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Your Guide to Lung Diseases.
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About.com Health's Disease and Condition content is reviewed by Sanja Jelic, MD

Definition: When absolute air volumes in the lungs must be measured, spirometry is insufficient. Therefore, your doctor may order body plethsmography to get more precise measurements of lung volumes. Body plethsmography can measure residual volume, function residual capacity, and total lung capacity.

During a body plethsmography, the patient sits in an airtight box, inhales or exhales a particular volume, and then a shutter closes off the breathing tube. The patient is then directed to breathe against the shutter's resistance, causing the chest volume to expand and decompress the air in the lungs. This increase in chest volume reduces the box volume and slightly increases the pressure in the box.