Describe the events in the cardiac cycle. Explain “double circulation”.

Answers (1)

The cardiac cycle comprises of one heartbeat, which can also be understood as contraction and relaxation of cardiac muscles per cycle. The contraction phase of atria and ventricles is called the systole, and the relaxation is called the diastole. The stages are as follows-

  1. Atrial systole: The contraction wave contracts the atria stimulated by the SA node, and the blood is moved into the ventricles as the bicuspids and tricuspids are open.
  2. Ventricular systole: The beginning of the ventricular systole is stimulated by the AV node. The bicuspid and tricuspid valves close, making the first heart sound. The complete ventricular contraction occurs, the semilunar valves open, and the blood is forced into the pulmonary trunk and aorta.
  3. Beginning of ventricular diastole: The ventricles relax and as the semilunar valves close, the second heart sound is created.
  4.  Joint diastole: As the ventricles relax, the tricuspid and bicuspid valves are forced open, and the blood can now move freely to the ventricles.

Double Circulation: In a cardiac cycle, blood flows through the heart twice, and this circulation is called double circulation. It results in complete separation of oxygenated and deoxygenated blood, which leads to the systemic and pulmonary circulation, allowing the more efficient supply of oxygen into the cells and not mixing the two blood.

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