Transient blood/plasma density and hematocrit effects after passive (sitting) thermoneutral water immersion in men

At the beginning and with ceasing of suction which is exerted upon the lower parts of the body, we have recently discovered transient hemodilution/hemoconcentration effects by use of the high-precision so-called mechanical oscillator technique. Since transient effects are useful for biological systems analysis, we addressed the question if such transients in blood and plasma mass density also occur with and immediately after water immersion in humans. Water immersion (WI) has been used as an analog of microgravity conditions. The effects on blood volume depend on the test conditions, particularly body position before immersion, and the time of measurement. Consequently, hemodilution has been found as an early effect of WI in many but not all studies. If test persons are immersed after sitting or standing on air, WI counteracts postural hemoconcentration and consistently produces a decrease in hematocrit, hemoglobin, and plasma protein concentration and, hence, plasma and blood mass density. In this paper we present evidence that, like with termination of lower body suction, there might be transient hemoconcentration in the first minutes of thermoneutral WI in a sitting position in men, but no evidence could be obtained for any counterdirected transient hemodilution when test subjects are passively brought out (in sitting position) from the water, as was found with starting lower body suction (in supine position).