A plethysmograph is any device used for measuring changes in volume within a body, either due to the presence of blood or, with the lungs, air.

The lung plethysmograph consists of a sealed chamber about the size of a telephone booth which contains a single mouthpiece. To run a lung test, the patient is asked to exhale through the tube. The tube is then sealed, and the patient is asked to try to inhale through the now sealed tube. This allows the patient's lungs to expand even though they aren't taking in any air. As the body expands, the air pressure in the chamber also increases and this increase is measured to determine total lung volume.

Plethysmographs can also be used to measure the circulation of blood through limbs, by immersing the limb in a sealed box full of either water or air.

A penile plethysmograph is used to measure blood flow in the penis.

Plethysmograph at Wikipedia

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