Chapter 7: Vascular Control

OBJECTIVES

The student understands the general mechanisms involved in local vascular control:

• Identifies the major ways in which smooth muscle differs anatomically and functionally from striated muscle.

• Lists the steps leading to cross-bridge cycling in smooth muscle.

• Lists the major ion channels involved in the regulation of membrane potential in smooth muscle.

• Describes the processes of electromechanical and pharmacomechanical coupling in smooth muscle.

• Defines basal tone.

• Lists several substances potentially involved in local metabolic control.

• States the local metabolic vasodilator hypothesis.

• Describes how vascular tone may be influenced by endothelin, prostaglandins, histamine, and bradykinin.

• Describes the myogenic response of blood vessels.

• Defines active and reactive hyperemia and indicates a possible mechanism for each.

• Defines autoregulation of blood flow and briefly describes the metabolic, myogenic, and tissue pressure theories of autoregulation.

• Defines neurogenic tone of vascular muscle and describes how sympathetic neural influences can alter it.

• Describes how vascular tone is influenced by circulating catecholamines, vasopressin, and angiotensin II.

• Lists the major influences on venous diameters.

• Describes how control of flow differs between organs with strong local metabolic control of arteriolar tone and organs with strong neurogenic control of arteriolar tone.

The student knows the dominant mechanisms of flow and blood volume control in the major body organs:

• States the relative importance of local metabolic and neural control of coronary blood flow.

• Defines systolic compression and indicates its relative importance to blood flow in the endocardial and epicardial regions of the right and left ventricular walls.

• Describes the unique features and major mechanisms of flow and blood volume control in each of the following systemic organs: skeletal muscle, brain, splanchnic organs, kidney, and skin.

• States why mean pulmonary arterial pressure is lower than mean systemic arterial pressure.

• Describes how pulmonary vascular control differs from that in systemic organs.

INTRODUCTION

Because the body’s metabolic needs are continually changing, the cardiovascular system must continually make adjustments in the diameter of its vessels. The purposes of these vascular changes are (1) to efficiently distribute the cardiac output among tissues with different current needs (the job of arterioles) and (2) to regulate the distribution of blood volume and cardiac filling (the job of veins). In this chapter, we discuss our current understanding of how all this is accomplished.

VASCULAR SMOOTH MUSCLE

Although long-term adaptations in vascular diameters may depend on remodeling of both the active (i.e., smooth muscle) and passive (i.e., structural, connective tissue) components of the vascular wall, short-term vascular diameter adjustments are made by regulating the contractile activity of vascular smooth muscle cells. These contractile cells are present in the walls of all vessels except capillaries. The task of the vascular smooth muscle is unique, because to maintain a certain vessel diameter in the face of the continual distending pressure of the blood within it, the vascular smooth muscle must be able to sustain active tension ...