I'm finishing up my really great book that I got myself for my birthday about the physiological effects of extreme environments on the body. I just finished the chapter about diving, and the author goes into detail about what actually happens to your body when you get "the bends." You get the bends when you're scuba diving and you ascend too quickly. The air that you have been breathing in your tank is really pressurized and and your blood becomes very full of nitrogen. This typically isn't a problem, except when you rise too quickly and the nitrogen separates from the blood and forms bubbles.
Basically, it's like you're a soda can, and when you go deep under water it's like you have been shook up a lot and are under a lot of pressure. If you open the can too quickly (a.k.a. ascend out of deep water too quickly) bubbles will actually form in your blood and every other fluid in your body. Your body doesn't know how to handle bubbles--it has never encountered them before. You blood starts to clot because it thinks you've been cut. You're blood vessels constrict because, hey, maybe that'll help. Bubbles form in the liquid in your joints and causes immense pain. People who get the bends try to flex and bend all their joints to relieve the pain (hence the name). Sometimes the bubbles cause permanent hearing loss and blindness. Sometimes it causes paralysis by bubbles forming in the fluid in the spinal cord. Sometimes it causes death by bubbles blocking certain pathways in the brain.
I guess what I think is fascinating about this is not how bubbles affect the body, but how something so simple as nitrogen bubbles can bypass they body's amazing and intricate defense system. This is a relatively new condition, one that humankind has brought upon itself. The only true cause is the human desire to push the natural limits and go to places that the human body can't handle.