Ventricular Systole and the Events Occurring

Ventricular systole refers to the phase of the cardiac cycle when the ventricles of the heart are contracting. During this phase, several events take place that facilitate the pumping of blood out of the heart and into the systemic circulation. Let's examine each of the given options to determine which event does not occur during ventricular systole.

a) Closure of tricuspid and bicuspid valves
- The closure of the tricuspid and bicuspid valves, also known as the atrioventricular valves, occurs during ventricular systole.
- These valves prevent the backflow of blood from the ventricles into the atria during contraction.
- Their closure produces the first heart sound, commonly referred to as "lub."

b) Flow of blood from atria to ventricles
- The flow of blood from the atria to the ventricles occurs during ventricular diastole, not ventricular systole.
- During ventricular diastole, the atria relax, and blood flows from the atria to the ventricles through the open tricuspid and bicuspid valves.
- This phase is commonly referred to as atrial systole.

c) Opening of the semilunar valves
- The opening of the semilunar valves occurs during ventricular systole.
- These valves, including the pulmonary and aortic valves, are located at the entrances of the pulmonary artery and aorta, respectively.
- When the ventricles contract, the pressure in the ventricles surpasses the pressure in the arteries, causing the semilunar valves to open, allowing blood to be ejected from the heart.

d) Atrial diastole
- Atrial diastole refers to the phase of the cardiac cycle when the atria are relaxed and filling with blood.
- This phase occurs during ventricular systole since the atria need time to refill with blood after contracting.
- During atrial diastole, the tricuspid and bicuspid valves are open, allowing blood to flow from the atria into the ventricles.

Conclusion
Based on the above explanations, the event that does not occur during ventricular systole is the flow of blood from atria to ventricles (option B). This event takes place during ventricular diastole, not systole.