Fluid and protein shifts after postural changes in humans.

With the use of a new mass density detection method, blood density (BD), plasma density (PD), and erythrocyte density (ED) were measured during different postures in euhydrated humans. ED remained stable under various head-up tilt (HUT) procedures. Changes of PD and BD mirrored the time course of hemodilution and hemoconcentration. The mass density of the shifted fluid (FD) was virtually identical for the outward filtrated fluid when upright and for the inward movement of fluid when supine; it averaged 1,008.3 g/l (37 degrees C), which is equivalent to a protein concentration of 30 g/l. PD and BD increased almost linearly with increasing angles of tilt. A stepwise increase from supine position to 90 degrees HUT within 2 h resulted in a mean plasma volume (PV) loss of 18%. Repeated sudden HUT to 70 degrees for 45 min, separated by 45-min supine (0 degree) periods, resulted in slightly reduced PV shifts which averaged -14% during 45 min of quiet HUT. The results indicate that erythrocyte volume remains constant after assuming different HUT positions in euhydrated subjects; a net loss of intravascular protein occurs during postural hemoconcentration, and protein gain occurs with postural hemodilution; the protein concentration of the shifted fluid resembles that of whole-body lymph; and microsample densitometry on blood and plasma is an accurate technique for measuring dynamic responses of rapid blood-volume changes in humans.