Cardiovascular Physiology
367
force-generating cross-bridges is activated. Details of the cel-
lular structure and function of cardiac muscle are discussed in
Chapter 9.
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
conducting system
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.
Innervation
The heart receives a rich supply of sympathetic and parasym-
pathetic nerve fi bers, the latter contained in the vagus nerves
(
Figure 12–9
). 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.
Blood Supply
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
coronary arteries,
and the blood fl
owing through them is the
coronary blood
ow.
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
right atrium.
Heartbeat Coordination
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
Pulmonary veins
Capillaries of lungs
Pulmonary arteries
Pulmonary trunk
Right atrium
Right AV valve
Right ventricle
Pulmonary valve
Left atrium
Left AV valve
Left ventricle
Aortic valve
Aorta
Arteries
Arterioles
Capillaries
Venules
Veins
Venae cavae
Figure 12–8
Path of blood fl ow through the entire cardiovascular system. All the
structures within the colored box are located in the heart.
Figure 12–8
physiological
inquiry
How would this diagram be different if it was drawn for a
systemic portal vessel?
Answer can be found at end of chapter.
β
Parasympathetic
Vagus nerves
Acetylcholine
Sympathetic
Atria
M
β
Ventricles
Thoracic
spinal
nerves
Norepinephrine
Epinephrine
Bloodstream
Figure 12–9
Autonomic innervation of heart. Neurons shown represent
postganglionic neurons in the pathways. M = muscarinic-type ACh
receptor;
β
= beta-adrenergic receptor.
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