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.

Numerous studies have focused on alterations in plasma volume (PV) on interventions like quiet standing, exercise, or heat stress. However, no method seems capable of truly estimating the PV alteration. Therefore, an attempt was made to validate commonly used indexes of PV changes. Quiet standing was used to cause graded PV reductions estimated from hemoglobin and hematocrit (Hb/Hct) and from serum concentrations of total protein, albumin, and "large proteins" (LP; total protein minus albumin). Results indicated the following. 1) Hb/Hct, with the merit that the marker (erythrocyte) stays within the circulation, reflect accurately a small-to-moderate PV loss (/=15-20% of control), however, F-cell shift can cause Hb/Hct to underestimate the response by up to 25-30%. 2) Albumin and total protein underrate PV loss due to protein escape (mainly albumin) from the circulation. 3) LP also underestimate the PV decline due to protein escape but can often predict large PV reductions clearly better than Hb/Hct. 4) Prolonged standing can lead to pronounced 25% PV decline.

Seven healthy males were exposed to quiet standing (15 min) after supine rest. Alterations in the total mass of plasma proteins were analysed from changes in plasma volume (PV; determination of control PV and subsequently of induced per cent PV changes using Hb/Hct) and protein concentration as revealed in arterial blood collected after standing. This approach adopted the concept that valid data on overall circulatory haemoconcentrations prevailing on standing can only be reached when blood is sampled on resumption of the recumbent posture, whereas conventional sampling from the standing subject provides erroneous information. The PV reduction on standing averaged 649 +/- 65 mL (16.9 +/- 1.0%). There were very similar net decreases in plasma (serum) total protein (7.6 +/- 0.8 g) and albumin (7.8 +/- 0.9 g). These findings permitted the following main conclusions of physiological and methodological pertinence: (1) Quiet standing leads to a clear-cut net decrease in the plasma protein content predominantly confined to albumin, in all probability via convection secondary to PV loss by filtration in dependent regions. (2) It is suggested that the albumin loss reflects a quite high capillary macromolecular permeability in the dependent limbs on standing preferentially confined to skin/subcutaneous tissues. (3) The albumin loss implies that plasma concentration changes of neither albumin nor of total protein can be used to describe the PV loss on standing. However, concentration changes of the plasma globulin fraction as a whole, expressed by the difference (total protein-albumin), seem to reflect PV alterations approximately.