Figure 6. (Top) Pressure-area curves for human thoracic aortas with moderate atherosclerosis (solid line) and severe atherosclerosis (dotted line) at the ages of 40, 60, and 80 yr, respectively. Area = aortic cross-sectional area, in cm²; pressure = aortic pressure, in mmHg; A^max, maximal cross-sectional area at high pressure; P⁰, position of the inflection point on the pressure axis; P¹, steepness of the curve. (Bottom) The matching compliance curves on a semilogarithmic scale, in 10^-3[middle dot] cm²[middle dot] mmHg^-1. Compliance decreases when pressure increases because an aorta can expand elastically only until the collagen fibers in its wall are fully stretched. (Modified from data in Langewouters et al. [  31 ]