Human Physiology, 14th edition (2016)

468 Chapter 14

in the brain and some other organs by the appropriate responses
of vascular smooth muscle. A decrease in arterial pressure causes
cerebral vessels to dilate, so that adequate blood flow can be
maintained despite the decreased pressure. High blood pressure,
by contrast, causes cerebral vessels to constrict, so that finer ves-
sels downstream are protected from the elevated pressure. These
responses are myogenic; they are direct responses by the vascular
smooth muscle to changes in pressure.

Metabolic Control Mechanisms

Local vasodilation within an organ can occur as a result of
the chemical environment created by the organ’s metabolism.
The localized chemical conditions that promote vasodilation
include (1) decreased oxygen concentrations that result from
increased metabolic rate; (2) increased carbon dioxide con-
centrations; (3) decreased tissue pH (due to CO 2 , lactic acid,
and other metabolic products); and (4) release of K 1 and
paracrine regulators (such as adenosine, nitric oxide, and
others) from tissue cells. Through these chemical changes,
the organ signals its blood vessels that it needs increased oxy-
gen delivery.
The vasodilation that occurs in response to tissue metabo-
lism can be demonstrated by constricting the blood supply to
an area for a short time and then removing the constriction.
The constriction allows metabolic products to accumulate by
preventing venous drainage of the area. When the constric-
tion is removed and blood flow resumes, the metabolic prod-
ucts that have accumulated cause vasodilation. The tissue thus
appears red. This response is called reactive hyperemia. A
similar increase in blood flow occurs in skeletal muscles and
other organs as a result of increased metabolism. This is called
active hyperemia. The increased blood flow can wash out
the vasodilator metabolites, so that blood flow can fall to pre-
exercise levels a few minutes after exercise ends.

14.4 BLOOD FLOW TO THE HEART

AND SKELETAL MUSCLES

Blood flow to the heart and skeletal muscles is regulated

by both extrinsic and intrinsic mechanisms. These mecha-

nisms provide increased blood flow when the metabolic

requirements of these tissues are raised during exercise.

| CHECKPOINT

6a. Describe the relationship between blood flow, arterial

blood pressure, and vascular resistance.

6b. Describe the relationship between vascular

resistance and the radius of a vessel. Explain how

blood flow can be diverted from one organ to

another.

7a. Explain how vascular resistance and blood flow

are regulated by (a) sympathetic adrenergic fibers,

(b) sympathetic cholinergic fibers, and

(c) parasympathetic fibers.

7b. Describe the formation and action of nitric oxide.

Why is this molecule considered a paracrine

regulator?

7c. Define autoregulation and explain how this

process occurs through myogenic and metabolic

mechanisms.

LEARNING OUTCOMES

After studying this section, you should be able to:

8. Explain the mechanisms that regulate blood flow to
   the heart and skeletal muscles.


9. Describe the circulatory changes that occur during
   exercise.

Survival requires that the heart and brain receive an adequate

supply of blood at all times. The ability of skeletal muscles to

respond quickly in emergencies and to maintain continued high

levels of activity also may be critically important for survival.

During such times, high rates of blood flow to the skeletal mus-

cles must be maintained without compromising blood flow to

the heart and brain. This is accomplished by mechanisms that

increase the cardiac output and divert the blood away from the

viscera and skin so that the heart, skeletal muscles, and brain

receive a greater proportion of the total blood flow.

Aerobic Requirements of the Heart

The coronary arteries supply an enormous number of capillaries,

which are packed within the myocardium at a density ranging

from 2,500 to 4,000 per cubic millimeter of tissue. For compari-

son, fast-twitch skeletal muscles have a capillary density of 300

to 400 per cubic millimeter of tissue. As a consequence of its

greater density of capillaries, each myocardial cell is within only

10  m m of a capillary, compared to an average distance of 70  m m

in other organs. The exchange of gases by diffusion between

myocardial cells and capillary blood thus occurs very quickly.

Contraction of the myocardium squeezes the coronary arter-

ies. For this reason, unlike other organs, the blood flow in the

coronary vessels is less during systole than diastole. For exam-

ple, only 15% to 20% of the blood flow through the left ventricle

occurs during systole when a person is not exercising. However,

the myocardium contains large amounts of myoglobin, a pig-

ment related to hemoglobin (the molecules in red blood cells

that carry oxygen). Myoglobin in the myocardium stores oxy-

gen during diastole and releases its oxygen during systole. In

this way, the myocardial cells can receive a continuous supply of

oxygen even though coronary blood flow is temporarily reduced

during systole.

In addition to containing large amounts of myoglobin,

heart muscle contains numerous mitochondria and aerobic

respiratory enzymes. This indicates that—even more than slow-

twitch skeletal muscles—the heart is extremely specialized for