Man breathes by drawing air into his lungs (inhaling) and letting it out again (exhaling). In breathing, a fresh supply of air is brought into contact in the lung with tissue very rich in blood. In the lung, gases are exchanged between blood and the air.

The lungs are large, soft organs, which fill both sides of the chest cavity. The lung tissue is like a fine sponge in some ways. The spaces, or air sacs, are the pockets where the air is received, the proper gases used, and the unwanted gases forced out.