Blood volume,hematocrit and hemorheology: The interrelationships

Abstract

There is not only good evidence that hematocrit alterations are intimately related to cardiovascular control, but there is also a wide clinical spectrum of relative anemia and relative polycythemia which are predominantly manifestations of interactions between the cardiovascular system and blood volume. For patients whose hematocrit levels remain within the normal range, even though there may be significantly fluctuating, few questions are usually asked. However, if the changes in plasma volume result in a patient developing relative anemia or relative polycythemia greater attention is taken as the patient has entered the “abnormal” hematocrit range. It is thus important to consider an individual's hematocrit in relation to cardiovascular and blood volume control. In most circumstances an alteration in hematocrit is an appropriate physiological adaptation. In other circumstances the changes may be maladaptive or if there are conflicted stresses on oxygen transport and/or other defects present (eg arterial, lung or cardiovascular disease) the changes in hematocrit to one stimulus may result in secondary undesirable effects which are not “part of the evolutionary plan”. Lastly, there may be circumstances when the organism is not able to respond appropriately to a stress due to disease (eg marrow failure, shock, cardiorespiratory disease etc) and in these circumstances the clinician should consider what response “nature” would like to achieve, but is unable.