Figure 7.2.1.

Sea lion lung. The smallest airways of deep-diving mammals such as the sea lion are buttressed by cartilage to ensure that all air is evacuated from the lungs when the animal is at depth to prevent the lungs bursting on rapid ascent. In humans there are several generations of airways that lack cartilage (the bronchioles) and consequently close before exhalation is complete. The trapped gas (the residual volume) is normally of no consequence but it poses a danger that the lungs may burst or of decompression sickness if ascent is rapid, whether in the sea or by air.

(Courtesy of Professor D Denison, Brompton, UK.)