The right atrium receives the blood first. The right atrium contracts and forces the blood into the right ventricle. When the right ventricle contracts the blood is pumped into both lungs via the pulmonary artery. This portion of the circulatory system is sometimes referred to as pulmonary circulation or lesser circulation. The pulmonary artery is the only artery in the body that carries deoxygenated blood.
Blood is returned from the lungs via the pulmonary veins.
These are the only veins in the body that carry oxygenated blood. The oxygenated blood is returned to the left atrium. When the atrium contracts the blood is forced into the left ventricle. The left ventricle is the strongest and most muscular portion of a healthy heart. This is due to the fact that the left ventricle works the hardest.