Blood becomes oxygenated in the lungs, then the oxygenated blood goes to the heart via the pulmonary veins and enters the left atrium
Before blood returns to the left atrium, it is collecting oxygen from the lungs. While in the lungs it is releasing carbon dioxide to be exhaled.
pulmonary veins
left atrium
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.
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
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.
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.
Simply the deoxygenated blood goes into right atrium into right ventricle. oxygenated blood goes into left atrium then into left ventricle
Blood transported by the pulmonary veins returns to the left atrium of the heart. The pulmonary vein carries oxygenated blood away from the lungs.
Cornary sinus