Blood returning from the periphery of the body is collected in the vena cava (the posterior vena cava coming from the caudal parts of the body and the anterior vena cava coming from the cranial parts of the body). This blood is low in oxygen and high in carbon dioxide.  It is collected in the right atrium and passed into the right ventricle, which then pumps it into the pulmonary circulation via the pulmonary artery into the lungs. In the lungs gas exchange takes place during which carbon dioxide is discharged into the air and fresh oxygen is taken up by the blood.

The oxygenated blood returns to the heart in several pulmonary veins and is collected in the left atrium before being passed into the left ventricle. The left ventricle, which possesses a very thick muscular wall, then pumps the blood into the systemic circulation via the largest artery in the body, the aorta. Several large blood vessels branch off the aorta close to the heart including the subclavian artery which supplies the shoulder, neck and head and from which the carotid arteries branch off to supply the head and the brain.