Ganong's Review of Medical Physiology, 23rd Edition

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 pathways,
and effector mechanisms involved.
■ Describe the direct effects of CO2 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 regulatory
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
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
C H A P T E R
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arterioles is adjusted in part by autoregulation (Table 33–1). It
is also increased in active tissues by locally produced vasodilator
metabolites, is affected by substances secreted by the endothelium,
and is regulated systemically by circulating vasoactive
substances and the nerves that innervate the arterioles. The caliber
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 generally
used to refer to constriction and dilation of the resistance
vessels. Changes in the caliber of the veins are referred to specifically
as venoconstriction or venodilation.
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 innervation
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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