A computational method developed by LaForte et al., 1992, and Lee, 1994, uses changes in hematocrit, plasma protein concentration, plasma density, or blood density to calculate the fluid restitution from the tissues to the blood, as well as the volume shift from micro- to macrocirculation; this microvascular volume shift is not taken into account by the preexisting methods for computing fluid shifts between the tissues and the blood based on changes in hematocrit. The new computational method was applied to published data obtained in two studies of posture change in humans by Hagan et al., 1978, and by Hinghofer-Szalkay and Moser, 1986. The results support the hypothesis that the microvasculature has the capacity to dilate or constrict so as to absorb a large part of the change in the total blood volume induced by posture change, thus ensuring minimal change in the filling of the macrocirculation.