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Air enters into the lungs by contraction of diaphragm and leaves the lungs by relaxation of diaphragm. Respiratory muscles are required for inhaling and exhaling air from the atmosphere. These muscles are found in the lungs. The most important respiratory muscles are the inter costal muscles which are found in sets. The external intercostal muscles that pull the ribs together thus raising the rib cage during inspiration and the internal intercostal muscles which depress the rib cage during expiration.