The pulmonary system carries blood between the heart and the lungs. Deoxygenated blood travels into the heart through the superior and inferior vena cava into the right atrium to the right ventricle. The right ventricle then contracts to force the blood into the lungs via the pulmonary arteries. The tricuspid and pulmonary valves prevent the backflow of blood. In the lungs an exchange of gases takes place, carbon dioxide is breathed out and the oxygen is breathed in. This oxygenated blood returns to the heart via pulmonary veins.

The systemic system carries blood from the heart to the rest of the body and back again. Oxygenated blood enters the heart via the pulmonary veins. The blood travels through the left atrium and into the left ventricle. The left ventricle then contracts and forces blood out of the heart via the aorta. The blood then travels around the body. During this journey, the oxygen diffuses itself from the blood cells to supply oxygen to all the cells in the body and in turn carbon dioxide links itself to the deoxygenated blood cells. This deoxygenated blood is then carried back to the heart.