Pulmonary vascular resistance.

where P represents the pressure gradient between the pulmonary artery and the pulmonary vein, F, the blood flow per unit time, and R. the vascular resistance. For a given pressure gradient and flow rate, R will be determined, among other things, by the physical dimensions of the chaniiels that the blood traverses. In a rigid hydraulic system this formula applies over a wide range of flow and pressure, and R remains constant. In the living subject, however, numerous mechanisms act on the vessels to change their caliber, hence R becomes a variable. The effect of passive distention on the vascular resistance has been studied recently by several investigators on various animal preparations. Thus, Phillips, Brind, and Levy' found that for a constant pressure gradient between the femoral artery and vein, the flow in the denervated hind leg of the anesthetized dog increased with increasing intraluminal pressures. They concluded that the resistance