Final answer:
Blood returning from the systemic circulation first enters the right atrium of the heart before being pumped to the lungs for oxygenation.
Step-by-step explanation:
When blood reenters the heart from systemic body circulation, it first goes through the right atrium. After circulating through the body, blood that is low in oxygen and high in carbon dioxide returns to the heart via two major systemic veins: the superior and inferior venae cavae, and also the coronary sinus which drains the blood supply of the heart muscle. The superior vena cava carries blood from the upper half of the body, while the inferior vena cava carries blood from the lower half. This oxygen-poor blood then flows from the right atrium into the right ventricle, from where it is pumped into the pulmonary circulation to be reoxygenated by the lungs.