b. Atrioventricular Node. The AV node is located between the right atrium
and the right ventricle. The AV node is responsible for the contraction of both the
ventricles. From the atrioventricular node, the impulse travels through the Bundle of
HIS to the purkinje fibers of the ventricles.
c. Bundle of HIS. The Bundle of HIS is a collection of cardiac fibers through
which the impulse travels on its way to the Purkinje fiber system. The Bundle of HIS is
located at the uppermost portion of the ventricular septum. The ventricular septum is
the thick muscular membrane that separates the right ventricle from the left ventricle.
d. Purkinje Fibers. The Purkinje fibers transverse and branch off within the
ventricular septum branching to supply both ventricles near the bottom of the septum.
By branching close to the bottom of the ventricular septum, the contractions of the
ventricles go in an upward direction that is necessary for proper blood flow.
Consequently, the contraction of the ventricles forces the blood upward to the aorta and
e. Control of the SA Node and AV Node. Both the SA and the AV node are
controlled by the autonomic nervous system. Parasympathetic stimulation, supplied by
the vagus nerve tends to decrease both the rate and force of contraction of the heart.
Sympathetic stimulation, from the cervical sympathetic ganglia, serves to increase both
the rate and force of contraction of the heart. The predominant sympathetic receptor is
a beta-receptor although it has been shown that a small amount of alpha-receptors are
present in the heart.
ELECTROLYTES OF SIGNIFICANCE IN HEART FUNCTIONING
As with all muscle and nervous tissue, a proper concentration of electrolytes is
essential for normal heart function. The three electrolytes essential for proper cardiac
function are potassium, calcium, and sodium.
a. Potassium. An increase in the level of potassium in the extra-cellular fluid
causes a decrease in the heart rate as well as a decrease in the force of contraction.
The heart becomes dilated and flaccid. An extremely large increase in potassium can
block nervous conduction through the atrioventricular bundle. If potassium levels are
increased two or three times above normal, the atrioventricular blockade is usually so
severe that death occurs. Potassium depletion also causes a decrease in the heart rate
and an increase in the force of contraction. This is of concern, especially in the patient
who has been taking digitalis. As you will remember, digitalis is valuable in the
treatment of heart failure because it decreases the heart rate as well as increases the
force of contraction, thus the efficiency of the heart is increased. If potassium levels are
depleted at too great a degree, digitalis intoxication can result in which case the heart
rate might decrease to too slow a rate.
b. Calcium. Calcium is primarily involved with the contractile processes of the
myocardium. An increase in calcium levels may cause over contraction of the heart and