Essentials of Anatomy and Physiology

Recall that the fibrous skeleton of the heart sepa-
rates the atrial myocardium from the ventricular
myocardium; the fibrous connective tissue acts as elec-
trical insulation between the two sets of chambers.
The only pathway for impulses from the atria to the

ventricles, therefore, is the atrioventricular bundle

(AV bundle), also called the bundle of His. The AV

bundle is within the upper interventricular septum; it

receives impulses from the AV node and transmits

them to the right and left bundle branches. From the

The Heart 281

SA node

Left atrium

Right

atrium

AV node

Right

ventricle

AV bundle

(Bundle of His)

Purkinje

fibers

Left ventricle

Left bundle branch

Right bundle branch

R

P wave

Q S

T wave

P-R Interval QRS

Complex ST SegmentS-T Interval

Figure 12–6. Conduction pathway of the heart. Anterior view of the interior of the

heart. The electrocardiogram tracing is of one normal heartbeat. See text and Box 12–3 for

description.

QUESTION:What structure is the pacemaker of the heart, and what is its usual rate of

depolarization?

BOX12–2 HEART MURMUR

close and prevent backflow during ventricular sys-

tole.

Some valve defects involve a narrowing (steno-

sis) and are congenital; that is, the child is born with

an abnormally narrow valve. In aortic stenosis, for

example, blood cannot easily pass from the left ven-

tricle to the aorta. The ventricle must then work

harder to pump blood through the narrow valve to

the arteries, and the turbulence created is also heard

as a systolic murmur.

Children sometimes have heart murmurs that

are called “functional” because no structural cause

can be found. These murmurs usually disappear

with no adverse effects on the child.

A heart murmur is an abnormal or extra heart sound

caused by a malfunctioning heart valve. The func-

tion of heart valves is to prevent backflow of blood,

and when a valve does not close properly, blood will

regurgitate (go backward), creating turbulence that

may be heard with a stethoscope.

Rheumatic heart disease is a now uncommon

complication of a streptococcal infection. In rheu-

matic fever, the heart valves are damaged by an

abnormal response by the immune system. Erosion

of the valves makes them “leaky” and inefficient,

and a murmur of backflowing blood will be heard.

Mitral valve regurgitation, for example, will be heard

as a systolic murmur, because this valve is meant to