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Healthy lungs exhaling. Frontal chest X-ray of healthy lungs (blue) during expiration (breathing out). Below the lungs lies the dome-shaped diaphragm (white), a muscular sheet between the chest cavity and abdominal cavity. The lungs are protected by the ribs (horizontal bands). During exhalation, the diaphragm relaxes into its dome- shape, while the intercostal muscles between the ribs also relax, letting the ribs drop down. These actions decrease the volume of the lungs but increase their internal pressure, which causes air to be forced out of the lungs. For an image of the lungs during inspiration (breathing in), see P580/166.
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