Human Physiology, 14th edition (2016)

470 Chapter 14

beta-adrenergic receptors by the hormone epinephrine, stimu-

late vasodilation as part of the fight-or-flight response to any

stressful state, including that existing just prior to exercise

( table 14.5 ). These extrinsic controls have been previously dis-

cussed and function to regulate blood flow through muscles at

rest and upon the anticipation of exercise.

As dynamic exercise progresses, the vasodilation and

increased skeletal muscle blood flow that occur are almost

entirely due to intrinsic metabolic control. The high metabolic rate

of skeletal muscles during exercise causes local changes, such as

increased carbon dioxide concentrations, decreased pH (due to

carbonic acid and lactic acid), decreased oxygen, increased extra-

cellular K^1 , and the secretion of adenosine. As in the intrinsic

control of the coronary circulation, these changes cause vasodila-

tion of arterioles in skeletal muscles. This decreases the vascu-

lar resistance and increases the blood flow. As a result of these

changes, skeletal muscles can receive as much as 85% of the total

blood flow in the body during maximal exercise ( fig. 14.20 ).

Circulatory Changes During Exercise

Both breathing and pulse rate increase within one second of

exercise, suggesting that the motor cortex responsible for orig-

inating the exercise also influences the cardiovascular adjust-

ments to exercise. However, cardiovascular changes during

exercise are also affected by sensory feedback from the con-

tracting muscles and by the baroreceptor reflex (discussed

shortly in conjunction with blood pressure regulation). These

mechanisms increase the activity of the sympathoadrenal sys-

tem and reduce parasympathetic nerve activity during exercise.

As a result, there is an increase in cardiac rate, stroke volume,

and cardiac output.

The vascular resistance through muscles decreases during

dynamic exercise but increases during static (isometric exer-

cise), while the resistance to flow through the visceral organs

and skin increases in both static and dynamic exercise. The

increased resistance through the viscera and skin is produced

by vasoconstriction due to increased activity of adrenergic sym-

pathetic fibers. In summary, the blood flow through dynami-

cally exercising muscles increases due to: (1) increased total

resistance at rest. Epinephrine released by the adrenal medulla
can stimulate the beta-adrenergic receptors to produce vasodila-
tion when the sympathoadrenal system is activated during the
fight-or-flight reaction.
During heavy exercise, the oxygen consumption of the
myocardium can increse from four to six times resting values.
This involves an increased b -oxidation of fatty acids and an
even greater increase in the oxidation of glucose, requiring a
four- to sixfold increase in coronary blood flow due to vasodila-
tion. The vasodilation and decreased resistance of the coronary
circulation during exercise is produced partly by sympatho-
adrenal system changes, but mostly by intrinsic metabolic
changes. As the metabolism of the myocardium increases, there
are increased local tissue concentrations of carbon dioxide,
K^1 , and released paracrine regulators that include nitric oxide,
adenosine, and prostaglandins. These act directly on the vas-
cular smooth muscle to cause vasodilation. Exercise training
(1) increases the density of coronary arterioles and capillaries,
(2) increases the production of nitric oxide for promoting vaso-
dilation, and (3) decreases the compression of the coronary ves-
sels in systole, due to the lower cardiac rate (and thus frequency
of systoles) in trained athletes.

Regulation of Blood Flow Through

Skeletal Muscles

The arterioles in skeletal muscles, like those of the coronary
circulation, have a high vascular resistance at rest as a result
of alpha-adrenergic sympathetic stimulation. This produces a
relatively low blood flow. Because muscles have such a large
mass, however, they still receive from 20% to 25% of the total
blood flow in the body at rest. Also, as in the heart, blood flow
in a skeletal muscle decreases when the muscle contracts and
squeezes its arterioles, and in fact blood flow stops entirely
when the muscle contracts beyond about 70% of its maximum.
Pain and fatigue thus occur much more quickly when an isomet-
ric contraction is sustained (in static exercise ) than when rhyth-
mic isotonic contractions are performed (in dynamic exercise ).
In addition to adrenergic fibers, which promote vasocon-
striction by stimulation of alpha-adrenergic receptors, there
are also cholinergic sympathetic fibers in skeletal muscles.
These cholinergic fibers, together with the stimulation of

Table 14.5 | Changes in Skeletal Muscle Blood Flow Under Conditions of Rest and Exercise

Condition Blood Flow (ml/min) Mechanism

Rest 1,000 High adrenergic sympathetic stimulation of vascular alpha receptors, causing

vasoconstriction

Beginning exercise Increased Dilation of arterioles in skeletal muscles due to cholinergic sympathetic nerve activity

and stimulation of beta-adrenergic receptors by the hormone epinephrine

Heavy exercise 20,000 Fall in alpha-adrenergic activity

Increased cholinergic sympathetic activity

Increased metabolic rate of exercising muscles, producing intrinsic vasodilation