Plethysmography is a medical technique used to measure changes in volume within an organ or body part. It is commonly employed to assess blood flow, lung function, or body composition. The method typically involves using a device that detects variations in pressure or electrical resistance as blood or air fills and empties from the area being studied. There are different types of plethysmography, including air plethysmography for measuring lung capacity and photoelectric plethysmography for monitoring blood flow in extremities. This non-invasive technique provides valuable information for diagnosing various health conditions and monitoring treatment effectiveness.