The mechanisms by which blood volume influences arterial pressure in situations of reduced excretory capacity are reviewed. The relationships between volume and arterial pressure are discussed as acute, transitional, and long-term phases, recognizing that they are part of a continuum of events leading to a steady state. The acute hydraulic effects, reflex and hormonal responses, and local autoregulatory responses and their interactions are reviewed. Important functional changes of the microcirculation that occur in the transitional phase (1-2 days) are presented. These include increased O2 sensitivity, elevated tone, increased vasomotion, and functional rarefaction, all of which appear to be mediated at the local tissue level. In situation of long-term chronic volume expansion, some of these functional changes evolve into permanent structural changes such as anatomical rarefaction. However, increased vascular sensitivity to oxygen remains. These changes appear to account for a maintained increase in total peripheral resistance with hypertension, which is sustained at relatively normal levels of blood volume and cardiac output.