During atrial systole, which statement best describes the atria's action?

During atrial systole, which statement best describes the atria's action?

• A. They contract to push blood into relaxed ventricles.
• B. They relax to fill the ventricles.
• C. They push blood into the lungs.
• D. They fill the ventricles by ejecting blood into the aorta.

Answer: (A)

During atrial systole the atria actively contract to push blood into the ventricles, which are relaxed and receiving that blood. This completes ventricular filling just before the ventricles themselves contract. The right atrium’s contraction moves blood into the right ventricle, which then sends blood to the lungs after ventricular systole, not directly from the atria. The idea that atria relax to fill describes atrial diastole, and the notion of ejecting into the aorta describes ventricular ejection, not atrial systole.

During atrial systole the atria actively contract to push blood into the ventricles, which are relaxed and receiving that blood. This completes ventricular filling just before the ventricles themselves contract. The right atrium’s contraction moves blood into the right ventricle, which then sends blood to the lungs after ventricular systole, not directly from the atria. The idea that atria relax to fill describes atrial diastole, and the notion of ejecting into the aorta describes ventricular ejection, not atrial systole.