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U N I T 5
Circulatory Function
The Heart as a Pump
The heart is a four-chambered muscular pump approxi-
mately the size of a fist that beats an average of 70 times
each minute, 24 hours each day, 365 days each year for
a lifetime. In 1 day, this pump moves more than 1800
gallons of blood throughout the body, and the work per-
formed by the heart over a lifetime would lift 30 tons to
a height of 30,000 ft.
Functional Anatomy of the Heart
The heart, which is enclosed in a loose-fitting sac called
the
pericardium
, is located between the lungs in the medi-
astinal space of the intrathoracic cavity. It is located pos-
terior to the sternum and anterior to the vertebral column
and extends about 5 inches from the second to fifth ver-
tebrae (Fig. 17-4A). The heart is suspended by the great
vessels, with its broader side (i.e., base) facing upward
and its tip (i.e., apex) pointing downward, forward, and
to the left. The heart is positioned obliquely, so that the
right side of the heart is almost fully in front of the left
side of the heart, with only a small portion of the lat-
intraluminal pressure, wall tension becomes
greater as the radius of a vessel increases; and
more pressure will be needed to overcome
the contractile tension in a vessel wall as the
diameter decreases. Wall tension is also affected
by wall thickness, increasing as the wall becomes
thinner and decreasing as it becomes thicker.
■■
The velocity or speed of blood flow through a
vessel is greatly affected by its cross-sectional
area, increasing as the cross-sectional area
decreases and decreasing as it increases. High
velocity can create turbulent blood flow, in which
the blood moves crosswise and lengthwise
in blood vessels; as opposed to laminar or
layered flow, in which the blood components are
arranged so that the plasma is adjacent to the
smooth surface of the inner lining of the vessel
wall and the blood components are in the center
of the bloodstream.
■■
Vascular compliance or capacitance reflects the
distensibility of blood vessels and total quantity
of blood that can be stored in a given part of
the circulatory system for a given change in
pressure. It is greater in the thin-walled vessels
of the venous system than in the thick-walled
vessels of the arterial system.
SUMMARY CONCEPTS
(continued)
eral left ventricle on the frontal plane of the heart (Fig.
17-4B).When the hand is placed on the thorax, the main
impact of the heart’s contraction is felt against the chest
wall at a point between the fifth and sixth ribs, a little
below the nipple and approximately 3 inches to the left of
the midline. This is called the
point of maximum impulse.
The wall of the heart is composed of an outer epicar-
dium, which lines the pericardial cavity; the myocardium
or muscle layer; and the smooth endocardium, which
lines the chambers of the heart (Fig. 17-5). A fibrous
skeleton separates the atria and ventricles and forms a
rigid support for attachment of the heart valves. The
interatrial and interventricular septa divide the heart
into a right and a left pump, each composed of two
muscular chambers: a thin-walled atrium, which serves
as a reservoir for blood coming into the heart, and a
thick-walled ventricle, which pumps blood out of the
heart. The increased thickness of the left ventricular wall
compared to the right ventricle (Fig. 17-4C) results from
the additional work this ventricle is required to perform.
Pericardium
The pericardium forms a fibrous covering around the
heart, holding it in a fixed position in the thorax and
providing physical protection and a barrier to infec-
tion. The pericardium consists of a tough outer fibrous
layer and a thin inner serous layer. The outer fibrous
layer is attached to the great vessels that enter and leave
the heart, the sternum, and the diaphragm. The fibrous
pericardium is highly resistant to distention; it prevents
acute dilatation of the heart chambers and exerts a
restraining effect on the left ventricle. The inner serous
layer consists of a visceral layer and a parietal layer. The
visceral layer, also known as the visceral pericardium or
epicardium,
covers the entire heart and great vessels and
then folds over to form the parietal layer that lines the
fibrous pericardium (see Fig. 17-5). Between the visceral
and parietal layers is the
pericardial cavity,
a potential
space that contains 30 to 50 mL of serous fluid. This
fluid acts as a lubricant to minimize friction between the
two layers as the heart contracts and relaxes.
Myocardium
The myocardium, or muscular portion of the heart,
forms the walls of the atria and ventricles. Cardiac mus-
cle cells, like skeletal muscle, are striated and composed
of
sarcomeres
that contain actin and myosin filaments
(see Chapter 1). They are smaller and more compact
than skeletal muscle cells and contain many large mito-
chondria, reflecting their continuous energy needs.
The contractile properties of cardiac muscle are simi-
lar to those of skeletal muscle, except the contractions
are involuntary and the duration of contraction is much
longer. Unlike the orderly longitudinal arrangement of
skeletal muscle fibers, cardiac muscle cells are arranged
as an interconnecting latticework, with their fibers divid-
ing, recombining, and then dividing again (Fig. 17-6A).
The fibers are separated from neighboring cardiac muscle
cells by dense structures called intercalated disks
. The
intercalated disks, which are unique to cardiac muscle,
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