Cardiac Cycle

Cardiac cycle has two phases-systole and diastole. Ventricular systole and diastole occur as a result of depolarization and chamber volume and pressure. The diastolic phase of both ventricles normally occurs simultaneously. 

This phase of ventricular myocardial relaxation permits emptying of atrial contents through the open atrio-ventricular valves (tricuspid and bicuspid or mitral). Rapid ventricular filling, a passive gravity flow of blood form atria to ventricles of starts when atrial pressure exceeds ventricular pressure and AV valves open. Increasing blood volume causes ventricular pressure to rise, which slows further filling. Atrial musculature contracts propelling additional blood into the ventricle before ventricular contraction (atrial systole ).

There is increase in myocardial tension and intra-ventricular pressure without change in blood volume. AV. Valves dose. For one short period, all the valves are closed until, with ventricular depolarization, the pressure in the ventricles exceeds that of pulmonary artery and aorta. (Isovolumetric ventricular contraction).

The systolic phase is the active contraction of the ventricular myocardium causing ejection of blood into the pulmonary artery and aorta. The right and left ventricle contract simultaneously. Systole begins when the semi- lunar valves open and end when they close. The higher pressure in the aorta and pulmonary artery than that in ventricles causes the closure of the semi-lunar valves. With each contraction, a volume of blood is ejected, called the stroke volume. Normal stroke volume (SV) is approximately 70 ml.

Cardiac output = SV x Heart rate /minute.

Three important factors affecting stroke volume and in turn cardiac output, are preload, contractility and after load. Preload is related to the volume of blood distending the ventricles at the end of diastole. Contractility refers to a change in the inotropic state of the muscle without a change in myocardial fiber length or preload. After load is the amount of tension the ventricle must develop during contraction to eject blood from the left ventricle into the aorta.

The 'lub-dub' sounds heard on auscultation corresponds with the closure of heart valves. The 'lub' sound corresponds with closure of AV valves at the beginning of ventricular systole and 'dub' sound corresponds with the closure of semilunar valves at the end of ventricular systole.