Microcirculation as a Dynamic Entity

Abstract

A substantial array of data on the molecular level, together with the application of new immunological probes, have reinforced the concept that the microcirculatory segment of the vascular system contributes in a significant way to the pathophysiologic manifestations of a number of major systemic disorders such as hypertension, diabetes, sepsis and shock. The design features and functional attributes which enable this segment of the vascular system to operate as an independent organic entity have intrigued investigators for many years.

The peripheral circulation is concerned with two levels of circulatory homeostasis — to maintain the systemic blood pressure in a prescribed range and to ensure the stability of the internal environment [the tissue interstitium] in the face of the continually shifting range of bodily activities. The capacity of the microvascular subdivisions of the tree to act as an independent organic entity is a consequence of structural as well as functional features.

The point where the supply arteries for the various organs of the body enter the tissue proper can be taken as the dividing line between the macro and micro-circulation. The paramount objective becomes the local distribution of blood in accord with changing needs of the parenchymal cells. In the ensuing hierarchical branching, the arterial vessels begin to interconnect with one another to form an arcade type of network from which the majority of the terminal arterioles are distributed as side-arm offshoots, a situation that in essence provides for a variable in-parallel circuit.

There is considerable non-uniformity in the design pattern and control features among the various tissues depending upon whether the tissue has a relatively stable level of metabolism as opposed to tissues which exhibit extremes in metabolic requirements when activated. Most terminal vascular beds exhibit intrinsic features that enable them to operate as an independent functional unit.

Key factors in the maintenance of tissue homeostasis are the selective distribution of the available volumetric flow in conjunction with the modulation of pressure levels relative to the blood-tissue exchange process. The latter involves an interplay of factors affecting volumetric flow delivery and the number of capillary vessels that are perfused so as to ensure a surface area compatible with an effective blood-tissue exchange. The feedback controls in this regard exert their effects through a local resetting of smooth muscle cell tone, an interaction of shear rate with the lining endothelial cells, as well as blood-borne and parenchymal cell cytokines.