Blood volume regulation

The maintenance of adequate cardiac output and tissue perfusion is dependent on intravascular blood volume and its adequate return to the heart. Considering the numerous functions of the cardiovascular system in providing adequate flow to the peripheral microcirculation, it is not surprising that there may be conflicts of interest when stresses are placed on the system. Of particular importance are the stresses in which there are increased oxygen demands, decreased oxygen availability and concomitant requirements for temperature regulation. In the context of depleted intravascular blood volume, the splanchnic circulation provides an autologous blood bank until transcapillary refill of the circulation occurs. On the other hand centralisation of blood from the venous capacitance compartment, as seen with acute hypoxia, leads to overfilling of the heart, release of atrial natriuretic peptide and subsequent reduction of the plasma volume by transcapillary efflux. In this review the basic physiology of blood volume, red cell mass and plasma volume regulation will be presented, followed by a consideration of the pathophysiological changes occurring in the context of a variety of clinical settings. Recent insights into neurohormonal aspects of blood volume regulation, the role of the spleen, the interstitial compartment, colloid oncotic pressure, vasomotion and red cell function will be presented.