Volume control during periodic changes of blood volume in the alert rat

Blood (3.4-13.5% of blood volume) was pumped in and out of the circulation of rats at different rates and period lengths during continuous measurements of blood conductivity (reciprocally related to hematocrit) and arterial pressure. Hct followed the same zig-zag course as the induced changes of blood volume in every case, indicating that fluid shifts (delta v) between interstitium and intravascular space closely follow blood volume changes. As the hct increase during reinfusion was not as great as the preceding decrease, hct dropped continuously during the 20-90 minutes of experimentation, so that a final volume increase (delta v) by about 4% was calculated, which was confirmed by a corresponding decrease of plasma protein concentration. Both final delta v and delta v during periodic volume change (delta % B.V.) were greater when arterial pressure dropped. delta v was directly related to delta % B.V. but not to its rate of change. Heart rate dropped slightly at the end of the reinfusion periods, whereas it rose to control at the end of the withdrawal periods. The results were regarded as evidence of blood volume regulation proportional to the absolute volume of blood lost in non-hypotensive hemorrhage.