Extracellular fluid and plasma volumes during water immersion in nephrectomized dogs

Extracellular fluid volume (ECF, [¹²⁵I]iothalamate space), blood volume (BV, ⁵¹Cr-labeled erythrocyte space), and hematocrit were measured continuously to study the kinetics of fluid movements between intracellular, interstitial, and plasma compartments during water immersion (WI) at 38°C in seven splenectomized and acutely nephrectomized dogs. ECF and plasma volume (PV) increased linearly during WI by 10 ± 2 ml/kg (4% of initial ECF volume, P < 0.05) and 12 ± 2 ml/kg (33% of initial PV, P < 0.05), respectively, above the control level by 120 min of WI. We estimate that 83% of the fluid entering the intravascular compartment is derived from the intracellular space at 120 min of WI. The results of this study indicate that WI leads to a sustained fluid movement of intracellular fluid toward the intravascular compartment. The increase in interstitial hydrostatic pressure (wick method) by 28.5 mmHg from the control level at 5 min of WI in response to the external water pressure exceeds the increase in mean capillary pressure by 10-11 mmHg relative to the control level. We postulate that this negative hydrostatic pressure gradient across the capillary wall leads to an increase in PV during WI.