The cardiac cycle is the cycle of contraction and relaxation of the atria and ventricles during a complete heartbeat. It begins with passive ventricular filling. The AV valves are open and the semilunar valves are closed. Blood enters the atria and ventricles due to venous pressure. When the atria contract, the filling of the ventricles is completed. The volume of blood in each ventricle after complete filling (the maximum volume) is the end-diastolic volume (EDV). The filling phase (diastole) has completed.
A Wiggers diagram shows the changes in ventricular pressure and volume during the cardiac cycle. Often these diagrams also include changes in aortic and atrial pressures, the EKG, and heart sounds. Diastole starts with the closing of the aortic valve (the second heart sound). Shortly after, ventricular filling begins when the mitral valve (and tricuspid valve) open. This is the lowest volume on the ventricular volume curve. Volume rapidly increases as the atria and ventricles fill. Ventricular pressure remains low during this filling. Ventricular volume increases a little more when the atria contract. This contraction causes a small increase in atrial and ventricular pressures (and is associated with the P wave of the ECG).
Once the ventricles are filled, they begin to contract (QRS in the ECG). The contraction pushes the AV valves closed, causing the first heart sound at the start of systole. At first the pressure is not high enough to open the semilunar valves. Thus, ventricular volume does not change (the valves are closed). It is an isovolumetric ventricular contraction. Once ventricular contraction builds enough pressure in the ventricles, the semilunar valves open. When the aortic and pulmonary valves open, blood is ejected from the ventricles into the great arteries.
Once the ventricular pressure falls, due to this ejection of blood, the semilunar valves close and ventricular pressure drops to allow filling to begin again with another cardiac cycle. The volume after contraction is the end-systolic volume (ESV). This is the volume left in the ventricles after ejection. The ESV is the minimum volume of blood in the ventricles during the cardiac cycle.
Stroke volume is the volume of blood ejected per heartbeat. It equals EDV – ESV. Cardiac output (CO) is the volume of blood ejected by the ventricles per minute, so it is the product of stroke volume and heart rate (CO = SV x HR). The efficiency of pumping is measure by the ejection fraction, which is the stroke volume divided by the maximum filling volume (EDV) expressed as a percentage (SV/EDV x 100%).
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