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   rdi ccycle  The repeating pattern of contraction and relaxation of the heart is called cardiac cycle.  It consists of two events – (i) Systole – contraction on heart chambers (either atrium or ventricles) is known as systole. (ii) Diastole – relaxation of heart chambers (either atrium or ventricles) is known as diastole  EVENTS OF CARDIAC CYCLE  Blood flows from vena cava and pulmonary artery and fills left and right atria respectively.  Pressure in atria increases which causes opening of AV  valves and blood flows into ventricles ( semilunarvalves are closed to prevent back flow of blood ).  SAN generates action potential (impulse) which stimulates atrialcontraction – atrialsystole  This increases the blood flow by 30% .  Now the pressure in both the ventricles is increasing.   As soon as the impulse reaches the ventricles, they undergo contraction –  ventricular systole .   At this time relaxation of atria occurs – atrialdiastole .  ã The ventricular systole causes opening of semilunarvalves. ( at this time the AV valve remains closed) ã Due to this, the blood flow from left and right  ventricles to pulmonary artery and aorta respectively. ã The blood gets circulated in the entire body. ã Now the ventricles relax –  ventricular diastole. ã  At this time, both atria and ventricles are in relaxed state –  joint diastole.  DURATIONS  Duration of one cardiac diastole is 0.4 seconds.  atrialsystole occurs for 0.1 seconds.   ventricular systole occurs for 0.3 seconds.  Thus one complete cardiac cycle occurs in 0.8 seconds.
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