554
UNIT FOUR
downward and to the left, terminating as a bluntly pointed
apex
at the level of the F fth intercostal space. ±or this rea-
son, it is possible to sense the
apical heartbeat
by feeling or
listening to the chest wall between the F
fth and sixth ribs,
about 7.5 centimeters to the left of the midline.
Coverings of the Heart
The
pericardium
(per
i-kar
de-um), or pericardial sac, is a
covering that encloses the heart and the proximal ends of
the large blood vessels to which it attaches. The pericardium
consists of an outer fibrous bag, the
fibrous pericardium,
that surrounds a more delicate, double-layered serous mem-
brane. The innermost layer of this serous membrane, the
vis-
ceral pericardium
(epicardium), covers the heart. At the base
of the heart, the visceral pericardium turns back upon itself
to become the
parietal pericardium,
which forms the inner
lining of the F
brous pericardium.
The fibrous pericardium is a tough, protective sac
largely composed of dense connective tissue. It is attached
to the central portion of the diaphragm, the posterior of the
sternum, the vertebral column, and the large blood vessels
emerging from the heart (see
f
gs.
1.9
b
,
15.4
and reference
plates 16 and 17). Between the parietal and visceral layers
of the pericardium is a space, the
pericardial cavity,
that
contains a small volume of serous fl uid that the pericardial
membranes secrete. This fl
uid reduces friction between the
pericardial membranes as the heart moves within them.
Alveolus
Oxygenated blood
Deoxygenated blood
Deoxygenated
blood pumped
to lungs via
pulmonary arteries
Oxygenated
blood pumped to
all body tissues
via aorta
Systemic circuit
delivers oxygen to all
body cells and carries away wastes.
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
Pulmonary circuit
eliminates carbon
dioxide via the lungs and oxygenates the
blood.
O
2
CO
2
CO
2
Right atrium
Deoxygenated blood returns
to heart via venae cavae
Right ventricle
Left atrium
Left ventricle
Oxygenated blood returns
to heart via pulmonary veins
FIGURE 15.1
The cardiovascular system transports blood between the body cells and organs such as the lungs, intestines, and kidneys that
communicate with the external environment. Vessels in the pulmonary circuit carry blood from the heart to the lungs and back to the heart,
replenishing oxygen (O
2
) and releasing the metabolic waste carbon dioxide (CO
2
). Vessels of the systemic circuit supply all of the other cells.
012345
c
m
FIGURE 15.2
Anterior view of a human heart. This photo is not life-
size, so a proportionately reduced ruler has been included to help the
student grasp the true size of the organ.
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