Blood flow in the pulmonary system
This circuit shows that deoxygenated blood (coloured blue) is taken from the right side of the heart to the lungs, where it can pick up oxygen. The newly oxygenated blood (coloured red) then returns to the left side of the heart, which then pumps the oxygenated blood around the body.
Because the diagram looks at the system from the front, the left side of the heart appears to be on the right, and the right on the left.
Here is a close-up diagram of the pulmonary system. The air we breathe in contains more oxygen and less carbon dioxide than the air we breathe out.