During atrial systole, what happens?

• A. The atria contract and force blood from the atria into the ventricles
• B. The ventricles contract and force blood from the ventricles into the arteries
• C. The atria relax and fill from the veins
• D. The ventricles relax and fill from the atria

Answer: (A)

During atrial systole, the atria contract to push blood into the ventricles. The ventricles are relaxed at this moment, so the atrioventricular valves are open and blood flows from the atria into the ventricles. The semilunar valves remain closed because ventricular pressure hasn’t risen to exceed arterial pressure yet. This active atrial contraction completes ventricular filling and increases preload for the upcoming ventricular contraction. In contrast, atrial diastole involves atrial relaxation and filling from the veins, and ventricular diastole involves the ventricles relaxing while filling from the atria or during passive filling.