The main function of a rat's lungs is to support breathing. Breathing is accomplished through respiration in which air enters the body through the nose or mouth, travels down the trachea and into the bronchi of the lungs.
The alveoli, small branches of the bronchi, are specifically designed to perform gas exchange at the cellular level. Oxygen in the alveoli enters the bloodstream through diffusion, and carbon dioxide enters the lungs. This carbon dioxide is expelled when the rat exhales.
The lungs are surrounded by ribs and the muscular diaphragm. The diaphragm contracts during inspiration, creating negative pressure and pulling air into the lungs. When the diaphragm relaxes, expiration occurs.