In the first phase of the cardiac cycle when both the atria are filled with blood, an action potential is generated by the SA node and then the atria contract. This is called as ________________. 
  1. atrial systole
  2. atrial diastole
  3. ventricular systole
  4. ventricular diastole


The correct option is A atrial systole
The relaxation of the heart chambers is called diastole and the contraction of the heart chambers is called systole.
At the beginning of the cardiac cycle, all the four chambers of the heart are in a relaxed state called joint diastole. Blood from the inferior vena cava, superior vena cava enters into the right atrium and blood from the pulmonary vein enters into the left atrium.

As the bicuspid and tricuspid valves are open so the blood enters into the ventricles. The semilunar valves remain closed during this period. The sinoatrial node generates an action potential due to which the left and right atrium contracts and undergoes atrial systole. 

The action potential is then further conducted to the atrioventricular node. Then it is conducted to the bundle of His and purkinje fibres. Due to the transmission of the action potential, the ventricles contract called ventricular systole.

At this stage, the atria also relax and undergo diastole. Due to the ventricular systole, the pressure inside the ventricles increase which results in closure of the bicuspid and tricuspid valve. As the pressure inside the ventricles increases, the semilunar valves open up and blood is released into the systemic circulatory system. The two semilunar valves are present in between the right ventricle and pulmonary artery and in between the left ventricle and the aorta. After this stage the pressure inside the ventricles falls and the ventricles relax. The semilunar valve closes, which prevents the backflow of blood into the ventricles. This stage is called ventricular diastole. 

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