I recently watched a wonderful documentary called The Wildest Dream, about natural born climber George Mallory who may or may not've been the first to summit Mt. Everest in 1924.
Brilliant, breath-taking photography, adventure, danger, love and tons of bright shiny glittery morbidness. Altitude sickness, loss of appetite, severe dehydration, snow blindness, coughing up frostbitten larynx, gangrene, oddly cheerful nicknames for landmarks like "Green Boots Cave" and "Rainbow Alley" named after the corpses left on the mountain side.
And then, there's the Death Zone.
The lack of oxygen makes it impossible to function for long, impossible for the human body to acclimatize. The body uses up its store of oxygen faster than it can be replenished. Above 26k ft the body enters into necrosis, one is dying, humans weren't meant to survive at this altitude and you're on borrowed time.
Sources: Here, Here and the documentary
Couple of additional comments about the documentary. Liam Neeson has the neatest voice. George Mallory has the neatest face. And the most romantic quote ever, "We can't Apart as we can Together."