670
UNIT 4
Maintenance of the Body
18
Te
right coronary artery
courses to the right side of the
heart, where it also gives rise to two branches:
Te
right marginal artery
serves the myocardium of the lat-
eral right side of the heart.
Te
posterior interventricular artery
runs to the heart apex
and supplies the posterior ventricular walls. Near the apex of
the heart, this artery merges (anastomoses) with the anterior
interventricular artery.
±ogether the branches of the right coronary artery supply the
right atrium and nearly all the right ventricle.
Te arterial supply of the heart varies considerably. For ex-
ample, in 15% of people, the le² coronary artery gives rise to
both
interventricular arteries. In about 4% of people, a single
coronary artery supplies the whole heart. Additionally, there
may be both right and le² marginal arteries. Tere are many
anastomoses (junctions) among the coronary arterial branches.
Tese fusing networks provide additional (
collateral
) routes for
blood delivery to the heart muscle, but are not robust enough to
supply adequate nutrition when a coronary artery is suddenly
occluded (blocked). Complete blockage leads to tissue death
and heart attack.
Te coronary arteries provide an intermittent, pulsating
blood flow to the myocardium. Tese vessels and their main
branches lie in the epicardium and send branches inward to
nourish the myocardium. Tey deliver blood when the heart
is relaxed, but are fairly ineffective when the ventricles are con-
tracting because they are compressed by the contracting myo-
cardium. Although the heart represents only about 1/200 of the
body’s weight, it requires about 1/20 of the body’s blood supply.
As might be expected, the le² ventricle receives the most plenti-
ful blood supply.
Right
ventricle
Left
ventricle
Interventricular
septum
Figure 18.10
Anatomical differences between the right and
left ventricles.
The left ventricle has a thicker wall and its cavity is
basically circular. The right ventricle cavity is crescent shaped and
wraps around the left ventricle.
Coronary Veins
A²er passing through the capillary beds of the myocardium,
the venous blood is collected by the
cardiac veins
, whose paths
roughly follow those of the coronary arteries. Tese veins join
to form an enlarged vessel called the
coronary sinus
, which
empties the blood into the right atrium. Te coronary sinus is
obvious on the posterior aspect of the heart (Figure 18.11b).
Te sinus has three large tributaries: the
great cardiac vein
in the anterior interventricular sulcus; the
middle cardiac vein
in the posterior interventricular sulcus; and the
small cardiac
vein
, running along the heart’s right inferior margin. Addition-
ally, several
anterior cardiac veins
empty directly into the right
atrium anteriorly.
Right
ventricle
Right
coronary
artery
Right
atrium
Right
marginal
artery
Posterior
interventricular
artery
Anterior
interventricular
artery
Circumflex
artery
Left
coronary
artery
Aorta
Anastomosis
(junction of
vessels)
Left
ventricle
Superior
vena cava
(a) The major coronary arteries
Left atrium
Pulmonary
trunk
Superior
vena cava
Anterior
cardiac
veins
Small cardiac vein
Middle cardiac vein
Great
cardiac
vein
Coronary
sinus
(b) The major cardiac veins
Figure 18.11
Coronary circulation.
In both drawings, lighter-
tinted vessels are more posterior in the heart.
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