Normal inspiration is accomplished by the expansion of thorax or chest cavity. It occurs when the muscles of inspiration contract.During contraction, the diaphragm descends and enlarges the vertical size of thoracic cavity. The external intercostal muscles contract and raise the ribs slightly, increasing the circumference of thorax. The activities of these muscles represent the "work" required to inspire.

 

Normal exhalation is passive and does not require any work. During normal exhalation, the muscles relax, the diaphragm moves upward to its resting position, and the ribs return to their normal position. The volume of thoracic cavity decreases, and air is forced out of alveoli.