The cardiac cycle
1. Both the atria and ventricle are relaxed. The right atrium receives blood from venae cavae while the left atrium receives blood from the pulmonary vein.
2. The atria contract, forcing blood into the ventricles.
3. After a short pause, the ventricle contract. This is called ventricular systole. The increase in blood pressure forces the bicuspid and tricuspid valves to close, preventing backflow of blood into the atria. This produces a loud ‘lub’ sound. When the pressure in the left ventricle becomes higher than the pressure in the aorta, the semi-lunar valves in the pulmonary and aortic arches are forced open. Blood flows from the left ventricle into the aortic arch. As the ventricle contract, the atria relax.
4. The ventricle then relax. This is ventricle diastole. The drop in pressure in the ventricle causes the semi-lunar valves in two arches to close. This produces a soft ‘dub’ sound. This prevents the backflow of blood into the ventricle. The drop in pressure in the ventricle also causes the bicuspid and tricuspid valves to open. The whole cycle (1-4) then repeats.
A ventricle systole and a ventricular diastole make up one heartbeat. It lasts for about 0.8 seconds. There is a short pause between to heartbeats. The rate of heartbeat varies with the age and size of the individual. The average normal heartbeat of an adult is about 72 times per minute. Note that the atria and the ventricle work alternately. When the atria contract, the ventricles relax and vice versa.