Hello,

The heart has an increasing rhythmic activity. It pumps blood by its contraction and relaxation. The contraction of the heart is called systole and the relaxation is called diastole. The contraction and relaxation together constitute the heart beat. The heart beats at the rate of 72 beats per minute. The changes that occur in the heart during the beat one is repeated in the same order in the next beat. This cyclical repetition is called cardiac cycle. During the cardiac cycle, blood flows through the cardiac chambers in a specific manner and direction, the backward flow being prevented by the valves. There are 3 main events in the cardiac cycle, namely Auricular systole, Ventricular systole, Joint Diastole.

- Auricular Systole: This phase involves the contraction of the 2 auricles, pushing the blood into the respective ventricles. There is no back flow of blood due to the presence of the bicuspid and the tricuspid valves. The atrial systole takes 0.1 second. This is followed by the atrial diastole when both the auricles relax simultaneously. This is about 0.7 seconds.

-Ventricular Systole: This takes place alongside auricular diastole. The pressure on the blood in the ventricles increases. The auriculo ventricular valves close rapidly to prevent the backward flow of blood into the auricles. This closing of the auriculo ventricular valves at the start of ventricular systole produces first heart sound called lubb.

As the pressure in the ventricle increases, than that in the great arteries, namely pulmonary artery and the aorta, the semilunar valves guarding the openings of these arteries open and blood enters them. From the right ventricle, the deoxygenated blood enters the pulmonary artery. From the left ventricle, the oxygenated blood enters the dorsal aorta, to be taken to all body parts. Ventricular systole takes about 0.3 seconds.

-Joint Diastole: Ventricular systole is followed by ventricular diastole. The auricles are already in diastole, so all the chambers of the heart are in diastole.

When the ventricles are in diastole, the pressure in the ventricles decreases more than that in the great arteries. So to prevent the backward flow of blood, the semilunar valves close rapidly. This produces the second heart sound called dup.

During a complete cardiac diastole, blood from the superior and inferior vena cava flows into the auricles slowly. The pressure in the ventricles decreases and finally becomes lower than atrial pressure. Then the AV values open and blood (auriculo ventricular valves) from the atria starts entering into the relaxing ventricles. A complete cardiac diastole takes only 0.4 seconds. An entire cardiac cycle is completed in 0.8 seconds.

Hope this helps, Good Luck