The air inside the lungs is never replaced completely. It is always a mixture of the air left inside and the air inspired. In other words, the air in the lungs is only becoming better and worse with each inspiration and expiration.

Quality wise, the expired air differs from inspired air in the following respects:

• It contains less oxygen.
• It contains more carbon dioxide.
• It contains more water vapor.
• It is warmer (or at the same temperature as that of the body).
• It may contain some bacteria.