Inter-compartmental body-fluid distribution is contingent upon posture
, exercise state and environmental temperature. This investigation aim
ed at quantifying the distribution of intra- and extravascular fluid v
olumes during postural manipulations. Fluid shifts were measured in ei
ght males utilizing a simultaneous, radionuclide dilution technique, i
n which radioiodinated serum fibrinogen, radiochromated erythrocytes,
radiobromine and tritiated water were used to measure plasma, red cell
, extracellular and total body water volumes. Subjects were exposed to
three postural changes [seated (control), supine and standing] for 30
min at an air temperature of 22.0 degrees C, with each posture separa
ted by 30 min seated rest. Total body water content remained stable th
roughout postural changes (P = 0.842). Relative to seated volumes, BV
increased by 89 mL when supine, and decreased by 406 mL while standing
(P = 0.003), with such shifts being primarily a result of plasma move
ment (P = 0.011). Red cell volume changes were not significant. Vascul
ar fluid lost during standing was filtered into the interstitial compa
rtment (P = 0.014), with the extracellular and intracellular volumes r
emaining unaffected (P = 0.271 and P = 0.800, respectively). These obs
ervations confirmed the influence of posture on inter-compartmental bo
dy-fluid distribution. The intravascular fluid loss when standing was
caused by the filtration of plasma into the interstitium, while, durin
g supine rest, intravascular volume increased, reflecting fluid flux f
rom the interstitium to the circulation.