The advantage of the cetacean respiratory system lies not in its size, but rather in its efficiency. The large tidal volume of the cetacean lung allows it to exchange nearly all of the depleted air in its lung with each exhalation. Tidal volume is the amount of air that passes in and out of the lung during normal breathing. In terrestrial mammals the tidal volume ranges from 10% to 15% of the total capacity of the lungs. Marine mammal lungs have an average tidal volume of 75% of the total capacity of the lungs. The vital capacity, or maximum tidal volume, even ranges up to 90% of total lung capacity for some cetaceans (Wartzok 2002). For a marine animal, increased tidal volume is superior to simply having larger lungs. With larger lungs, more energy would be needed to counteract the resultant increased buoyancy. For this reason, increased tidal volume is both a spatial and energetic advantage for cetaceans.
Mass-corrected representation of maximum lung capacity and tidal volume change in various mammals. Figure adapted from Wartzok 2002.
The cetacean lung is able to replace so much of its tidal air because of the elasticity of its tissues. Like the conduction tract, the lungs are composed of a large amount of cartilage which will expand to facilitate large volumes of air and will also forcefully expel used gases during exhalation. While the lungs and alveoli collapse under pressure, the trachea and conduction tract does not. This same mechanism that prevents decompression sickness also aids in the rapid and extensive turn-over of tidal air. Because the airway remains open when placed under great stress, the flow rates into and out of the lung are extremely high compared to terrestrial mammals (Wartzok 2002).
