Cardiac cycle

Circulatory System of Class 11

In a normal heart beat the two auricle contract and the ventricles relax. When the auricle relax, the ventricles contract. Sysotle is the phase of contraction and diastole is relaxation. A complete heart beat consists

of the systole and diastole of both auticle and systole and diastole of both ventricles.

In the cardiac cycle near the end of auricular diastole, AV valves are open and semilunar valves are closed. Throughout most of auricular diastole, the ventricles receive blood from the auricle. These are constantly filling with blood. The right auricle has deoxygenated blood and left auricle has oxygenated blood. The left auricle gets oxygenated blood from the pulmonary veins. About 75% of the filling occurs during auricular diastole.

When the SA node fibres at the end of auricular diastole the auricle depolarize and contract. This is the start of auricular systole. Contraction of the auricle forces the remaining blood into the ventricles.

Deoxygenated blood passes from the right auricle passes into the right ventricle through the open tricuspid valve. Oxygenated blood passes from the left auricle into the left ventricle through the mitral valve.

While the auricle are contracting the ventricles are in diastole. During venticular diastole, the ventricles are filling with blood and semilunar valves in the aorta and pulmonary trunk are closed.

When auricualr systole and ventricular diastole are completed, the events the reversed, the auricle go into diastole and ventricles go into systole. During auricular diastole, deoxygenated blood from the various

parts of the body enters the right auricle. At the same time left auricle gets filled with oxygenated blood from the lung. During the first part of auricular diastole, the auriculoventricular valves are closed because the ventricle are in systole.

In ventricular systole, the ventricles contract and the blood is pumped into the vessels. The right ventricle pumps the blood to the lungs through the semilunar valve of the pulmonary trunk and left ventricle

pumps oxygenated blood through the open semilunar valve of the aorta. At the end of the ventricular systole, the semilunar valves close and both the auricle and ventricles relax. In a  complete cardiac cycle the auricle are in systole 0.1 second and in diastole 0.7 second. By contrast, the ventricles are in systole 0.3 second and in diastole 0.5 second. The opening and closing of the valves and the contraction and relaxation of the myocardium control the movement of blood through the heart.

If we assume that the average heart beats 72 times/minute, then each cardiac cycle requires about 0.8 seconds. During the first 0.1 second, the auricle contract and the ventricles relax. The auriculoventricular valves are open and the semilunar valves are closed. For the next 0.7 seconds, auricle relaxing and the ventricles are contracting. During first part of this period all the valves are closed. During the second part, the semilunar valves are open. The last 0.4 second of the cycle is the relaxation period. All the chambers are in diastole. For the first part of the relaxation period all the valves are closed and during the latter part the AV valves open and blood starts draining into the ventricles.

Heart sounds

The first heart sound is low pitched, less loud lubb (0.15 second) created by the closure of the AV valves soon after ventricular systole begins. The second, short, louder, sharp sound in called dupp (0.1 second) and it is created as the semilunar valves close towards the end of ventricular systole. A pause also come between the second sound and first sound of the next cycle.

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