Air reaches tiny lung air sacs through the airways. Oxygen crosses from air in the sacs into nearby blood; carbon dioxide crosses the other way. Blood goes from the lungs to the left side of the heart, then to the body. Blood returning from the body goes through the right side of the heart to the lungs. The two sides belong to one heart.
Worked example
An arrow labeled oxygen points from a lung air sac toward nearby blood. It shows gas exchange, not the whole air sac moving and not the heart making oxygen.
A route poster to check
A museum draft says: airways bring air to lung air sacs; oxygen moves into nearby blood; that blood returns to the heart before it is pumped toward body cells. A second draft says the heart makes new oxygen. Use the model to check the drafts. This is a paper activity: do not change or hold your breathing, measure anyone, or collect health information.