ICSE Class 10 Answered

Each cardiac cycle involves:

1. Atrial systole (0.15 sec): In this phase atria contract and blood flows into the ventricles. During this phase, atrio-ventricular valves open while the semilunar valves are closed.

2. Ventricular systole (0.30 sec): In this phase ventricles contract atrio-ventricular valves close to prevent back flow of the blood and semi lunar valves open. Deoxygenated blood from the right ventricle is sent to lungs through pulmonary artery and oxygenated blood from the left ventricle is sent to the body through aorta.

3. Joint diastole (0.40 sec): At the end of ventricular systole, ventricles begin to relax at the same time atria are also in relaxed state for a short period. During this period, left atrium receives oxygenated blood from the lungs and the right atrium receives deoxygenated blood from the superior and inferior vena cava.