Earth’s Hypobaric Environment
I entered the world of mountaineering inwhen I climbed Mount Rainier via the Camp Muir route. There were seven of us, five novice mountaineers and two accomplished guides. Our first day involved a leisurely hike from Paradise Lodge at to Camp Muir at ,ft, via the snowfields. After dinner, we slept in our tents until : P.M. put on our climbing gear, roped up, and went for the summit attempt. The moon was almost full and the alpine glow was spectacular as we ascended Disappointment Cleaver. At about ,ft, I began to experience the effects of chronic hypoxia: fatigue, irritability, and a headache in the frontal lobe. As we approached the false summit at ,ft, my headache spread to the top of my head and felt like it was securely fastened inside a vice grip. I was very nauseous and wanted to vomit, but did not. My body was extremely dehydrated and my urine was a dark yellow color. After shaking hands and taking pictures, we started down. The descent was fast. My headache and nausea went away and the fatigue was less noticeable.
The previous chapter described earth’s only hyperbaric environment. Here we consider the opposite challenge to homeostasis: the reduced pressure hypobaria and oxygen content hypoxia of high terrestrial environments, known in everyday terms as ”thin air. When the body acclimatizes to high altitude, primarily the respiratory and circulatory systems respond to hypobaria and hypoxia. Several unique illnesses may result from high-altitude exposure; their incidence increases with the speed of the ascent, altitude reached, and the length of stay. Safety recommendations focus on nutritional, fluid, and pharmacologic interventions that optimize health and exercise performance.
The Nature of Air at High Altitude
The opening description of acute mountain sickness was recorded during a conversation with Donald Winant, who is a determined triathlete as well as a former collegiate swimmer and Air Force officer. He is one of the few people on earth who has had the opportunity to experience hypoxia low levels in blood or tissues in three different ways: flying high-performance jet aircraft acceleration-induced hypoxia, resting in an altitude chamber acute hypobaric hypoxia, and climbing a mountain chronic altitude-induced hypoxia.
In contrast to chapterwhich focused on hyperbaria in underwater environments, chapter deals with the physiologic responses and medical problems associated with hypobaria low pressure at high terrestrial altitudes. The atmospheric pressure at these heights is considerably less than at sea level which is known as atmosphere, or atm. Figureillustrates this fact. A young woman is holding a balloon at various locations above and below sea level; the temperature is identical at all locations. At sea level, the volume of the gas inside the balloon is affected by the atmospheric pressure of that location, atm. As she descends into a mine, the volume of the balloon decreases due to increasing air pressure Boyle’s law; see chapterConversely, as she ascends a mountain, the balloon expands
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The volume of air inside a balloon changes at different altitudes, above and below sea level. Although the percentage of oxygen is constant, the density of the gas changes due to differences in atmospheric pressure as the atmospheric pressure decreases. This is due to the fact that the column of air compressing the balloon on the mountain weighs less than that inside the deep mine.
Above sea level, the atmospheric pressure diminishes by about for an increase of approximately mft. Thus, air has a lower density at higher altitudes fewer particles per cubic inch because the gas has expanded. Even though the percentage of oxygen and other gases in air does not change as altitude increases up to an altitude ofm, ft, the thinner air presents less oxygen to the lungs, alters physiologic responses, and produces unique illnesses because of the decrease in gas density.
Three definitions will help you understand the physiologic responses and medical problems associated with hypobaric-hypoxic environments those with low air pressure and low O. In the paragraphs below, the term high altitude refers to sites that are – -, ft above sea level. Very high altitude refers to locations that are – m-, ft above sea level; more than million people live at this elevation range. The term extreme altitude refers to any locale above mft; few humans reside above this elevation. It is likely that about mft is the upper limit for human habitation, although the caretakers of the Aucanquilcha mine in northern Chile live at mft. When no specific elevation is implied or when a principle applies to high, very high, and extreme altitudes, the general term altitude is utilized.