CHAPTER 7 • ORGAN BLOOD FLOW
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Metabolic Activity
Time (min)
■ FIGURE 7.5 Active hyperemia. The le ft panel shows that increasing tissue metabolism for 2 minutes
transiently increases blood flow (active or functional hyperemia). The rig ht panel shows that the steady-
state increase in blood flow during active hyperemia is directly related to the increase in m etabolic activity
until the vessels become maximally dilated and flow can no longer increase.
as shown in the right panel. At high levels of
metabolic activity, the vasculature becomes
maximally dilated, resulting in a maximal
increase in blood flow. Active hyperemia is
important because it increases oxygen deliv-
ery to tissues at a time of increased oxygen
demand. Furthermore, the increased blood
flow enhances the removal of metabolic waste
products from the tissue.
The vasodilatory capacity during active
hyperemia differs considerably among organs.
In skeletal muscle, blood flow can increase
more than 20- to 50-fold during exercise,
depending on the type of muscle. Cerebral
blood flow, in contrast, increases no more
than twofold at maximal metabolic activity.
The reason for this difference is that resting
skeletal muscle has a high degree of vascular
tone in contrast to the cerebral circulation,
which has a relatively low degree of vascu-
lar tone because of its higher metabolic rate
under basal conditions.
SPECIAL CIRCULATIONS
Coronary Circulation
In order to supply sufficient oxygen to support
the high oxidative metabolism of the beating
heart, there must be an extensive network of
vessels that provide blood flow throughout the
myocardium. These vessels are the coronary
arteries and veins. Regulatory mechanisms exist
to ensure that adequate oxygen is delivered to
the myocardium. Coronary artery disease or
the failure of regulatory mechanisms can lead
to insufficient oxygen delivery to the myocar-
dium, which will impair cardiac function.
CORONARY VASCULAR ANATOMY
The two major branches of the coronary cir-
culation are the left main and right main
coronary arteries
(Fig.
7.6).
These vessels
arise from coronary ostia, which are small
openings in the wall of the ascending aorta
just distal to the aortic valve. The left main
coronary artery is relatively short in length
(~1 cm). After coursing behind the pulmonary
artery trunk, it divides into the left anterior
descending artery, which travels along the
interventricular groove on the anterior surface
of the heart, and the circumflex artery, which
travels posteriorly along the groove between
the left atrium and ventricle. These branches of
the left coronary artery supply blood primar-
ily to the left ventricle and atrium. The right
main
coronary artery travels between the
right atrium and ventricle (left atrioventricu-
lar groove) toward the posterior regions of the
heart. This vessel and its branches serve the
right ventricle and atrium, and in most individ-
uals, the inferoposterior region of the left ven-
tricle. Significant variation is possible among
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