How do seals avoid getting bent?

Many divers are under the impression that decompression sickness can only occur if you breathe while under water. (Either with conventional scuba equipment or possibly a rebreather) This is perhaps understandable, as the entry level dive manuals of PADI and many other scuba agencies have paragraphs that could be construed to indicate this.  In actual fact breath hold divers are no different than scuba divers – they are subject to the exact same laws of physics as scuba divers are.  Given enough exposure (a combination of depth and time) freedivers will develop the bends  just as any scuba diver would.  While this is extremely unlikely to occur on any single breath held dive (baring the most extreme dives) there are well documented cases of it happening to free divers conducting series of fairly long and deep dives consecutively.

So with this in mind, why is it that diving mammals such as seals and whales don’t develop decompression sickness? Well, there are essentially three ways which marine mammals can adapt to circumvent decompression sickness. 1) Either they have to be tolerant to the bubbles that form,  2) Prevent the supersaturation of gas  or 3) develop a mechanism to suppress the formation of bubbles to begin with.

Seals (with a few exceptions such as the artarctic fur seal) limit their susceptibility by exhaling air from the lungs as they dive. (that’d be method two above) which limits how much nitrogen can get absorbed into the blood stream.  This works for seals – although it obviously affects with decreased performance, same as it would if we were to exhale before breath hold diving.

How different whales do it isn’t fully understood – It is not known for example if whales exhale fully or partially as they dive. However, they do have oversize tracheae – when extreme depths are reached, the pressure will expel air from the lungs into the trachea, thereby preventing (or, at least, limiting to a great extent) nitrogen absorbtion via the lungs (the trachea is reinforced with bone rings to withstand the pressure).

If you’re interested in diving physics and fysiology you may wish to browse through the material of the PADI divemaster course, which offers a fairly decent overview of the theoretical aspects of both scuba diving and free diving.