It refers to the heart's repeated cycle of systole (contraction) and diastole (relaxation). Each atrial and ventricular systole and diastole cycle lasts 0.8 seconds.
Systole lasts just 0.1 seconds in auricles, but diastole lasts 0.7 seconds. Ventricles, on the other hand, have prolonged systole, i.e. 0.3 sec, due to their extremely muscular walls. As a result, diastole in the ventricles lasts just 0.5 seconds. The subsequent events of the cardiac cycle are outlined shortly below. (i) Atrial systole: The atria contract during atrial systole as a result of a wave of contraction induced by the SA node. Because the bicuspid and tricuspid valves are open, blood is driven into the ventricles. It just takes 0.1 seconds. (ii) Ventricular systole: Systole of the ventricle Both ventricles contract, causing blood to flow through the capillary artery to the lungs and the aorta to the rest of the body. It just takes 0.3 seconds. (iii) Atrial diastole: The atria relax, while the ventricles remain constricted. Blood enters the atria via the major veins that exit the body. This process takes 0.3 seconds. (iv) Ventricular diastole: As the next cycle begins, the ventricular relaxes and prepares to fill with blood from the atria. This takes around 0.5 seconds.
