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whats meant by cardiac cycle
 A cardiac cycle is one ventricular systole followed by one ventricular diastole. A single cycle of cardiac activity can be divided into two basic phases - diastole and systole.  The cardiac cycle includes a description of the systolic and diastolic activities of the atria and ventricles, the blood volume and pressure changes within the heart, and the action of the heart valves.


The isovolumetric ventricular relaxation is the period during which the ventricles are relaxed and both AV and semilunar valves are still closed. The volume of the ventricles remains unchanged (isovolumetric) during this period. Isovolumetric contraction occurs when the AV valves are forced shut. During this brief period, while the semilunar valves are still closed, the volume of the ventricles remains unchanged.

    Ventricular filling begins as the AV valves open and blood fills the ventricles. The ventricles remain in diastole during this period. The filling of the ventricles can be described as three successive events:

        Rapid ventricular filling occurs as blood flows into the empty and relaxed ventricles. Volume of the ventricles increases rapidly.

        Diastasis is a slower filling event than that of the preceding because most of the volume of the ventricle is already occupied by blood.

        Atrial systole (and the P wave of the ECG) occurs and forces the remaining blood from the atria into the ventricles. The blood volume at the end of this interval is called the end-diastolic volume (EDV).

    Ventricular contraction (ventricular systole) begins as the action potential from the AV node enters the ventricles, the ventricles depolarize, and the QRS complex is observed on the ECG. The following intervals during this phase are observed:

       

        Ventricular ejection occurs as the continuing contraction of the ventricles increases the pressure in the ventricles and forces the semilunar valves open. At this point, blood is forced out of the ventricles. This interval ends when the ventricles begin to relax, and the semilunar valves close.  The amount of blood remaining in the ventricles at this time is called the end-systolic volume (ESV).

The heart sounds associated with the beating of the heart can be heard by auscultating (listening to) the thorax with a stethoscope. The two major heart sounds, described as “lub-dub,” originate from blood turbulence generated by the closing of the AV valves and the semilunar valves, respectively).
   
   


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