The lability of blood volume and its relationship to locomotor activity was investigated in two species of snakes Elaphe obsoleta, Say and Crotalus viridis, Rafinesque. Repetitive measurements of blood volume, determined from changes in the specific activity of circulating 51Cr-labelled erythrocytes, indicated that 15 min of locomotor activity reduced blood volume by 21% due to filtration of plasma from capillaries. This magnitude of plasma translocation exceeds that measured in exercising mammals by factors of 2 to 7, depending on the intensity and duration of muscular activity. Activity produced changes in arterial blood pressure, heart rate and haematocrit that were proportionately similar in both species, increasing approximately 57, 85 and 25%, respectively, above resting values. Arterial infusion of norepinephrine increased arterial pressure by more than 100% and reduced blood volumes 8.6 and 17.0% in Elaphe and Crotalus, respectively. These data demonstrate that blood volume varies substantially in relation to activity or the hypertensive state of these reptiles. Thus care must be employed in interpreting or comparing reported measurements of blood volume in these and probably other vertebrates.

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