Physiological Reviews

Effects on thermal stress and exercise on blood volume in humans

M. H. Harrison


The opening remarks of this review emphasize, somewhat pessimistically, the serious disagreements, going back over a hundred years, between different investigators regarding effects of exercise and thermal stress on intravascular volume. However, the concluding remarks may legitimately assume a more optimistic, positive aspect, and a certain degree of order and sense may be brought to what had superficially appeared to be the chaos and confusion of conflicting observations and conclusions. Krebs and Meyer's (162) "marked differences in findings between one investigator and another," and Senay's (245) comment 77 years later that "disagreements abound" can now be seen as an inevitable consequence of the widely differing experimental protocols and procedures that have been adopted. Of particular importance in this respect is the failure to standardize conditions before subjects are exposed to thermal stress or begin to exercise--notably in terms of posture and environmental temperature; both may profoundly influence blood volume responses, quantitatively and qualitatively. Then there is subject status; physical fitness, heat acclimatization, and dehydration are important factors contributing to the variability of individual responses to thermal stress and exercise. With the causes of disagreement at least identified, it is now possible to answer the question posed in section I: Is thermal- and exercise-induced hemoconcentration fact or fantasy? Undeniably it is fact, but only under certain circumstances. For example, in resting subjects reduction in blood volume is associated only with high environmental temperatures above the upper limit of the prescriptive zone; within the upper part of the zone, blood volume commonly increases. If heat exposure is preceded by a control period in a cool environment, transient hemodilution is generally observed, followed by hemoconcentration after entry into the heat as skin temperature rises, cutaneous blood flow increases, and sweating begins. Exercise too causes hemoconcentration, but only if the exercise is performed in a supine or seated, not in an upright (standing), position. Hence cycling is almost always associated with a reduction in plasma volume, as is arm exercise and swimming. Bench stepping, walking, and running, on the other hand, are associated with an extremely variable intravascular volume response. If allowance is made for the reduction in plasma volume that occurs when moving to an upright position from a supine or seated position, the initial rapid hemoconcentration seen at the onset of cycling exercise is absent with bench stepping, walking, and running.(ABSTRACT TRUNCATED AT 400 WORDS)