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Spirometry is used to test lung function. The person breathes normally, then quickly inhales deeply through a mouthpiece with attached tubing. She next exhales quickly until she has emptied her lungs as much as possible. These breaths are measured by the spirometer (at the right), which has a black plastic attachment that rises and falls with each complete breath. Many modern spirometers are electronic and do not have a piston or bellow that moves as a person breathes.
Video from Lahey Clinic Media Center.