Physiology of the Microcirculation

Abstract

Functional integrity of the blood vessels is an essential ingredient of a normal circulatory system. In its simplest terms, physiological “normality” of the microcirculation includes the capacity to bring about dynamic changes in the function of the arterioles, the precapillary sphincters, the muscular venules, and the veins, so as to ensure appropriate perfusion of the relatively passive capillaries under the variable conditions of daily life. Arteriolar contraction and relaxation serve both to maintain an adequate diastolic blood pressure for the organism as a whole and to redistribute blood flow to different vascular beds as metabolic requirements dictate. In most circumstances requiring increased tissue perfusion—say, to the muscles during vigorous exercise—dilatation of arterioles and precapillary sphincters increases the flow of blood into an expanded capillary bed. After perfusion of the tissues, the return of blood to the heart is enhanced by tonic contraction of the muscular venules. Venous return is further facilitated by the negative intrathoracic pressure and by the massaging action of skeletal muscular contraction on venous flow directed centrally by venous valves.