592
UNIT FOUR
chemoreceptors that sense blood concentrations of oxygen
and carbon dioxide.
Three major arteries originate from the
arch of the aorta
(aortic arch). They are the brachiocephalic artery, the left
common carotid artery, and the left subclavian artery. The
aortic arch has baroreceptors that detect changes in blood
pressure.
The
brachiocephalic
(brak
e-o-se
˘-fal
ik)
artery
supplies
blood to the tissues of the upper limb and head, as its name
suggests. It is the F rst branch from the aortic arch and rises
through the mediastinum to a point near the junction of the
sternum and the right clavicle. There it divides, giving rise to
the right
common carotid
(kah-rot
id)
artery,
which carries
blood to the right side of the neck and head, and the right
subclavian
(sub-kla
ve-an)
artery,
which leads into the right
arm. Branches of the subclavian artery also supply blood to
parts of the shoulder, neck, and head.
The left
common carotid artery
and the left
subclavian
artery
are respectively the second and third branches of the
aortic arch. They supply blood to regions on the left side of the
body corresponding to those supplied by their counterparts on
the right (
f g. 15.42
and reference plates 21, 22, and 23).
The upper part of the
descending aorta
is left of the mid-
line, but it gradually moves medially and lies directly in front
of the vertebral column at the level of the twelfth thoracic
vertebra. The portion of the descending aorta above the
diaphragm is the
thoracic aorta
(tho-ras
ik a-or
tah), and it
gives off many small branches to the thoracic wall and the
thoracic viscera. These branches, the
bronchial, pericardial,
and
esophageal arteries,
supply blood to the structures for
which they were named. Other branches become
mediasti-
nal arteries,
supplying various tissues in the mediastinum,
and
posterior intercostal arteries,
which pass into the tho-
racic wall.
Below the diaphragm, the descending aorta becomes
the
abdominal aorta,
and it branches to the abdominal wall
and various abdominal organs. These branches include the
following:
1.
Celiac
(se
le-ak)
artery.
This single vessel gives rise
to the left
gastric, splenic,
and
hepatic arteries,
which
supply upper portions of the digestive tract, the spleen,
and the liver, respectively. (
Note:
The hepatic artery
supplies the liver with about one-third of its blood fl
ow,
and this blood is oxygen-rich. The remaining two-thirds
of the liver’s blood fl
ow arrives by means of the hepatic
portal vein and is oxygen-poor.)
2.
Phrenic
(fren
ik)
arteries.
These paired arteries supply
blood to the diaphragm.
3.
Superior mesenteric
(mes
en-ter
ik)
artery.
The
superior mesenteric artery is a large, unpaired vessel
that branches to many parts of the intestinal tract,
including the jejunum, ileum, cecum, ascending colon,
and transverse colon.
4.
Suprarenal
(soo
prah-re
nal)
arteries.
This pair of
vessels supplies blood to the adrenal glands.
of the blood. Alternatively, lymphatic vessels (see chapter
16, pp. 617–618) may return it to the circulation.
As a result of the gas exchanges between the blood and
the alveolar air, blood entering the venules of the pulmonary
circuit is rich in oxygen and low in carbon dioxide. These
venules merge to form small veins, and these veins in turn
converge to form larger veins. ±our
pulmonary veins,
two
from each lung, return blood to the left atrium, and this com-
pletes the vascular loop of the pulmonary circuit.
Pulmonary edema,
in which lungs f
ll with F
uid, can accompany a ±ail-
ing le±t ventricle or a damaged mitral valve. A weak le±t ventricle may
be unable to move the normal volume o± blood into the systemic cir-
cuit. Blood backing up into the pulmonary circuit increases pressure
in the alveolar capillaries, F
ooding the interstitial spaces with F
uid.
Increasing pressure in the interstitial ±luid may rupture the alveo-
lar membranes, and F
uid may enter the alveoli more rapidly than it
can be removed. This reduces the alveolar sur±ace available ±or gas
exchange, and the person may su²
ocate.
Systemic Circuit
±reshly oxygenated blood moves from the left atrium into the
left ventricle. Contraction of the left ventricle forces this blood
into the systemic circuit, which includes the aorta and its
branches that lead to all of the body tissues, as well as the com-
panion system of veins that returns blood to the right atrium.
PRACTICE
57
Distinguish between the pulmonary and systemic circuits o± the
cardiovascular system.
58
Trace the path o± blood through the pulmonary circuit ±rom the
right ventricle.
59
Explain why the alveoli normally do not f
ll with F
uid.
15.7
ARTERIAL SYSTEM
The
aorta
is the largest diameter artery in the body. It
extends upward from the left ventricle, arches over the heart
to the left, and descends just anterior and to the left of the
vertebral column.
Principal Branches of the Aorta
The part of the aorta attached to the heart is called the aor-
tic root. ±rom there, the F rst part of the aorta is called the
ascending aorta.
At the root are the three cusps of the aor-
tic valve, and opposite each cusp is a swelling in the aortic
wall called an
aortic sinus.
The right and left
coronary arter-
ies
arise from two of these sinuses. The elastic recoil of the
aortic wall following contraction of the left ventricle drives
blood fl
ow into these arteries.
Several small structures called
aortic bodies
lie in the
epithelial lining of the aortic sinuses. These bodies house
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