I had a campfire discussion not long ago with a man that got pulmonary edema while attempting the summit of Aconcagua, and we got into the discussion of the effects of high elevation. I had always assumed that after you adapted to the thin air, the risk to your body was how thick your blood got with the production of extra red blood cells to compensate for the lack of oxygen. The thicker blood making frostbite and edema more likely because of poor blood flow, and heart attacks and strokes more likely because of increased chances of forming blood clots. But my friend said it had less to do with thick blood and more to do with the lack of air pressure...

What does low pressure do to you? What *exactly* happens to your body at high altitude?