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.
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.