Breathing consists of two phases. The first phase is the inspiration phase. Inspiration allows air to flow into the lungs. The second phase is expiration. Expiration involves gases leaving the lungs. During inspiration, the diaphragm and intercostal muscles contract allowing air to enter the lungs. During expiration, the inspiration muscles relax forcing gases to flow out of the lungs.
Last reviewed 7/16/2011 by Jacob L. Heller, MD, MHA, Emergency Medicine, Virginia Mason Medical Center, Seattle, Washington. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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