In this month's installment of Field Notes, Scott Bowe of Kemp Station discusses how the body adapts to recreation in high elevations.
Last month I had the opportunity to spend some time in Colorado’s Flat Tops Wilderness Area, about two hours west of Denver. We were not successful in our pursuit of elk, but we had a great time and met a lot of nice people. One thing that always amazes me are the challenges presented by camping at higher elevations. We don’t see those types of challenges here in the lowlands of Wisconsin. Let’s talk about elevation and the challenges and changes it brings to the human body.
The lowest point in Wisconsin is at the shores of Lake Michigan, 581 feet above sea level. The highest point is Timms Hill in Price County, about 40 miles northwest of Wausau. Timms Hill rises to 1,951 feet above sea level. In contrast, our campsite in Colorado was at 10,600 feet above sea level. That is 9000 feet higher than where I work at Kemp Station.
The first thing you notice at 10,600 feet is you become short of breath performing what should be simple activities. I guess this is not surprising for someone as slow and out of shape as I am. At sea level, air is made up of 78% nitrogen, 21% oxygen, and 1% other gases. At higher elevations, these percentages are the same, but the as elevation increases, the air pressure decreases. In other words, there are fewer molecules present in a given volume of air. Even though the percentage of oxygen is the same, 21%, there are fewer oxygen molecules at higher elevation because of the lower air pressure.
Atmospheric pressure decreases exponentially with elevation. As I mentioned earlier, the oxygen fraction remains constant, so the pressure of oxygen decreases exponentially with elevation. Here is a comparison. At sea level, the atmospheric pressure is a measured standard of 1 atmosphere. At 16,000 feet, atmospheric pressure is about ½ atmosphere, or ½ that of sea level. At our camp elevation of 10,600 feet, it is about 0.7 atmospheres, which translates into a 30% reduction of available oxygen from sea level.
What I find amazing is how quickly the human body adapts to higher elevations. A hike that is extremely taxing on day 1, gets much easier each successive day. The human body can adapt to high elevation through both immediate and long-term strategies. At high elevation, in the short term, the lack of oxygen is sensed by a cluster of specialized cells called carotid bodies. These cells are found in carotid arteries (the main arteries that run along both sides of your throat). It makes sense that the body would have these oxygen sensing cells near the brain, a strategy to keep enough oxygen going to the brain. When the body senses the decrease in available oxygen, its strategy is to increase your breathing depth and rate. In addition, at high elevation, the heart beats faster. Even while at rest in your sleeping bag, you notice your heart is beating faster than normal to deliver the need oxygen to the body.
In a weeklong trip to the Colorado mountains, you never fully acclimate. Full adaptation to high elevation is achieved when the body produces more red blood cells. Eventually, the increase of red blood cells reaches a plateau and stops. The length of time to full adaption can be estimated by multiplying the elevation in kilometers by 11.4 days. To adapt to our campsite at 10,600 feet, you would need to camp for about 37 days. In my mind, that is a long time to tent camp when the nighttime temperatures drop into the single digits. At night, you have to keep you water bottles in your sleeping bag to keep them from freezing solid!
The human body is an amazing and adaptable machine. The next time you head for the mountains, bring plenty of sun block and be prepared for some strenuous hikes and heavy breathing.
For Field Notes, this is Scott Bowe from the University of Wisconsin-Madison’s Kemp Natural Resources Station.