Figure 18-6 Pressure-volume loop. Point a depicts the start of isovolumic contraction. The aortic valve opens at point b, and ejection of blood follows (b→c). The mitral valve opens at d, and ventricular filling ensues. External work is defined by a, b, c, and d and internal work by e, d, and c. The pressure-volume area is the sum of external and internal work. (From Opie LH: Ventricular function. In The Heart. Physiology from Cell to Circulation, 3rd ed. Philadelphia, Lippincott-Raven, 1998, pp 343–389.)