Final answer:
Blood arriving in the right atrium has just come from the venae cavae and coronary sinus.
Step-by-step explanation:
The blood arriving in the right atrium has just come from the venae cavae and coronary sinus. The right atrium is the initial chamber that receives deoxygenated blood from the systemic circuit, which includes the upper body via the superior vena cava, and the lower body via the inferior vena cava, as well as deoxygenated blood from the heart muscle itself via the coronary sinus. From the right atrium, blood moves into the right ventricle, which then pumps it to the lungs for gas exchange within the pulmonary circuit.