The pulmonary circulation is a high-flow and low-pressure circuit. The functional state of the pulmonary circulation is defined by pulmonary vascular pressure-flow relationships conforming to distensible vessel models with a correction for hematocrit. The product of pulmonary arterial compliance and resistance is constant, but with a slight decrease as a result of increased pulsatile hydraulic load in the presence of increased venous pressure or proximal pulmonary arterial obstruction. An increase in left atrial pressure is transmitted upstream with a ratio ≥1 for mean pulmonary artery pressure and ≤1 the diastolic pulmonary pressure. Therefore, the diastolic pressure gradient is more appropriate than the transpulmonary pressure gradient to identify pulmonary vascular disease in left heart conditions. Exercise is associated with a decrease in pulmonary vascular resistance and an increase in pulmonary arterial compliance. Right ventricular function is coupled to the pulmonary circulation with an optimal ratio of end-systolic to arterial elastances of 1.5-2.