Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 473

after a few minutes. For these reasons, the cerebral blood flow

is held remarkably constant at about 750 ml per minute. This

amounts to about 15% of the total cardiac output at rest.

Unlike the coronary and skeletal muscle blood flow, cere-

bral blood flow is not much influenced by sympathetic nerve

activity under normal conditions. Only when the mean arterial

pressure rises to about 200 mmHg do sympathetic nerves cause

a significant degree of vasoconstriction in the cerebral circula-

tion. This vasoconstriction helps to protect small, thin-walled

arterioles from bursting under the pressure, and thus helps to

prevent cerebrovascular accident (stroke).

In the normal range of arterial pressures, cerebral blood flow

is regulated almost exclusively by local intrinsic mechanisms—

a process called autoregulation, as previously mentioned. These

mechanisms help to ensure a relatively constant blood flow

despite changes in systemic arterial pressure. The autoregulation

of cerebral blood flow is achieved by both myogenic and meta-

bolic mechanisms.

Myogenic Regulation

Myogenic regulation occurs when there is variation in sys-

temic arterial pressure. When the blood pressure falls, the cere-

bral arteries automatically dilate; when the pressure rises, they

constrict. This helps to maintain a constant flow rate during the

normal pressure variations that occur during rest, exercise, and

emotional states.

The cerebral vessels are also sensitive to the carbon

dioxide concentration of arterial blood. When the carbon

14.5 Blood Flow to the Brain and Skin

Intrinsic control mechanisms help maintain a relatively

constant blood flow to the brain. Blood flow to the skin, by

contrast, can vary tremendously in response to regulation

by sympathetic nerve stimulation.

Table 14.7 | Cardiovascular Changes During Moderate Exercise

Variable Change Mechanisms

Cardiac output Increased Increased cardiac rate and stroke volume

Cardiac rate Increased Increased sympathetic nerve activity; decreased activity of the vagus nerve

Stroke volume Increased Increased myocardial contractility due to stimulation by sympathoadrenal system;

decreased total peripheral resistance

Total peripheral resistance Decreased Vasodilation of arterioles in skeletal muscles (and in skin when thermoregulatory

adjustments are needed)

Arterial blood pressure Increased Increased systolic and pulse pressure due primarily to increased cardiac output; diastolic

pressure rises less due to decreased total peripheral resistance

End-diastolic volume Unchanged Decreased filling time at high cardiac rates is compensated for by increased venous

pressure, increased activity of the skeletal muscle pump, and decreased intrathoracic

pressure aiding the venous return

Blood flow to heart and muscles Increased Increased muscle metabolism produces intrinsic vasodilation; aided by increased cardiac

output and increased vascular resistance in visceral organs

Blood flow to visceral organs Decreased Vasoconstriction in digestive tract, liver, and kidneys due to sympathetic nerve stimulation

Blood flow to skin Increased Metabolic heat produced by exercising muscles produces reflex (involving hypothalamus)

that reduces sympathetic constriction of arteriovenous shunts and arterioles

Blood flow to brain Unchanged* Autoregulation of cerebral vessels, which maintains constant cerebral blood flow despite

increased arterial blood pressure

*There can be slight changes in cerebral blood flow (see text), but the extent of these changes is buffered by autoregulation due to myogenic control mechanisms.

LEARNING OUTCOMES

After studying this section, you should be able to:

10. Explain how blood flow to the brain is regulated.


11. Explain how blood flow to the skin is regulated.

The examination of cerebral and cutaneous blood flow is a
study in contrasts. Cerebral blood flow is regulated primarily
by intrinsic mechanisms; cutaneous blood flow is regulated by
extrinsic mechanisms. Cerebral blood flow is relatively con-
stant; cutaneous blood flow exhibits more variation than blood
flow in any other organ. The brain is the organ that can least
tolerate low rates of blood flow, whereas the skin can tolerate
low rates of blood flow better than any other organ.

Cerebral Circulation

When the brain is deprived of oxygen for just a few seconds, a
person loses consciousness; irreversible brain injury may occur