ABSTRACT
The heart has four valves whose main function is to control the direction of blood žow through the heart, permitting forward žow and preventing back žow. On the right side of the heart, the tricuspid and pulmonary valves regulate the žow of blood that is returned from the body to the lungs for oxygenation. The mitral and aortic valves control the žow of oxygenated blood from the left side of the lungs back to the body. The aortic and pulmonary valves allow blood to be pumped from the ventricles into arteries on the left and right sides of the heart, respectively. Similarly, the mitral and tricuspid valves lie between the atria and ventricles of the left and right sides of the heart, respectively. The aortic and pulmonary valves open during systole when the ventricles are contracting and close during diastole when the ventricles are Ÿlling with blood that enters through the open mitral and tricuspid valves. During isovolumic contraction and relaxation, all four valves are closed (Figure 7.1a). In Figure 7.1b and c, the pressure and volume information for the left heart is shown to illustrate the timings of a normal cardiac cycle. The details of the cardiac events and the opening of the valves are included in Section 3.3.
