The function of the circulatory system is to move blood throughout the body. To achieve this goal the heart muscle contracts rhythmically and systematically, pumping over 1,000 gallons of blood a day. To perform such a miraculous feat day in and day out the valves and every chamber of the heart must be perfectly synchronized for every beat. Each of these beats take approximately 0.8 seconds and the same sequence of events occurs in every beat. One of these complete sequences is known as the cardiac cycle.
The cycle can be divided into three separate phases: diastole (relaxation), atrial systole (contraction of atria), and ventricular systole (contraction of ventricles). This is a timeline of the cardiac cycle which shows the relative and approximate absolute lengths of each phase of the cycle.
To see an animation of the cardiac cycle with explanations of the mechanical
activity and blood flow occurring in each phase click
During this phase blood flows into the heart. Blood comes into the atria through the pulmonary veins (left atrium) and the superior and inferior vena cava (right atrium). From the left and right atria the blood flows directly into both ventricles because both the mitral and tricuspid valves are open. The ventricles accept this blood and passively dilate (expand). By the end of the diastole the ventricles are filled to about 80%. All cardiac muscle is relaxed throughout this phase. The aortic and pulmonary valves are closed during this phase to prevent the reverse flow of blood from the aorta and pulmonary artery.
This is a relatively short phase. During this phase the blood from the left and right atria (approximately 20%) is pumped into the ventricles.
During this phase the ventricles contract. As they begin to contract the mitral and tricuspid valves close to prevent any backflow into the atria. At the end of this phase the pulmonary and aortic valves close and diastole begins again.