when does isovolumetric relaxation and ventricular filling take place
Cardiac cycle "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 (S2). 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).
Heart sounds The first heart sound, or S1, forms the "lub" of "lub-dub" and is composed of components M1 (mitral valve closure) and T1 (tricuspid valve closure). Normally M1 precedes T1 slightly. It is caused by the closure of the atrioventricular valves, i.e. tricuspid and mitral (bicuspid), at the beginning of ventricular contraction, or systole. When the ventricles begin to contract, so do the papillary muscles in each ventricle. The papillary muscles are attached to the cusps or leaflets of the tricuspid and mitral valves via chordae tendineae (heart strings). When the papillary muscles contract, the chordae tendineae become tense and thereby prevent the backflow of blood into the lower pressure environment of the atria. The chordae tendineae act a bit like the strings on a parachute, and allow the leaflets of the valve to balloon up into the atria slightly, but not so much as to evert the cusp edges and allow back flow of blood. It is the pressure created from ventricular contraction that closes the valve, not the papillary muscles themselves. The contraction of the ventricle begins just prior to AV valves closing and prior to the semilunar valves opening. The sudden tensing of the chordae tendineae and the squeezing of the ventricles against closed semilunar valves, sends blood rushing back toward the atria, and the parachute-like valves catch the rush of blood in their leaflets causing the valve to snap shut. The S1 sound results from reverberation within the blood associated with the sudden block of flow reversal by the valves. The delay of T1 even more than normally causes the split S1 which is heard in a right bundle branch blockage.