Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 475

requires less blood. As a result of exposure to extreme cold,

however, blood flow to the skin can be so severely restricted

that the tissue dies—a condition known as frostbite. Blood

flow to the skin can vary from less than 20 ml per minute at

maximal vasoconstriction to as much as 3 to 4 L per minute at

maximal vasodilation.

As the temperature warms, cutaneous arterioles in the

hands and feet dilate as a result of decreased sympathetic

nerve activity. Continued warming causes dilation of arte-

rioles in other areas of the skin. If the resulting increase in

cutaneous blood flow is not sufficient to cool the body, sweat

gland secretion may be stimulated. Perspiration helps cool the

body as it evaporates from the surface of the skin. The sweat

glands also secrete bradykinin, a polypeptide that stimulates

vasodilation.

In usual ambient temperatures, the cutaneous vascular

resistance is high and the blood flow is low when a person

is not exercising. In the pre-exercise state of fight or flight,

sympathetic nerve activity reduces cutaneous blood flow still

further. During exercise, however, the need to maintain a deep-

body temperature takes precedence over the need to maintain

an adequate systemic blood pressure. As the body temperature

rises during exercise, vasodilation in cutaneous vessels occurs

together with vasodilation in the exercising muscles.

This can cause an even greater lowering of total periph-

eral resistance during exercise. However, the mean arterial

pressure is still high during exercise because of the increased

cardiac output. If a person exercises in hot and humid weather,

especially if restrictive clothing prevents adequate evaporative

cooling of the skin (from perspiration), the increased skin tem-

perature and resulting vasodilation can persist after exercise

has ceased. If the total peripheral resistance remains very low

as the cardiac output declines toward resting values, the blood

pressure may fall precipitously; people have lost consciousness

and even died as a result.

Changes in cutaneous blood flow occur as a result of changes

in sympathetic nerve activity. Because the activity of the sympa-

thetic nervous system is controlled by the brain, emotional states,

acting through control centers in the medulla oblongata, can

affect sympathetic activity and cutaneous blood flow. During fear

reactions, for example, vasoconstriction in the skin, along with

activation of the sweat glands, can produce a pallor and a “cold

sweat.” Other emotions may cause vasodilation and blushing.

Figure 14.23 Circulation in the skin showing

arteriovenous anastomoses. These vessels function as

shunts, allowing blood to be diverted directly from the arteriole to

the venule, and thus to bypass superficial capillary loops.

Epidermis

Dermis

Vein

Artery

Arteriole

Venule

Arteriovenous

anastomosis

Capillary loop

| CHECKPOINT

10a. Define the term autoregulation and describe how this

process is accomplished in the cerebral circulation.

10b. Explain how hyperventilation can cause dizziness.

11. Explain how cutaneous blood flow is adjusted to
    maintain a constant deep-body temperature.

14.6 BLOOD PRESSURE

The pressure of the arterial blood is affected by the blood

volume, total peripheral resistance, and the cardiac rate.

These variables are regulated by a variety of negative

feedback control mechanisms to maintain homeostasis.

Arterial pressure rises and falls as the heart goes through

systole and diastole.

LEARNING OUTCOMES

After studying this section, you should be able to:

12. Explain how blood pressure is regulated.


13. Describe how blood pressure is measured.

Resistance to flow in the arterial system is greatest in the arteri-

oles because these vessels have the smallest diameters. Although

the total blood flow through a system of arterioles must be equal

to the flow in the larger vessel that gave rise to those arterioles,

the narrow diameter of each arteriole reduces the flow in each

according to Poiseuille’s law. Blood flow and pressure are thus

reduced in the capillaries, which are located downstream of the

high resistance imposed by the arterioles. (The slow velocity

of blood flow through capillaries enhances diffusion across the