Maintenance of the Body
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The Pulmonary and Systemic Circuits
Te right side of the heart is the pulmonary circuit pump. It
pumps blood through the lungs, where the blood picks up
oxygen and dumps carbon dioxide. Te leF side of the heart is the
systemic circuit pump. It pumps blood through the body’s tissues,
supplying them with oxygen and nutrients and removing carbon
Size, Location, and Orientation
Te human heart, about the size of a clenched ﬁst, is located
obliquely within the mediastinum of the thorax.
Coverings of the Heart
Te heart is enclosed within a double sac made up of the outer
ﬁbrous pericardium and the inner serous pericardium (parietal
and visceral layers). Te pericardial cavity between the serous
layers contains lubricating serous ﬂuid.
Layers of the Heart Wall
Layers of the heart wall, from the interior out, are the
endocardium, myocardium (reinforced by a ﬁbrous cardiac
skeleton), and epicardium (visceral layer of the serous
Chambers and Associated Great Vessels
Te heart has two superior atria and two inferior ventricles.
±unctionally, the heart is a double pump.
Entering the right atrium are the superior vena cava, inferior vena
cava, and coronary sinus. ±our pulmonary veins enter the leF
Te right ventricle discharges blood into the pulmonary trunk;
the leF ventricle pumps blood into the aorta.
Te atrioventricular (AV) valves (tricuspid and mitral) prevent
backﬂow into the atria when the ventricles are contracting; the
semilunar (SL) valves (pulmonary and aortic) prevent backﬂow
into the ventricles when the ventricles are relaxing.
Pathway of Blood Through the Heart
Oxygen-poor systemic blood enters the right atrium, passes into
the right ventricle, through the pulmonary trunk to the lungs,
and back to the leF atrium via the pulmonary veins. Oxygen-
laden blood entering the leF atrium from the lungs ﬂows into
the leF ventricle and then into the aorta, which provides the
functional supply of all body organs. Systemic veins return the
oxygen-depleted blood to the right atrium.
Te right and leF coronary arteries branch from the aorta and via
their main branches (anterior and posterior interventricular, right
marginal, and circumﬂex arteries) supply the heart itself. Venous
blood, collected by the cardiac veins (great, middle, and small),
empties into the coronary sinus.
Blood delivery to the myocardium occurs during heart relaxation.
Cardiovascular System; Topic: Anatomy Review: The Heart, pp. 1–8.
Cardiac Muscle Fibers
Cardiac muscle cells are branching, striated, generally uninucleate
cells. Tey contain myoﬁbrils consisting of typical sarcomeres.
Intercalated discs containing desmosomes and gap junctions
connect adjacent cardiac cells. Te myocardium behaves as a
functional syncytium because of electrical coupling provided by
Mechanism and Events of Contraction
As in skeletal muscle, the membrane depolarization of contractile
myocytes causes opening of sodium channels and allows sodium
to enter, which is responsible for the rising phase of the action
potential curve. Depolarization also opens slow Ca
entry prolongs the period of depolarization (creates
the plateau). Ca
released by the SR and entering from the
extracellular space couples the action potential to sliding of the
myoﬁlaments. Compared to skeletal muscle, cardiac muscle has a
prolonged refractory period that prevents tetany.
Cardiovascular System; Topic: Cardiac Action Potential, pp. 11–18.
Cardiac muscle has abundant mitochondria and depends almost
entirely on aerobic respiration to form A²P.
Certain noncontractile cardiac muscle cells exhibit automaticity and
rhythmicity and can independently initiate action potentials. Such
cells have an unstable resting potential called a pacemaker potential
that gradually depolarizes, driFing toward threshold for ﬁring.
Tese cells compose the intrinsic conduction system of the heart.
Te conduction system of the heart consists of the SA and AV
nodes, the AV bundle and bundle branches, and the subendocardial
conducting network. Tis system coordinates the depolarization of
the heart and ensures that the heart beats as a unit. Te SA node has
the fastest rate of spontaneous depolarization and acts as the heart’s
pacemaker; it sets the sinus rhythm.
Defects in the intrinsic conduction system can cause arrhythmias,
ﬁbrillation, and heart block.
Te autonomic nervous system innervates the heart. Cardiac
centers in the medulla include the cardioacceleratory center,
which projects to the ²
region of the spinal cord, which in
turn projects to the cervical and upper thoracic sympathetic