The work of the heart is to receive the blood and then pump it out of the heart with great force. In order to do this function heart keeps on relaxing and contracting throughout the life of a person. The contraction of the heart is known as Systole and relaxation is termed as Diastole. The systole and diastole alternate with each other. The contraction of the heart chambers decreases the volume of the heart chambers and forces the blood out of them. The relaxation of heart chambers bring the heart back to normal size and then it starts receiving blood. During systole, the whole heart doesn’t contract at the same time. The auricles systole earlier than the ventricles. A regular sequence of three events i.e. Auricular systole, Ventricular systole and Joint diastole is known as the cardiac cycle. In other words, the sequence of events which takes place during the completion of one heartbeat is known as the cardiac cycle. The human heart beats at a rate of 72 times per minute at normal conditions.
During this phase, blood continues to flow into the atria (auricles) through the great veins i.e. superior and inferior vena cava and pulmonary vessels. Some blood also flows slowly from the atria to respective ventricles through atrioventricular valves. But no blood flows from the ventricles to the great arteries and the semilunar valves remain closed. The ventricles may be half-filled with blood by the end of joint diastole. At the end of this phase, the next heartbeat starts with the auricular systole.
Auricular or Atrial Systole:
Contraction of the Atria (auricles) drives most of the blood into the respective ventricles which are still in diastole (ventricular diastole). The two atria act as a pump to collect and force the venous blood to the ventricles. During atrial systole, blood does not pass back from the atria into the great veins because the roots of great veins are compressed to block their openings. After the atrial systole, it comes back into its diastole and receives blood again from the great veins- the right auricle from the vena cava and left auricle from pulmonary veins. Atrial systole takes about 0.1 seconds while atrial diastole takes about 0.7 seconds.
It involves simultaneous relaxation of atria (atrial diastole) and contraction of ventricles (ventricular systole). Ventricular Systole is due to action potentials generated by the atrioventricular node and the atrioventricular bundle. Due to ventricular systole, the pressure on the blood in the ventricles is increased compared to that in the atria. The atrioventricular valves close rapidly at the beginning of ventricular systole to prevent the backflow of blood from ventricles to auricles. So, at the onset of ventricular systole, the first heart sound ‘LUBB‘ commences due to sudden closure of the AV valves.
Finally, the pressure in the ventricles increases than that in the great arteries, so semilunar valves open and blood enters great arteries. Deoxygenated blood enters the pulmonary arch which carries it to the lungs. Oxygenated blood enters the aortic arch which carries it to all other body parts. Each ventricle pumps out about 70 ml of blood (called stroke volume). Ventricular systole takes about 0.3 seconds while ventricular diastole takes about 0.5 seconds.
At the end of ventricular systole, the ventricles undergo diastole. The auricles are already in diastole stage, so all the heart chambers enter into diastole and called as joint diastole. As the ventricular diastole starts, the pressure in the ventricles falls down than that in the great arteries. Therefore, the semilunar valves become closed in the aorta and pulmonary artery to prevent the backflow of blood into the ventricles. The sharp closure of semilunar valves at the onset of ventricular diastole produces the second heart sound- ‘DUP‘. The pitch of this sound is higher but for a shorter duration than the first sound.
The ventricles become a closed chamber due to closure of semilunar and atrioventricular valves and their pressure is still higher than the auricular pressure. As the ventricular diastole continues, the ventricular pressure falls down sharply than the auricles. So, the atrioventricular valves become open and blood again starts flowing down from the relaxing atria to the relaxing ventricles. This phase lasts for 0.4 seconds.