555

CHAPTER

336

Cardiovascular

Regulatory Mechanisms

OBJECTIVES

After studying this chapter, you should be able to:

■

Outline the neural mechanisms that control arterial blood pressure and heart rate,

including the receptors, afferent and efferent pathways, central integrating path-

ways, and effector mechanisms involved.

■

Describe the direct effects of CO

2

and hypoxia on the vasomotor areas in the

medulla oblongata.

■

Describe how the process of autoregulation contributes to control of vascular caliber.

■

Identify the paracrine factors and hormones that regulate vascular tone, their

sources, and their mechanisms of action.

INTRODUCTION

In humans and other mammals, multiple cardiovascular regu-

latory mechanisms have evolved. These mechanisms increase

the blood supply to active tissues and increase or decrease

heat loss from the body by redistributing the blood. In the

face of challenges such as hemorrhage, they maintain the

blood flow to the heart and brain. When the challenge faced is

severe, flow to these vital organs is maintained at the expense

of the circulation to the rest of the body.

Circulatory adjustments are effected by altering the output

of the pump (the heart), changing the diameter of the resistance

vessels (primarily the arterioles), or altering the amount of

blood pooled in the capacitance vessels (the veins). Regulation

of cardiac output is discussed in Chapter 31. The caliber of the

arterioles is adjusted in part by autoregulation (Table 33–1). It

is also increased in active tissues by locally produced vasodila-

tor metabolites, is affected by substances secreted by the endo-

thelium, and is regulated systemically by circulating vasoactive

substances and the nerves that innervate the arterioles. The cal-

iber of the capacitance vessels is also affected by circulating

vasoactive substances and by vasomotor nerves. The systemic

regulatory mechanisms synergize with the local mechanisms

and adjust vascular responses throughout the body.

The terms

vasoconstriction

and

vasodilation

are gener-

ally used to refer to constriction and dilation of the resistance

vessels. Changes in the caliber of the veins are referred to spe-

cifically as

venoconstriction

or

venodilation.

NEURAL CONTROL OF THE

CARDIOVASCULAR SYSTEM

NEURAL REGULATORY MECHANISMS

Although the arterioles and the other resistance vessels are
most densely innervated, all blood vessels except capillaries

and venules contain smooth muscle and receive motor nerve

fibers from the sympathetic division of the autonomic nervous

system. The fibers to the resistance vessels regulate tissue blood

flow and arterial pressure. The fibers to the venous capacitance

vessels vary the volume of blood “stored” in the veins. The in-

nervation of most veins is sparse, but the splanchnic veins are

well innervated. Venoconstriction is produced by stimuli that

also activate the vasoconstrictor nerves to the arterioles. The