In order to understand the question let's retrace the path of blood in the right side of the heart. From the superior and inferior vena cava, the blood is recieved body, and is deoxygenated. From there the atrium pushed blood into the right ventricle. The right ventricle and left ventricle contract during systole pushing blood into two vessels. The vessel your concerned about, needs to be oxygenated, so it is presumed the blood is pumped into the pulmonary artery.
