572
UNIT FOUR
The most important ions that infl uence heart action are
potassium (K
+
) and calcium (Ca
+2
). Potassium affects the
electrical potential of the cell membrane, altering its ability
to reach the threshold for conducting an impulse (see chap-
ter 10, p. 368). The sarcoplasmic reticula of cardiac muscle
F
bers have less calcium than do the sarcoplasmic reticula of
skeletal muscle F bers. Therefore, cardiac muscle depends
more on extracellular (blood-borne) calcium. Although
homeostatic mechanisms normally maintain the concentra-
tions of these ions within narrow ranges, these mechanisms
sometimes fail, and the consequences can be serious or even
fatal. Clinical Application 15.1 examines abnormal heart
rhythms.
receptors signal the cardioaccelerator center, and sympathetic
impulses reach the heart. As a result, heart rate and force of
contraction increase, and the venous pressure is reduced.
Impulses from the cerebrum or hypothalamus also infl
u-
ence the cardiac control center. Such impulses may decrease
heart rate, as occurs when a person faints following an emo-
tional upset, or they may increase heart rate during a period
of anxiety.
Two other factors that influence heart rate are tem-
perature change and certain ions. Rising body tempera-
ture increases heart action, which is why heart rate usually
increases during fever. On the other hand, abnormally low
body temperature decreases heart action.
impulses originating from unusual (ectopic)
regions of the heart probably cause a premature
beat. That is, the impulse originates from a site
E
ach year, thousands of people die from a
fast or irregular heartbeat. These altered
heart rhythms are called
arrhythmias.
In
f
brillation,
small areas of the myocardium
contract in an uncoordinated, chaotic fashion (F
g.
15B). As a result, the myocardium fails to contract
as a whole, and blood is no longer pumped. Atrial
fibrillation is not life threatening, because the
ventricles still pump blood, but ventricular F
bril-
lation is often deadly. Ventricular fibrillation can
be caused by an obstructed coronary artery, toxic
drug exposure, electric shock, or traumatic injury
to the heart or chest wall. A deF
brillator device can
deliver a shock to restore a normal heartbeat, as
described in the vignette that opens this chapter.
An abnormally fast heartbeat, usually more
than 100 beats per minute, is called
tachycardia.
Increase in body temperature, nodal stimulation
by sympathetic F
bers, certain drugs or hormones,
heart disease, excitement, exercise, anemia, or
shock can cause tachycardia. ±igure 15C shows
the ECG of a tachycardic heart.
Bradycardia
means a slow heart rate, usu-
ally fewer than sixty beats per minute. Decreased
body temperature, nodal stimulation by para-
sympathetic impulses, or certain drugs may
cause bradycardia. It also may occur during
sleep. ±igure 15D shows the ECG of a bradycardic
heart. Athletes sometimes have unusually slow
heartbeats because their hearts pump a greater-
than-normal volume of blood with each beat. The
slowest heartbeat recorded in a healthy athlete
was twenty-F
ve beats per minute!
A
premature beat
occurs before it is expected
in a normal series of cardiac cycles. Cardiac
other than the SA node. Cardiac impulses may
arise from ischemic tissues or from muscle F
bers
irritated by disease or drugs.
15.1
CLINICAL APPLICATION
Arrhythmias
FIGURE 15B
²Ventricular²F
brillation is rapid, uncoordinated depolarization of the ventricles.
FIGURE 15C
Tachycardia is a rapid heartbeat.
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