The plasma volume (PV) decline upon 1.5, 3, 5, 8, 10, 15 and 35 min periods of quiet standing was studied (Hb/Hct) in male volunteers (n = 7). This approach permitted detailed definition of the time-course of the volume change. PV decreased by as much as 8.5 +/- 0.4% (328 +/- 15 mL) after 3 min standing and by no less than 11.7 +/- 0.4% (466 +/- 22 mL) after 5 min. The reduction was 14.3 +/- 0.7, 16.8 +/- 0.8, 17.7 +/- 0.8 and 17.4 +/- 0.9% after 8, 10, 15 and 35 min, or 568 +/- 30, 671 +/- 39, 707 +/- 41 and 691 +/- 44 mL. These data, in conjunction with the 1.5 min experiments, indicated a very rapid approximately 125 mL min-1 fluid loss initially on standing. However, the PV loss showed marked decline with time and was virtually completed within 10 min. Finally, the observation was made that the rate of PV recovery after standing was inversely related to the duration of standing. It is suggested that (a) the transcapillary hydraulic conductivity in the dependent limbs, the predominant targets for fluid filtration on standing, is about 0.010 mL min-1 100 mL-1 mmHg-1 and much greater than indicated previously. However, protective mechanisms restrict rapid fluid loss to early phases of standing. (b) Decrease in PV may contribute importantly to haemodynamic stress and to orthostatic, fainting reactions during short quiet standing. Apparently, PV loss may be equally important as pooling of blood, traditionally regarded as a dominant cause of adverse orthostatic reactions. (c) The duration of standing, as such, may be critical for the rate of PV recovery after standing.

Our previous studies strongly indicate that the capillary filtration coefficient (CFC) in skeletal muscle and skin of man is much larger than previously believed, or about 0.050 ml min-1 100 ml-1 mmHg-1. The hypothesis that this large capillary fluid permeability is a factor of primary importance for plasma volume control was approached. Experimental hypovolaemia induced by lower body negative pressure (LBNP of 70-95 cmH2O) was associated with a rapid net fluid gain from the studied upper arm into the circulation of 0.17 ml min-1 100 ml-1 tissue. The transcapillary driving force for this fluid transfer, probably caused by adrenergic adjustment of vascular resistance, with a decline of capillary pressure, was relatively small, or 1.7 mmHg on average. CFC was instead very high during LBNP, increasing from a control value of 0.054 +/- 0.004 (SE) to no less than 0.097 +/- 0.007 ml min-1 100 ml-1 mmHg-1, probably reflecting an increased number of effectively perfused capillaries. It is suggested that the large capillary fluid permeability in skeletal muscle and skin of man, with large tissue mass and fluid reservoir, may be of great functional importance for plasma volume control after blood loss and also in other (patho)physiological situations. As demonstrated, it can thus permit rapid transfer of large fluid volumes into the circulation and, perhaps of special importance, with only small transcapillary driving force (capillary pressure decline).(ABSTRACT TRUNCATED AT 250 WORDS)

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