In the opening of this chapter we discussed the remarkable abilities of elephant seals to dive to great depths for long times and then spend only minutes breathing at the surface. We asked how elephant seals avoid getting the bends, a very painful condition that develops in scuba divers who surface too rapidly after spending time at depths. The pressure of the water on the body increases by 1 atmosphere for every 10 meters of depth. Body tissues are not compressible, but air in the respiratory system is, so scuba divers breathe air through a regulator that equalizes the air pressure in their lungs to the pressure of the water on the body. As the total air pressure goes up, the partial pressures of the component gases go up, increasing their rate of diffusion into the tissues. Eighty percent of the air is nitrogen, so the amount of nitrogen dissolved in tissues increases. If the diver comes up too rapidly, the dissolved nitrogen comes out of solution in the tissues, creating small bubbles that block blood vessels, damage tissues, cause pain, and can even result in death. To avoid the bends, divers use decompression tables that tell them how to ascend depending on how deep and how long their dives have been. For example, if a diver has spent 1 hour at a depth of 60 meters, he would have to spend 3 hours gradually ascending to shallower and shallower depths to allow the nitrogen dissolved in his tissues to diffuse out through the respiratory system rather than form bubbles in his tissues. Why does a seal who makes repeated dives not face the same consequences? The seal exhales before diving and the residual air in its respiratory system gets compressed into its airways, which are not respiratory exchange surfaces. So, when seals dive they do not have compressible air in their lungs that can be forced into tissues at depth and then come out of solution upon resurfacing.