386
U N I T 5
Circulatory Function
facilitated by a nodular thickening at the apex of each
leaflet (Fig. 17-10C). Immediately behind each of the
semilunar cusps, the walls the pulmonary trunk and
aorta are slightly dilated, forming a sinus (Fig. 17-10A).
In these sinuses, eddy currents develop that tend to keep
the valve cusps away from the vessel wall. The opening
for the right coronary artery is located in right aortic
sinus, and the opening for the left coronary artery is
located in left aortic sinus. Were it not for the presence
of the sinuses and eddy currents, the coronary artery
openings would be blocked by the valve cusps.
There are no valves at the atrial sites (i.e., venae cavae
and pulmonary veins) where blood enters the heart (see
Fig. 17-8). This means that excess blood is pushed back
into the veins when the atria become distended. For
example, the jugular veins typically become distended
when inflow into the right atria is impeded in right-sided
heart failure, whereas normally they are flat or col-
lapsed. Likewise, the pulmonary venous system becomes
congested when outflow from the left atrium is impeded.
Electrical Activity of the Heart
Heart muscle is unique among other muscles in that it
is capable of generating and rapidly conducting its own
electrical impulses or action potentials. These action
potentials result in excitation of muscle fibers through-
out the myocardium. Impulse formation and conduction
result in weak electrical currents that spread through the
entire body.
Cardiac Conduction System
In certain areas of the heart, the myocardial cells have
been modified to form the specialized cells of the con-
duction system (Fig. 17-11). Although most myocardial
cells are capable of initiating and conducting impulses,
it is this specialized conduction system that maintains
the pumping efficiency of the heart. Specialized pace-
maker cells generate impulses at a faster rate than other
myocardial cells, and the cells of the conduction system
transmit impulses at a faster rate than other myocardial
cells. Because of these properties, the conduction system
normally controls the rhythm of the heart.
The conduction system consists of (1) the sinoatrial
(SA) node, where the rhythmic impulse is generated; (2)
the internodal pathways, which conduct the impulse
from the SA node to the AV node; (3) the AV node, in
which the impulse from the atria is delayed before pass-
ing to the ventricles; (4) the AV bundle, which conducts
the impulse from the atria to the ventricles; and the (5)
left and right bundles of the Purkinje system, which con-
duct the impulses to all parts of the ventricles.
The SA node has the fastest intrinsic rate of firing
(60 to 100 beats per minute) and normally functions
as the pacemaker of the heart. From the SA node, the
impulse travels radially throughout the right atrium,
ultimately reaching the AV node. A special pathway,
the anterior interatrial pathway, conducts the impulse
to the left atrium.
The heart essentially has two conduction systems:
one that controls atrial activity and one that controls
ventricular activity. The AV node connects the two sys-
tems and normally provides for a one-way conduction
between the atria and ventricles. Within the AV node,
atrial fibers connect with very small junctional fibers in
the node itself. Because of these connections, the zone
surrounding and including the AV node and the adja-
cent atrial and ventricular conduction pathways is often
referred to as the
AV junctional area
. The velocity of con-
duction through these fibers is very slow (approximately
one-half that of normal cardiac muscle), which greatly
Right
coronary
artery
Right
aortic
sinus
A
Anterior view of aortic valve
B
Valve open
C
Valve closed
Bloodflow
Backflow of blood due to recoil of
elastic aorta (closes valve and
causes filling of coronary arteries
when myocardium is relaxed)
Left
coronary
artery
To heart
muscle
(myocardium)
To heart
muscle
(myocardium)
Beginning of
ascending
aorta
Posterior
aortic sinus
Lunule
Nodule
Left
aortic
sinus
FIGURE 17-10.
Aortic valve, aortic sinuses, and coronary arteries.
(A)
Like the pulmonary valve,
the aortic valve has three semilunar cusps: right, posterior, and left.
(B)
Blood ejected from the left
ventricle forces the cusps open.
(C)
When the valve closes, the valve edges and nodules meet in the
center. (From Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 6th ed. Philadelphia, PA:
Wolters Kluwer Health | Lippincott Williams &Wilkins; 2010:144.)
