To survive, your body needs oxygen which you get from the air you breathe. The lungs help take the oxygen from the air, through the air sacs (alveoli) and into the body.
The air sacs are surrounded by tiny blood vessels (capillaries), which crisscross the walls of the air sacs. The air sacs are where oxygen, which is a gas, is absorbed into the bloodstream.
Oxygen is then carried along in the bloodstream to the heart. The heart then pumps the blood to where it is needed in the body.
Carbon dioxide (CO2) is a waste gas product that is produced by the body. Carbon dioxide moves from the bloodstream back into the air sacs and through the breathing tubes or airways, where it is breathed out.