Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 469

aerobic respiration. Almost all of the ATP produced in the heart
is a result of aerobic respiration and oxidative phosphorylation
within mitochondria (chapter 5, section 5.2). This is so effective
that scientists estimate that the ATP within myocardial cells is
completely turned over (broken down and resynthesized) every
10 seconds. The resting heart obtains 50% to 70% of this ATP
from acetyl CoA produced by the b -oxidation of fatty acids
(chapter 5, figure 5.14). The balance of ATP is derived almost
equally from the aerobic respiration of glucose and lactate.
The normal heart always respires aerobically, even dur-
ing heavy exercise when the metabolic demand for oxygen can
rise to six times resting levels (largely due to increased cardiac
rate). This increased oxygen requirement is met primarily by
a corresponding increase in coronary blood flow, from about
80 ml at rest to about 400 ml per minute per 100 g tissue during
heavy exercise.

Regulation of Coronary Blood Flow

The coronary arterioles contain both alpha- and beta-adrenergic
receptors, which promote vasoconstriction and vasodilation,
respectively. Norepinephrine released by sympathetic nerve
fibers stimulates alpha-adrenergic receptors to raise vascular

CLINICAL APPLICATION

An angiogram of the coronary arteries might reveal narrowing

caused by atherosclerotic plaque (chapter 13, section 13.7), a

thrombus, or a spasm ( fig. 14.18 ). An angiogram is an X-ray

picture taken after a catheter is inserted into a brachial or fem-

oral artery, threaded under guidance by a fluoroscope to the

desired site at the coronary arteries, and iodine contrast (dye)

is injected. A coronary angiogram is the standard method for

assessing coronary artery disease.

Coronary angioplasty is the technique of inserting a

catheter with a balloon into the occluded site of a coro-

nary artery and then inflating the balloon to push the artery

open. However, restenosis (recurrence of narrowing) often

occurs after this balloon angioplasty, and for that reason, a

stent —a metallic mesh tube—is often inserted to support

the opened section of the coronary artery. If required, a

coronary bypass grafting ( CABG ) surgery may be per-

formed. This is the most common open-heart surgery,

involving the grafting of a vessel taken from the patient

onto the aorta so that it bypasses the narrowed coronary

artery ( fig. 14.19 ).

Figure 14.18 An angiogram of the coronary

arteries. The red arrow points to an area where the contrast

material of the angiogram is occluded by an obstruction, likely

due to atherosclerosis. This can cause myocardial ischemia and

lead to a myocardial infarction.

Figure 14.19 A diagram of coronary artery bypass

surgery. Segments of the saphenous vein of the patient are

commonly used as coronary bypass vessels.