Fluid shifts after hemorrhage

Fluid shifts were followed in acutely splenectomized dogs by recording changes in hematocrit, plasma specific gravity, total cell volume (Cr51), and plasma volume (T-1824). The dogs were subjected to two serial bleedings, not sufficient in amount to produce any consistent change in pressure, and then a third bleeding to lower the mean pressure to about 60 mm Hg. The first two bleedings produced small declines in plasma gravity in about half the dogs. The hematocrit changed less, which seemed to be evidence for a recruitment of red cells from some unknown locus. This recruitment was verified by total cell volume measurements. When the added cells were taken into account, the tagged cell technique followed the known loss of blood with an average error of but 1%. After the third hemorrhage most dogs showed a plasma gain of ultrafiltrate. Hematocrit and cell volume changes now often depicted a loss of red cells from the circulation. Agreement between expected and measured plasma volumes was not good. A large part of this discrepancy can be ascribed to a steepening of the disappearance slope, a result of the bleeding itself.