The heart acts as a pump to ensure that blood is distributed appropriately throughout the body. This is accomplished through a coordinated contraction of the heart muscles causing the blood to be pumped through four cardiac chambers. During the beginning of each cardiac cycle-a period known as diastole-deoxygenated blood from the body fills the right atrium and newly oxygenated blood from the lungs fills the left atrium. The flow of blood then proceeds through the tricuspid valve from the right atrium into the right ventricle or through the mitral valve from the left atrium into the left ventricle. Ventricular contraction-or systole-is the main impetus for moving blood out of the heart. The contraction of the ventricles forces blood from the right ventricle into the pulmonary artery to be delivered to the lungs for oxygenation and delivery of blood to the systemic circulation from the left ventricle through the aortic valve. The cycle completes when blood returns to the heart during the next diastolic period. Within the heart, the four valves maintain unidirectional flow of blood (Figure 10.1) by the coordinated action of leaflets that open and close during each cardiac cycle. In fact, the leaflets within each valve will open and close over 3 billion times in an average lifetime. 1 Therefore, the leaflets must be able to withstand dynamic, cyclic stresses while maintaining the structural integrity that is crucial to their function. The interplay between the forces caused by the fluid mechanics of blood flow and the biomechanical properties of the valve leaflets make the210 tissues that form the heart valve leaflets interesting subjects in mechanobiology. For introductory purposes, we will consider the two separate types of heart valves-the atrioventricular valves and the semilunar valves-and discuss the corresponding characteristics of each class.
