From the front door of our tent, Bob Ayers and I gazed up at Mount Parinacota, a symmetrical, ice-covered cone rising 6000 feet above Lake Chungara in Northern Chile. Our camp was at 15,000 feet, higher than the summit of Mount Whitney, the highest point in the continental United States. It had taken us only a few hours to drive to this high lake, traveling across the Atacama Desert from the seacoast town of Arica. We had come to climb mountains and to rendevous with the shadow of the moon during a total solar eclipse.
Both of us are mountaineers who live near sea level, and both of us had trained for this trip. We had bicycled and hiked, climbed stairs, and slept high--at 10,000 feet in the Sierra Nevada of California. Our muscles and cardiovascular systems were, we thought, quite fit.
I went to bed that first night confident of my ability to deal with the stresses of Chile's higher elevation. I thought I was coping adequately with significatly less oxygen than I was used to. But, as I slept, my breathing rate slowed, decreasing the amount of oxygen in my brain even more.
I woke up at midnight with the worst headache I've ever experienced. Whenever I moved my eyes, lights flashed inside my head. My stomach felt queasy, and I couldn't sleep. In the morning, I still had the headache and was tired too. Bob looked as bad as I felt. We both had the classic symptoms of mountain sickness.
The cure is easy--just go down 3,000 feet. But we didn't want to go down, we wanted to go up. We knew, though, that it would be foolhardy to climb higher. Mountain sickness may be followed by two serious illnesses, high-altitude pulmonary edema and cerebral edema, in which the lungs or brain fill with fluid. Both can be fatal.
We compromised. We decided to stay at camp for a few days to give our bodies a chance to adapt to the high altitude, although even staying at 15,000 feet involved some risk. To those who will not go down, the medical advice is to drink plenty of fluids, take aspirin, eat high-carbohydrate, low-fat foods, avoid alcohol, and exercise gently. That's what we did.
As mountaineers go up, they are climbing above part of the atmosphere. By 18,000 feet, a climber is above half of the atmosphere, which means that atmospheric pressure and density are half of their sea-level values. Each breath of air contains half as many molecules as at sea-level--in particular, only half as many molecules of oxygen. Breathing at 18,000 feet feels as if someone had ripped out one of your two lungs. (It's even worse on the summit of 29,000 foot Mount Everest, where climbers are above two-thirds of the atmosphere.)
Body tissues need a certain amount of oxygen to function properly, but a body can adapt to less oxygen using the available oxygen more efficiently. Shortly after arriving at a high elevation your body begins to make changes. You begin to breathe more often, and each breath is deeper. Tiny air sacs in the lungs called alveoli, which are normally collapsed when you breathe at rest at sea-level, begin to inflate with each breath. Your heart rate increases, and the blood pressure in your lungs goes up.
In about three days, your body completes these short-term changes and also begins to produce more red blood cells. After six weeks, the increase in blood cells reaches a maximum amount. Although these changes boost the amount of oxygen your body tissues receive, your performance as a runner or hiker will still be lower than at sea level.