What is the purpose of atrial contraction in the cardiac cycle?

Atrial contraction plays a critical role in the cardiac cycle, specifically during the phase known as atrial systole. During this phase, the atria contract and push blood into the ventricles, effectively filling them. This process is essential for ensuring that the ventricles have adequate volume to pump out more blood during their subsequent contraction.

When the atria contract, they create a slight increase in pressure that aids in the efficient filling of the ventricles. This is particularly important in the context of the heart's overall efficiency, as a well-filled ventricle can eject a greater volume of blood during ventricular systole (the contraction of the ventricles). Atrial contraction facilitates this process, especially during the later part of diastole when the ventricles are in a relaxed state and are receiving blood.

By ensuring that the ventricles are full, atrial contraction contributes to maintaining optimal cardiac output and ensuring that the tissues receive adequate oxygen and nutrients through the blood. Thus, the primary purpose of atrial contraction is indeed to fill the ventricles with blood, making the correct answer B.