describe the events in cardiac cycle
Answers
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole , followed by a period of robust contraction and pumping of blood, dubbed systole. After emptying, the heart immediately relaxes and expands to receive another influx of blood returning from the lungs and other systems of the body, before again contracting to pump blood to the lungs and those systems. A normally performing heart must be fully expanded before it can efficiently pump again. Assuming a healthy heart and a typical rate of 70 to 75 beats per minute, each cardiac cycle, or heartbeat, takes about 0.8 seconds to complete the cycle.
Cardiac Cycle: The sequential events of contraction and relaxation of heart chambers constitute cardiac cycle.
Systole is contraction of heart chambers. Chambers eject blood during systole.
Diastole is relaxation of heart chambers. Chambers receive blood during diastole.
All four chambers of heart can relax together in cardiac cycle but can never contract together.
Atrial chambers contract and relax together. Likewise ventricular chambers contract and relax together.
To begin with, all the four chambers of heart are in relaxed state, i.e., they are in joint diastole.
Tricuspid and bicuspid valves remain open during joint diastole.
Blood from vena cava and pulmonary veins flows into right and left ventricle respectively through right and left atria. Semilunar valves are closed during joint diastole.
SAN now generates an action potential which stimulates both atria to undergo simultaneous contraction, i.e., atrial systole.
This increases atrial pressure, as a result atria empty remaining blood into the ventricles.
Note: During joint diastole ventricles receive 70% (two-third) of the blood from atria. The remaining 30% of the blood is poured by atria into ventricles during atrial systole. Thus atrial systole increases the flow of blood into ventricles by 30%.
The action potential generated by SAN is conducted to ventricles by AVN and bundle of HIS from where Purkinje fibres transmit it throughout the ventricular musculature.
This initiates ventricular systole. Atria undergo relaxation simultaneously.
Ventricular systole increases ventricular pressure causing closure of tricuspid and bicuspid valves to prevent backflow of blood into the atria.
As the ventricular pressure increases further, the semilunar valves guarding pulmonary arch and aorta are forced open, allowing the blood to flow from ventricles to these vessels.
The ventricles now relax (ventricular diastole) and ventricular pressure falls causing closure of semilunar valves which prevents backflow of blood into the ventricles.
As the ventricular pressure declines further, tricuspid and bicuspid valves are pushed open by pressure in atria exerted by blood which is being poured into them by the veins.
The ventricles and atria are now again in a relaxed state (joint diastole).
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