In the embryo, the heart begins to beat at 4 weeks of

age, even before its nerve supply has been established.
If a person lives to be 80 years old, his or her heart
continues to beat an average of 100,000 times a day,
every day for each of those 80 years. Imagine trying to
squeeze a tennis ball 70 times a minute. After a few
minutes, your arm muscles would begin to tire. Then
imagine increasing your squeezing rate to 120 times a
minute. Most of us could not keep that up very long,
but that is what the heart does during exercise. A
healthy heart can increase its rate and force of con-
traction to meet the body’s need for more oxygen,
then return to its resting rate and keep on beating as if
nothing very extraordinary had happened. In fact, it
isn’t extraordinary at all; this is the job the heart is
meant to do.
The primary function of the heart is to pump blood
through the arteries, capillaries, and veins. As you
learned in the previous chapter, blood transports oxy-
gen and nutrients and has other important functions
as well. The heart is the pump that keeps blood circu-
lating properly.

LOCATION AND

PERICARDIAL MEMBRANES

The heart is located in the thoracic cavity between the
lungs. This area is called the mediastinum. The base
of the cone-shaped heart is uppermost, behind the
sternum, and the great vessels enter or leave here. The
apex (tip) of the heart points downward and is just
above the diaphragm to the left of the midline. This is
why we may think of the heart as being on the left
side, because the strongest beat can be heard or felt
here.
The heart is enclosed in the pericardial mem-
branes, of which there are three layers (Fig. 12–1).
The outermost is the fibrous pericardium, a loose-
fitting sac of strong fibrous connective tissue that
extends inferiorly over the diaphragm and superiorly
over the bases of the large vessels that enter and leave
the heart. The serous pericardium is a folded mem-
brane; the fold gives it two layers, parietal and visceral.
Lining the fibrous pericardium is the parietal peri-
cardium. On the surface of the heart muscle is the
visceral pericardium, often called the epicardium.
Between the parietal and visceral pericardial mem-

branes is serous fluid, which prevents friction as the

heart beats.

CHAMBERS—VESSELS AND VALVES

The walls of the four chambers of the heart are made

of cardiac muscle called the myocardium. The cham-

bers are lined with endocardium, simple squamous

epithelium that also covers the valves of the heart and

continues into the vessels as their lining (endothe-

lium). The important physical characteristic of the

endocardium is not its thinness, but rather its smooth-

ness. This very smooth tissue prevents abnormal

blood clotting, because clotting would be initiated by

contact of blood with a rough surface.

274 The Heart

Endocardium

Parietal

Myocardium pericardium

(heart muscle)

Epicardium

(visceral pericardium)

Fibrous pericardium

(pericardial sac)

Pericardial cavity

Figure 12–1. Layers of the wall of the heart and the

pericardial membranes. The endocardium is the lining of

the chambers of the heart. The fibrous pericardium is the

outermost layer.

QUESTION:What is found between the parietal and vis-

ceral pericardial layers, and what is its function?