The activity of the sympathetic nervous system provides for centrally coordinated control of vascular tone (see Chapters 27 and 28) and serves to maintain constant arterial blood pressure. However, there are additional mechanisms that regulate vascular tone. Local mechanisms arise either from within the blood vessel itself, or from the surrounding tissue. These local mechanisms function primarily to regulate flow. Regulation tends to be most important in organs that require a constant blood supply, or in which metabolic needs can increase markedly (brain, kidneys, heart, skeletal muscles).
Local mechanisms have two main functions. First, under basal conditions they regulate local vascular resistance to maintain the blood flow in many types of vascular beds at a nearly constant level over a large range of arterial pressures (50–170 mmHg). This tendency to maintain a constant flow during variations in pressure is termed autoregulation. Autoregulation prevents major fluctuations in capillary pressure which would lead to uncontrolled movement of fluid into the tissues.
Second, when a tissue requires more blood to meet its metabolic needs, local mechanisms cause dilatation of resistance vessels and upregulate blood flow. This response is referred to as metabolic vasodilatation. Autoregulation may persist under these conditions, but is adjusted to maintain flow around the new set point.
Autoregulation
Figure 23a illustrates the phenomenon of autoregulation. When the upstream pressure driving blood through a resistance artery is suddenly increased to P1 from its starting level P0, the artery dilates passively and blood flow immediately rises as predicted by Poiseuille’s law (arrow 1). However, within a minute the resistance artery responds to the increased pressure by actively constricting (arrow 2), thereby bringing blood flow back down towards its initial level (solid line). Similarly, decreases in upstream pressure cause rapid compensatory dilatations to maintain flow. Autoregulation ensures that under basal conditions blood flow remains nearly constant over a wide range of pressures, and is particularly important in the heart, the brain and the kidneys. Two homeostatic negative feedback mechanisms are involved: the myogenic response and the effect of vasodilating metabolites (Figure 23b).
The myogenic response is probably controlled by sensors in the plasma membrane of vascular smooth muscle cells which react to changes in pressure and/or stretch. There is increasing evidence that integrins, membrane-spanning proteins that act as adhesion molecules linking the extracellular matrix with the cytoskeleton (see Chapter 4), may constitute one class of such sensors. Integrins have two subunits (i.e. they are dimers
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