Cardiac cycle is the cycle of initiation of a beat till the beginning of another heartbeat.

Events of a Cardiac Cycle:

1. Atrial Systole (0.7 sec): In this event atria contracts due to the wave of contraction initiated by the SA node.

2. Now, the blood is forced into the ventricles as the bicuspid and tricuspid valves are open.

3. Ventricle Systole (0.5 sec): After this, contraction of ventricles start taking place due to the wave of contraction initiated by the AV node.

4. This leads to the closing of bicuspid and tricuspid valve and the first heart sound is produced ‘lub’.

5. Complete Ventricular Systole: After ventricular contraction the blood flows into the pulmonary trunk and aorta as the semilunar valves are opened.

6. Ventricular Daistole : In this event the ventricles relax and the semilunar valves close and the second heart sound is made ‘dub’.

7. Complete Ventricular Diastole: The opening of bicuspid and tricuspid valves take place as the pressure of ventricles lowers down and blood flows from atria to the ventricles.

8. The duration of this cycle is about 0.8 seconds.

CARDIAC CYCLE

Double Circulation:

1.Birds and mammals have four chambered heart which are two atria and two ventricles.

2.The two atria are separated by inter-atrial septum and the two ventricles are separated by inter-ventricular septum.

3.Oxygenated blood received by the left atria passes to the left ventricle and Deoxygenated blood received by the right atria passes to the right ventricle without getting mixed

4.The blood is circulated to the body organs through the systemic circulation and to the lungs through Pulmonary circulation.

5.This shows that birds and mammals have two separate circulatory pathways which are Systemic and Pulmonary Pathway

Advantage of Double Circulation:

1. Deoxygenated and oxygenated blood remains separated and do not mix.

DOUBLE CIRCULATION