The heart, in a continuous cycle, fills with blood and then actively contracts. It is constantly going through an involuntary cardiac cycle, which has two main phases: diastole and systole

During diastole the entire heart is relaxed and blood flows into all four chambers. This is also referred to as ventricular diastole. The ventricular walls are relaxed. The blood enters the atria. The atrioventricular valves are open at this point and so blood flows through into the ventricles until they are nearly filled. The semi-lunar valves are closed to prevent any blood from flowing out of the heart into the pulmonary artery or aorta. The closing of these semi-lunar valves is what produces the second sound of the heart beat, referred to as “dub”.

Then systole takes place. Atrial systole begins when the atria contract and finish completely filling the ventricles with blood. Then ventricular systole takes place in which the ventricles contract, the force of which closes the atrioventricular valves to prevent the backflow of blood to the atria. This causes the first heartbeat sound, referred to as “lub”. This also opens the semi-lunar valves. Blood is pumped into the arteries and at the same time, more blood is starting to refill the atria.

Note that every step takes place simultaneously in the left and right sides of the heart.