Assessing microvascular volume change and filtration from venous hematocrit variation of canine liver and lung.

The volume increase of canine liver after 1 min of a 10 mmHg elevation in hepatic venous pressure has been reported as 251 ml/kg tissue. An analysis of the transient hematocrit variation in hepatic venous blood indicated that 16% of the volume change results from transcapillary filtration, 72% from microvascular expansion, and 12% from macrovascular expansion. In the analysis, we first used the temporal change of the liver volume to determine the time course of the filtration and microvascular and macrovascular volume change. We next deduced, for a permeable microcirculation with a microvascular hematocrit lower than the feed hematocrit (the Fahraeus effect), how the filtration and microvascular volume change (MVC) produce a hematocrit variation in the blood leaving microcirculation. By accounting for the dispersion of the blood flow, the analysis predicted a hematocrit variation in the hepatic venous blood that matched well with the measured variation over the 1-min course of experiment. A reasonable fit with the hematocrit variation of pulmonary blood also was obtained for experiment with an 8 mm/Hg increase in the arterial and venous pressure perfusing the canine left lower lung lobe. The tissue and vascular volume increase at 1 min was 149 ml/kg tissue with 4% as a result of filtration, 41% as a result of microvascular expansion, and 55% as a result of macrovascular expansion. The large MVCs from the hepatic and pulmonary circulation indicate their microcirculations function as a reservoir in controlling blood volume redistribution.