CHANGE OF PLASMA VOLUME IN MAN DURING IMMERSION IN A THERMO-INDIFFERENT WATER BATH

1. Urine flow, change in weight, plasma volume (Evan's Blue) and hematocrit were measured in 24 subjects before and after immersion up to the chin in a thermo-indifferent bath of 8 hours' duration. 2. The diuresis of immersion [6] was accompanied by a weight loss of approximately 1,2 kg and a reduction in plasma volume in the order of 500 ml (14%). 3. The present results show considerable scatter and do not allow a precise correlation between weight loss and the mean reduction in plasma volume. It can, however, safely be stated that the shrinkage of plasma volume amounting to about 40% of the weight loss appears disproportionally great when considering that the plasma volume represents only one quater of the total extracellular space from which the fluid loss is derived. This observation may tentatively be explained by a reflex readjustment of pre- and postcapillary resistance in favor of outward capillary filtration during immersion. 4. The diuresis following expansion of the intrathoracic blood volume has been interpreted as an expression of a control mechanism arising in the atrial receptor zones for the reflex regulation of total blood volume [cf. 5]. This concept seems now fully justified after it could be demonstrated directly that expansion of the intrathoracic blood volume does indeed cause a significant (p<0.005) reduction in plasma volume even though this effect cannot be explained as a direct consequence of the diuresis. 5. Orthostatic weakness at the end of immersion was observed. It may be considered as a natural correlate of the reduction of blood volume. © 1969 Springer-Verlag.