The Cardiovascular System: The Heart
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.
and Systemic Circuits
Stripped of its romantic cloak, the
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
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
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
Te heart has two receiving chambers, the
, that receive blood returning from the systemic and pulmo-
nary circuits. Te heart also has two main pumping chambers, the
, 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.
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
Snugly enclosed within the
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
, or posterior surface, is about 9 cm
(3.5 in) wide and directed toward the right shoulder. Its
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
caused by your beating heart’s
apex where it touches the chest wall.
of lungs where
beds of all
The systemic and pulmonary circuits.
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.