Venous return and clinical hemodynamics: how the body works during acute hemorrhage.

Venous return is a major determinant of cardiac output. Adjustments within the venous system are critical for maintaining venous pressure during loss in circulating volume. This article reviews two factors that are thought to enable the venous system to compensate during acute hemorrhage: 1) changes in venous elastance and 2) mobilization of unstressed blood volume into stressed blood volume. We show that mobilization of unstressed blood volume is the predominant and more effective mechanism in preserving venous pressure. Preservation of mean circulatory filling pressure helps sustain venous return and thus cardiac output during significant hemorrhage.