Blood travels to the lungs via the pulmonary circulation, the part of the cardiovascular system that circulates blood from the heart through the lungs in order to oxygenate it (see Module 4 for more information).

At the thin membranes of the alveoli, oxygen diffuses from the air into the blood while carbon dioxide moves in the opposite direction.

The oxygenated blood (blood that is rich in oxygen) returns to the heart. It is then pumped to the tissue cells via the systemic circulation, the part of the cardiovascular system that circulates blood around the rest of the body.

Press PLAY to see the video illustration of the pulmonary and systemic circulation in action.