The phases of the cardiac cycle are:
1) Atrial Systole
This begins with the P-wave of the ECG. The atrio-ventricular valves are open and the ventricles are relaxed (diastole) and are filling with blood. The semilunar valves are closed and blood does not leak back from the aorta into the ventricles.
2) Isovolumetric Contraction
This term means that the internal volume of the ventricle is not changing. This phase starts with the R-wave on the ECG and the 1st heart sound. It is the brief initial phase of ventricular systole when the atrio-ventricular and semilunar valves are closed and the ventricular pressure rapidly builds up to open the semilunar valves.
3) Ejection
During the ejection phase of systole the semilunar valves are opened when the pressure gradient of the ventricles exceeds the pressure in the aorta and pulmonary artery. There is a phase of rapid ejection and a later phase of reduced ejection. The ventricular volume comes down and the T-wave on the ECG corresponds to the reduced ejection. During ejection the ventricular and great artery pressures equalize. At the end of ejection a quantity of blood remains in the ventricle and this is the residual or end systolic volume.
4) Isovolumetric Relaxation
This is the beginning of ventricular relaxation or diastole between the closure of the semilunar valves and opening of the atrio-ventricular valves. As the ventricular pressure falls after ejection there is a pressure gradient between the great arteries and ventricles and as these pressures cross the semilunar valves close. This corresponds to the 2nd heart sound. During isovolumetric relaxation the semilunar and atrio-ventricular valves are both closed. The volume of the ventricle remains the same and the ventricular pressure falls. 5) Ventricular (Passive) Filling When the diastolic pressure in the ventricle falls below that in the corresponding atrium, the atrio-ventricular valves open and there is passive filling of the ventricules. There is at first a rapid inflow (which may be accompanied by the 3rd heart sound) followed by a slow inflow or diastasis when the ventricular pressure rises gradually. Towards the end of this phase the atrium contracts and there is an augmented flow of blood into the ventricule. The 4th heart sound when heard corresponds to atrial systole.
6) Atrial Pressure Wave Form
In the left atrium the three positive waves are the 'a', 'c', and 'v' waves and the troughs are 'x', 'y', and 'z'.1 The 'a' is due to atrial contraction and is followed by the 'z' trough of atrial relaxation. The 'c' wave is due to upward movement of the mitral cusps and the 'x' descent due to the downward movement of the mitral valve ring with ventricular contraction. The 'v' wave is due to atrial filling during the latter part of ventricular systole with the mitral valve closed. The rising pressure ends when the mitral valve opens. The 'y' descent follows the flow into the ventricule in diastole. From the opening of the mitral valve to its closure the atrial and ventricular pressures are nearly identical.
7) Right Heart Pressures and Events
Events on the right side of the heart mirror the changes on the left side but occur slightly later. So tricuspid valve closure follows mitral closure and pulmonary closure follows aortic valve closure. Inspiration increases the negative pressure in the thorax thus increasing the venous return to the right side which results in delayed pulmonary valve closure during inspiration. This explains the physiological widening of the splitting of the 2nd heart sound on inspiration.