Pulmonary volumes

The volume of air in the lungs can be measured with a technique called spirometry. A spirometer measures the volumes of air inspired and expired and therefore changes in lung volume. Although more sophisticated spirometers are used today, a simple spirometer contains a bell filled with air that is partially submerged in water. A tube runs from the person's mouth under the water and emerges inside the bell, just above the water level. As the person exhales, air flows down the tube and into the bell, causing the bell to rise. The bell is attached to a pen, and its movement is recorded on a simple rotating drum.

This technique is used clinically to measure lung volumes, capacities, and flow rates as an aid in diagnosing such respiratory diseases as asthma.

The amount of air entering and leaving the lungs with each breath is known as the tidal volume. The vital capacity(VC) is the greatest amount of  air that can be expired after a maximal inspiration. The amount of air remaining in the lungs after a maximal expiration is the residual volume(RV). The residual volume cannot be measured with spirometry. The total lung capacity(TLC) is the sum of the vital capacity and the residual volume.