Stages of the cardiac cycle
1. A. Atrial walls contract, increasing atrial pressure. AV valves are open and semilunar valves remain closed
2. B. Atrial contraction complete, ventricles begin to contract, ventricular pressure > atrial pressure - AV valves close (first heart sound)
3. C. Continued contraction of ventricles - ventricle pressure > arterial pressure, semilunar valves open
4. D. Arterial pressure > ventricular pressure - semilunar valves close due to loss of blood from ventricles (second heart sound)
5. E. Ventricular pressure falls as little blood present and walls begin to relax - atrial pressure > ventricular pressure - AV valves open
6. F. Atrial pressure > ventricle pressure as blood flowing into atria - AV valves remain open, blood passively flows into the ventricles from the atria