Our studies on PBV indicate that: (1) TBV and SV may have, in various conditions, an important role in the control of the size of PBV. (2) The effect of MPWP on this volume is a complex one and more often the distending action of an elevated intravascular pressure is opposed and overrun by passive and active phenomena elicited by the intravascular pressure rise itself; and (3) the determination of PBV may be of interest not only in the investigation of the pulmonary circulation but also in the study of the left and right heart hemodynamics.