<P> The first and the fourth stages, together constitute the "ventricular diastole" stage, involve the movement of blood from the atria into the ventricles . Stages 2 and 3 involve the "ventricular systole" i.e. the movement of blood from the ventricles to the pulmonary artery (in the case of the right ventricle) and the aorta (in the case of the left ventricle). </P> <P> "Ventricular diastole," begins when the ventricles starts to relax . At this point, some blood of the previous cycle's systole is still flowing out of the ventricles through the semilunar valves, due to the inertia of the moving blood column, which overcomes the higher pressure in the aorta / pulmonary trunk with respect to the pressure in the ventricles . This short lasting phase, called "protodiastole" ends with the closure of the semilunar valves, producing the second heart sound (S). Now that both the AV valves and the semilunar valves are closed, the ventricles are now closed chambers . Hence, this phase is known as isovolumetric (also called isovolumic, isometric) relaxation phase . Then the atrioventricular (AV) valves (the mitral valve and the tricuspid valve) open, allowing blood to fill the ventricles . This ventricular inflow phase can be sub-divided into the' first rapid filling phase' as blood rushes in from the atria as a result of ventricular dilation; a phase of slow ventricular filling called' Diastasis', and the' last rapid filling phase' due to atrial contraction (systole). </P> <P> As the ventricular systole begins, pressure within the ventricle rises and the AV valves close producing the' first heart sound' (S). The semilunar valves remain closed . The contracting ventricles become closed chambers again and this phase is termed as "isovolumic contraction". As the name implies, there is no change in volume, but intra-ventricular pressure rises . The outflow phase, "ventricular ejection," is when the intra-ventricular pressure has achieved a higher pressure than the blood in the aorta (or the pulmonary trunk), the corresponding semilunar valves open . Ejection phase begins . </P> <P> Throughout the cardiac cycle, blood pressure increases and decreases . The cardiac cycle is coordinated by a series of electrical impulses that are produced by specialised pacemaker cells found within the sinoatrial node and the atrioventricular node . The cardiac muscle is composed of myocytes which initiate their own contraction without the help of external nerves (with the exception of modifying the heart rate due to metabolic demand). The duration of the cardiac cycle is the reciprocal of heart rate . Assuming a heart rate of 75 beats per minute, each cycle takes 0.8 seconds . </P>

During which phase of the cardiac cycle does the first heart sound occur