Chapter 18
The Cardiovascular System: The Heart
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18
body. Day and night, tissue cells take in nutrients and oxygen and
excrete wastes. Cells can make such exchanges only with their
immediate environment, so some means of changing and renew-
ing that environment is necessary to ensure a continual supply of
nutrients and prevent a buildup of wastes. Te cardiovascular sys-
tem provides the transport system “hardware” that keeps blood
continuously circulating to fulfill this critical homeostatic need.
The Pulmonary
and Systemic Circuits
Stripped of its romantic cloak, the
heart
is no more than the
transport system pump, and the hollow blood vessels are the
delivery routes. In fact, the heart is actually two pumps side by
side
(Figure 18.1)
.
Te
right side
of the heart receives oxygen-poor blood from
body tissues and then pumps this blood to the lungs to pick
up oxygen and dispel carbon dioxide. Te blood vessels that
carry blood to and from the lungs form the
pulmonary cir-
cuit
(
pulmo
5
lung).
Te
lef side
of the heart receives the oxygenated blood re-
turning from the lungs and pumps this blood throughout
the body to supply oxygen and nutrients to body tissues. Te
blood vessels that carry blood to and from all body tissues
form the
systemic circuit
.
Te heart has two receiving chambers, the
right atrium
and
lef
atrium
, that receive blood returning from the systemic and pulmo-
nary circuits. Te heart also has two main pumping chambers, the
right ventricle
and
lef ventricle
, that pump blood around the two
circuits. Using blood as the transport medium, the heart continu-
ally propels oxygen, nutrients, wastes, and many other substances
into the interconnecting blood vessels that service body cells.
Heart Anatomy
Describe the size, shape, location, and orientation of the
heart in the thorax.
Name the coverings of the heart.
Describe the structure and function of each of the three
layers of the heart wall.
Size, Location, and Orientation
Te modest size and weight of the heart belie its incredible
strength and endurance. About the size of a fist, the hollow,
cone-shaped heart has a mass of 250 to 350 grams—less than a
pound
(Figure 18.2)
.
Snugly enclosed within the
mediastinum
(me
0
de-ah-sti
9
num), the medial cavity of the thorax, the heart extends ob-
liquely for 12 to 14 cm (about 5 inches) from the second rib
to the fiFh intercostal space (±igure 18.2a). As it rests on the
superior surface of the diaphragm, the heart lies anterior to the
vertebral column and posterior to the sternum. Approximately
two-thirds of its mass lies to the leF of the midsternal line; the
balance projects to the right. Te lungs flank the heart laterally
and partially obscure it (±igure 18.2b, c).
Its broad, flat
base
, or posterior surface, is about 9 cm
(3.5 in) wide and directed toward the right shoulder. Its
apex
points inferiorly toward the leF hip. If you press your fingers
between the fiFh and sixth ribs just below the leF nipple, you
can easily feel the
apical impulse
caused by your beating heart’s
apex where it touches the chest wall.
Oxygen-rich,
CO
2
-poor blood
Oxygen-poor,
CO
2
-rich blood
Capillary beds
of lungs where
gas exchange
occurs
Capillary
beds of all
body tissues
where gas
exchange
occurs
Pulmonary
veins
Pulmonary
arteries
Pulmonary Circuit
Systemic Circuit
Aorta and
branches
Left
atrium
Heart
Left
ventricle
Right
atrium
Right
ventricle
Venae
cavae
Figure 18.1
The systemic and pulmonary circuits.
The right
side of the heart pumps blood through the pulmonary circuit* (to
the lungs and back to the left side of the heart). The left side of the
heart pumps blood through the systemic circuit to all body tissues
and back to the right side of the heart. The arrows indicate the
direction of blood flow.
*For simplicity, the actual number of two pulmonary arteries and four
pulmonary veins has been reduced to one each.
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