The heart is the pump that provides the energy necessary for transporting the blood through the circulatory system in order to facilitate the exchange of oxygen, carbon dioxide, and other metabolites through the thin-walled capillaries. The contraction of the heart produces changes in pressures and flows in the heart chambers and blood vessels. The mechanical events of the cardiac cycle can be divided into four periods; late diastole, atrial systole, ventricular systole, and early diastole.6 In late diastole, the mitral and tricuspid valves are open and the pulmonary and aortic valves are closed. Blood flows into the heart throughout diastole thus filling the atria and ventricles. The rate of filling declines as the ventricles become distended, and the cusps of the atrioventricular valves start to close. The pressure in the ventricles remains low throughout late diastole.8 In atrial systole, contraction of the atria forces additional blood into the ventricles, but approximately 70 percent of the ventricular filling occurs passively during diastole. Contraction of the atrial muscle that surrounds the openings of the superior and inferior vena cava and pulmonary veins, narrows their orifices and the inertia of the blood moving towards the heart tends to keep blood in the heart. However, there is some regurgitation of blood into the veins during atrial systole.2&5 At the start of ventricular systole, the AV valves close. The muscles of the ventricles initially contract relatively little, but intraventricular pressure rises sharply as the muscles squeezes the blood in the ventricle. This period of isovolumetric ventricular contraction lasts about 0.05 seconds until the pressures in the ventricles exceed the pressure in the aorta and in the pulmonary artery, and the aortic and pulmonary valves (semilunar valves) open. During this isovolumetric contraction, the AV valves bulge into the atria, causing a s...
