Mean arterial pressure plotted against flow (cardiac output) in the systemic (A) and pulmonary (B) circulations. The blue curve in each panel represents the normal condition of the circulation, and the red curve a hypertensive condition. (A) In the systemic circulation, the mean pressure (P)-flow (Q) plot is well described as a linear (Ohmic) relationship. The two points identified (open circles) show a normal cardiac output and a reduced cardiac output, respectively, in the hypertensive condition. At each of these cardiac outputs, it is clear that the ratio of P to Q is the same and therefore can be used to easily characterise the resistance of the systemic circulation. (B) In the pulmonary circulation, the plot of mean pressure against flow is curvilinear with an intercept on the pressure axis that is equal to left atrial pressure. The blue curve represents a normal pressure flow curve (healthy lung), while the red curve represents pressure flow curve in the presence of hypoxic pulmonary hypertension. The two points identified (open circles) show a normal cardiac output and a reduced cardiac output, respectively, in the hypertensive condition. At each cardiac output the pulmonary vascular resistance, (Ppa-LAP)/Q, is illustrated as the slope of the straight dashed line. Even though the two points are each on the same pressure flow curve, the calculated pulmonary vascular resistance is different at the different cardiac outputs. Psa, systemic arterial pressure (mean); Ppa, pulmonary arterial pressure (mean); Q, cardiac output (flow).