We investigated the effect of a 10% cyclic blood volume change with a period of 2 or 4 min to study the short-term control of blood volume. In experiments with pentobarbital-anesthetized rabbits, the blood density variation over a 2-min cycle is 0.94 +/- 0.04 (SE) g/l, and the plasma density variation is 0.17 +/- 0.04 g/l. The plasma density variation could result from a fluid restitution from the extravascular space (with a density 1,005 g/l), with a volume equal to 14% of the withdrawn blood volume. This restitution cannot account, however, for the entire observed density change in arterial blood. Because of the Fahraeus effect in microvascular flow, a shift in blood volume from the microvasculature is another mechanism that could lead to a decrease in the density of arterial blood. An analysis of the blood and plasma density variations indicates that a blood volume (49% of the shed volume) is shifted from the micro- to macrocirculation. This volume compensation by fluid restitution and volume shift acts to minimize the effect of hemorrhage on the filling of the venous system. We found that the blood density waveform parallels the change in blood volume. When the blood volume change reverses its direction, the density change also reverses direction with a time delay less than 8 s. The blood density variations are not altered by bilateral vagotomy or its combination with hexamethonium (a sympathetic ganglionic blocker). These observations of anesthetized rabbits indicate that the short-term compensation is primarily due to the volume shift from the microcirculation and is not regulated by humoral or neural mechanisms but by local mechanisms such as autoregulation and the passive response due to changes in microvascular pressure.
