The inferior vena cava
Two veins return deoxygenated blood to the right atrium. The superior vena cava returns blood from the head and upper body to the right atrium. The inferior vena cava returns blood from the legs and lower body to the right atrium.
Right atrium
As deoxygenated blood returns from the body, it flows from the vena cava into the right atrium. From the right atrium the blood flows into the right ventricle which pumps the blood through the pulmonary artery to the lungs. Oxygenated blood returns from the lungs via the pulmonary vein and enters the left atrium. The left atrium empties into the left ventricle which pumps the blood through the aorta and to the body.
right atrium
Simply the deoxygenated blood goes into right atrium into right ventricle. oxygenated blood goes into left atrium then into left ventricle
Blood is bright red when it returns to the heart from the lungs. It is darker red when it returns to the right atrium from the systemic circulation.
Blood returning from pulmonary circulation returns to the right atrium via the pulmonary vein. Blood returning from the systemic circulation returns to the right atrium via the Vena Cava.
Coronary sinus
atrium
right atrium
The right atrium receives deoxygenated blood, then the blood moves into the right ventricle. So both of them receive deoxygenated blood. Once the blood returns from the lungs it is oxygenated and comes into the heart through the pulmonary veins into the left atrium then to the left ventricle then out to the body.
The coronary sinus is a large vein that collects deoxygenated blood from the coronary circulation and returns it to the right atrium of the heart. This blood has supplied oxygen and nutrients to the heart muscle and is now in need of oxygenation.