Final answer:
Blood arriving in the right atrium comes from the systemic circulation, including the superior and inferior vena cavae and the coronary sinus.
Step-by-step explanation:
The right atrium receives blood from the systemic circulation, which includes the superior and inferior vena cavae and the coronary sinus. The superior vena cava drains blood from the head, neck, chest, shoulders, and upper limbs, while the inferior vena cava drains blood from the abdominal and pelvic regions and the lower limbs. The coronary sinus drains the blood supply of the heart muscle.
From the right atrium, the blood moves into the right ventricle, which pumps it to the lungs for gas exchange. This circulation of blood from the right atrium to the right ventricle is part of the pulmonary circuit.