Blood volume and body haematocrit of rats native to a simulated altitude of 3500 m

Circulating blood volume (BV) as the sum of circulating red cell volume (RCV) and plasma volume (PV) was estimated in rats native to a simulated altitude of 3500 m (“natives”), in rats born at sea level and later in life transferred to the simulated high altitude (“newcomers”), and in control sea-level rats. RCV per kg body weight (b.w.) was significantly larger in both “newcomers” and “natives” than in controls. PV per kg b.w. was in the “newcomers” insignificantly and in the “natives” significantly smaller than in the controls. BV per kg b.w. in both high altitude groups tended to be larger than in controls but the difference was not significant. Arterial haematocrit (Ahct) in the “newcomers” was significantly higher than in the controls, and in the “natives” significantly higher than in both other groups. Body haematocrit (the ratio of RCV and BV in per cent) was smaller than Ahct in all groups; this was more pronounced in the “newcomers” than in the controls and even more so in the “natives”. Apparently the haematocrit in the minute vessels of the organs of animals exposed to chronic hypoxic hypoxia increases much less than might be expected from changes of the Ahct. An attempt was made to evaluate the possible error of the more commonly used method of estimating BV, when only RCV, or only PV, is measured, and BV and its complementary fraction are calculated from arterial or venous haematocrit. When, in our results, BV was calculated from RCV and Ahct, the absolute values and also the differences between groups were somewhat underestimated. When BV was calculated from PV and Ahct, the BV itself, and particularly the differences between groups, were overestimated quite considerably. It is suggested that the only safe way to estimate BV is to measure RCV and PV separately.