The right atrium receives the de-oxygenated blood from the body. From the right atrium blood passes to the right ventricle.
The right ventricle propels this de-oxygenated blood to the lungs though pulmonary artery, for oxygenation. The deoxygenated blood loses carbon-di-oxide and gains oxygen in the lungs. This process is called gaseous exchange.
After the gaseous exchange, blood becomes oxygenated. This oxygenated blood comes to left atrium of the heart through pulmonary veins.
From the left atrium blood comes to the left ventricle. Left ventricle sends this oxygenated blood through aorta and arteries to all organs of the body.