Question

Which of the following represents a correct sequence of events in a cardiac cycle?

• A. Ventricular systole $\to$ atrial systole $\to$joint diastole $\to$ ventricular diastole
• B. Ventricular systole $\to$ atrial diastole $\to$joint diastole $\to$ ventricular diastole
• C. Joint diastole $\to$ ventricular systole $\to$ atrial systole $\to$ ventricular diastole
• D. Joint diastole $\to$ atrial systole $\to$ ventricular systole $\to$ventricular diastole $\to$ joint diastole

Answer 1

The correct option is D Joint diastole $\to$ atrial systole $\to$ ventricular systole $\to$ventricular diastole $\to$ joint diastole

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 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 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.

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. Now the atria and the ventricles are both relaxed, so they are in joint diastole again.