the arteries is actively pumped by the ventricles.
For this reason, the proper functioning of the ventri-
cles is much more crucial to survival than is atrial
functioning.
You may be asking “All this in one heartbeat?” The
answer is yes. The cardiac cycle is this precise
sequence of events that keeps blood moving from the
veins, through the heart, and into the arteries.
The cardiac cycle also creates the heart sounds:
Each heartbeat produces two sounds, often called lub-
dup, that can be heard with a stethoscope. The first
sound, the loudest and longest, is caused by ventricu-
lar systole closing the AV valves. The second sound is
caused by the closure of the aortic and pulmonary
semilunar valves. If any of the valves do not close
properly, an extra sound called a heart murmurmay
be heard (see Box 12–2: Heart Murmur).
CARDIAC CONDUCTION PATHWAY
The cardiac cycle is a sequence of mechanical events
that is regulated by the electrical activity of the
myocardium. Cardiac muscle cells have the ability
to contract spontaneously; that is, nerve impulses are
not required to cause contraction. The heart generates
its own beat, and the electrical impulses follow a
very specific route throughout the myocardium. You
may find it helpful to refer to Fig. 12–6 as you read the
following.
The natural pacemaker of the heart is the sinoatrial
(SA) node, a specialized group of cardiac muscle cells
located in the wall of the right atrium just below the
opening of the superior vena cava. The SA node is
considered specialized because it has the most rapid
rate of contraction, that is, it depolarizes more rapidly
than any other part of the myocardium (60 to 80 times
per minute). As you may recall, depolarization is the
rapid entry of Naions and the reversal of charges on
either side of the cell membrane. The cells of the SA
node are more permeable to Naions than are other
cardiac muscle cells. Therefore, they depolarize more
rapidly, then contract and initiate each heartbeat.
From the SA node, impulses for contraction travel
to the atrioventricular (AV) node, located in the
lower interatrial septum. The transmission of impulses
from the SA node to the AV node and to the rest of the
atrial myocardium brings about atrial systole.