Viesturs also has an anaerobic threshold of about 90 percent, meaning he can climb for hours on end using 90 percent of his maximum exercise capacity without collapsing. Most people collapse at levels around 60 percent.
But more than this physiology, Schoene said, Viesturs moves with incredible efficiency. At extreme altitude, any exertion, even getting out of a sleeping bag, leaves climbers short of breath. Efficient movement is essential, Schoene said.
This efficiencycombined with training, mountain savvy, and a desire to succeedputs Viesturs on top of the world time and again. He reached the summit of Mount Everest for the sixth time in May.
"I picked the right sport," Viesturs said.
While Viesturs does excel at high-altitude climbing, not even he can fly from his home in sea-level Seattle to Nepal and climb Everest the very next day. "For anybody to do anything at these altitudes, the body needs to adapt," Schoene said.
The body's task is to deliver oxygen from the air to mitochondria in the cells of muscle tissue, where it is turned into energy, Schoene explained. Getting the oxygen to the mitochondria, however, is a step-by-step process, each step affected by a change in altitude.
A person at high altitude will first notice an increase in their breathing rate. This reflects the need to get more air into their lungs. Their heart has to pump more blood to pick up and deliver oxygen from their lungs to body tissues where cells reside. There, oxygen must diffuse from tiny blood vessels to cells, where the gas can be metabolized by mitochondria.
"Everyone is a little bit different in terms of speed of acclimation and in terms of coping," Schoene said. A general rule of thumb for mountain climbers is to spend two nights at the same elevation for every 3,000 feet (915 meters) gained above about 10,000 feet (3,000 meters).
People who ascend too high too quickly may experience what is known as acute mountain sickness. Common symptoms are shortness of breath, headaches, loss of appetite, and insomnia, which Schoene said are all related to the lack of oxygen at altitude.
Once climbers venture above about 20,000 feet (6,000 meters), they enter what is known as the death zone. It is the point at which mountaineers can no longer escape the effects of altitude and can only hope to minimize them.
Above the death zone is where many mountaineers begin to lose their focus and make fatal errors of judgment, an affliction that Viesturs escapes.
"I do really well with less oxygen," he said. "I've gone to the top of Everest without oxygen and sat on top for an hour and had lunch. I was not crawling on my hands and knees."
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