Cardiac Cycle

The heart is actually composed of two separate pumps, one on the right side that supplies the pulmonary circulation and one on the left that supplies the systemic circulation. The principles that regulate flow into and out of the heart's ventricles are somewhat different from the pump in Fig. 1, which has a fixed stroke volume. The temporal relationships between ventricular contraction and blood flow in the heart are illustrated in Fig. 2. When the ventricular muscle is relaxed, a period referred to as diastole, pressures within the veins and atria exceed the pressures in the ventricles, causing blood to flow into the ventricles through the atrioventricular (mitral on the left and tricuspid on the right) valves. Unlike the mechanical pump in Fig. 1, however, the relaxed ventricle cannot create a negative pressure to pull blood into it. Instead, the ventricular lumen can only be distended passively with blood under a positive pressure. That pressure must be generated in

FIGURE 2 Mechanical events in the heart during a single cardiac cycle. Blood flow and wall motion are indicated by arrows. Atrial and ventricular activation are symbolized by color in the respective wall. RA, LA, RV, and LV refer to right atrium, left atrium, right ventricle, and left ventricle, respectively.

FIGURE 2 Mechanical events in the heart during a single cardiac cycle. Blood flow and wall motion are indicated by arrows. Atrial and ventricular activation are symbolized by color in the respective wall. RA, LA, RV, and LV refer to right atrium, left atrium, right ventricle, and left ventricle, respectively.

the veins that feed the heart. Because ventricular filling is in proportion to venous pressure, the heart's stroke volume is quite variable. At the end of diastole, the atria contract. Because there are no valves between the atria and the veins, much of the atrial blood is actually forced back into the veins. Nevertheless, some additional blood is also pushed forward into the ventricles, causing further increases in ventricular pressure and volume. Although the benefit of atrial contraction at normal heart rates may be negligible, it can substantially increase ventricular filling at high heart rates when diastolic filling time is curtailed.

As the ventricular musculature contracts, a period termed systole, the force in the walls is transmitted to the blood within the ventricular lumen. Ventricular pressure increases and, as it rises above venous pressure, the atrioventricular valves close. The heart now begins a period of isovolumetric contraction as pressure builds in the lumen. No blood can enter or leave the ventricle because both the inflow and the outflow valves are closed, hence the term isovolumetric. When pressure in the ventricular lumen finally exceeds that in the outflow vessel (the aorta for the left heart and the pulmonary artery for the right heart), the semilunar valves (aortic on the left and pulmonic on the right) open and blood is ejected.

As systole ends, the ventricular musculature relaxes and the force exerted on the blood in the ventricular lumen subsides. Ventricular pressure falls below outflow pressure in the outflow vessel and the semilunar valves close. At this point both the semilunar and the atrioventricular valves are closed so that a second isovolumetric period occurs. Atrial blood will not flow into the ventricles until relaxation has proceeded to the point when ventricular pressure falls below atrial pressure. When that occurs, the atrioventricular (AV) valves open and the filling phase of the cardiac cycle once again repeats itself.

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