A generalisation of the equation used bySalazar (1964) to fit the pressure-volume relationship of normal human lungs between FRC and TLC is found to fit the pressure-volume relationship of a variety of circulatory elements. The equation contains only two fundamental physiologic parameters: one a compliance constant k and the other a maximum element volume Vm. Fits of this equation to published atrial, ventricular, veneous, and arterial data are presented. Combined with the Poiseulle relationship, this equation is shown to fit data of flow resistance against transmural pressure for the pulmonary vascular system of a dog.