Ganong's Review of Medical Physiology, 23rd Edition

(Chris Devlin) #1
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
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