force-generating cross-bridges is activated. Details of the cel-
lular structure and function of cardiac muscle are discussed in
Approximately 1 percent of cardiac cells do not function
in contraction, but have specialized features that are essential
for normal heart excitation. These cells constitute a network
known as the
of the heart and are in
contact with the cardiac muscle cells via gap junctions. The
conducting system initiates the heartbeat and helps spread the
impulse rapidly throughout the heart.
One ﬁ nal point about cardiac muscle is that certain cells
in the atria secrete the family of peptide hormones collectively
called atrial natriuretic peptide, described in Chapter 14.
The heart receives a rich supply of sympathetic and parasym-
pathetic nerve ﬁ bers, the latter contained in the vagus nerves
). The sympathetic postganglionic ﬁ bers, which
innervate the entire heart, release primarily norepinephrine,
whereas the parasympathetics terminate mainly on cells
found in the atria and release primarily acetylcholine. The
receptors for norepinephrine on cardiac muscle are mainly
beta-adrenergic. The hormone epinephrine, from the adrenal
medulla, combines with the same receptors as norepinephrine
and exerts the same actions on the heart. The receptors for
acetylcholine are of the muscarinic type.
The blood being pumped through the heart chambers does
not exchange nutrients and metabolic end products with the
myocardial cells. They, like the cells of all other organs, receive
their blood supply via arteries that branch from the aorta. The
arteries supplying the myocardium are the
and the blood ﬂ
owing through them is the
The coronary arteries exit from the very ﬁ rst part of the
aorta (see Figure 12–7a) and lead to a branching network of
small arteries, arterioles, capillaries, venules, and veins similar
to those in other organs. Most of the cardiac veins drain into
a single large vein, the coronary sinus, which empties into the
The heart is a dual pump in that the left and right sides of
the heart pump blood separately, but simultaneously, into the
systemic and pulmonary vessels. Efﬁ cient pumping of blood
requires that the atria contract ﬁ rst, followed almost imme-
diately by the ventricles. Contraction of cardiac muscle, like
that of skeletal muscle and many smooth muscles, is triggered
by depolarization of the plasma membrane. Gap junctions
interconnect myocardial cells and allow action potentials to
spread from one cell to another. Thus, the initial excitation
of one cardiac cell eventually results in the excitation of all
Capillaries of lungs
Right AV valve
Left AV valve
Path of blood ﬂ ow through the entire cardiovascular system. All the
structures within the colored box are located in the heart.
How would this diagram be different if it was drawn for a
systemic portal vessel?
Answer can be found at end of chapter.
Autonomic innervation of heart. Neurons shown represent
postganglionic neurons in the pathways. M = muscarinic-type ACh
= beta-adrenergic receptor.