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Uploaded on Aug 3, 2007
The body's breathing apparatus is enclosed in an expansile bony cage, formed by the ribs, the spinal column and the sternum. The floor of the cage is closed by the muscular diaphragm. Air passes down the windpipe, or trachea, to enter the lungs. During inspiration, or inhaling, the diaphragm moves downward and becomes flatter, and the rib cage expands. This creates a partial vacuum in the lungs and, to equalize the pressure, air is drawn in. Expiration, or exhaling, is passive. The lungs have elastic tissue in their walls and during inspiration this is stretched. In expiration its natural recoil causes the lungs to partially deflate.