Plasma volume changes during head-out water immersion in conscious dogs

Blood volume (⁵¹Cr-erythrocyte dilution method), hematocrit, and arterial (P(a)), central venous (P(v)), plasma colloid osmotic (II(pl)), and interstitial fluid hydrostatic (P(cps), Guyton's capsule method) pressures were measured continuously to determine the kinetics of the transvascular fluid shaft during 100 min of water immersion (WI) at 37° C in six splenectomized dogs. Urine flow increased by 180% above control levels (P < 0.05) by 30 min of WI. Plasma volume (PV) started to increase at 5 min of WI and rose by 7.2% (P < 0.05) above control levels by 35 min of WI, and then it decreased gradually. PV returned to control levels immediately after WI. Plasma protein concentration and II(pl) decreased significantly by 0.2 g/100 ml and 1.2 mmHg, respectively, at 35 min of WI, while plasma osmolality and Na⁺ concentration were constant. P(a) and P(v) increased (P < 0.05) by 25 and 12 mmHg, respectively. Mean capillary pressure, which was calculated from P(a), P(v), and an estimated pre-to-postcapillary resistance ratio of 5-12, increased by 13-14 mmHg while P(cps) increased (P < 0.05) by 17 and 26 mmHg at upper hindlimb and lower forelimb, respectively. The changes in mean capillary pressure and II(pl) tend to promote capillary filtration in WI; however, the greater elevation of P(cps) more than offsets these forces and leads to a net transvascular shift into the plasma compartment.