[MUSIC] At high altitudes, where the rate of diffusion of oxygen into our blood is slower, we experience a condition in which our tissues are deprived of oxygen, called hypoxia. Physiological responses in our bodies help us to adjust to low levels of oxygen in a process called acclimatization. While some physiological responses to hypoxia kick in immediately, other responses occur more slowly, often following days to weeks of exposure to low levels of oxygen. So first, let's consider immediate physiological responses that attempt to increase oxygen intake. During normal respiration, the amount of air that's moved in and out of the lungs in each breath is called the tidal volume. The average tidal volume of an adult is about 500 milliliters. However, of this 500 milliliters, only about 350 milliliters, or 70%, reaches the alveoli to deliver oxygen to the bloodstream. The other 150 milliliters, or 30% of inhaled air, is called dead space, because it remains in the airway where gas exchange does not occur. We can maximize oxygen intake by breathing deeper, which increases our tidal volume and through hyperventilation. Although we often associate hyperventilation with a health problem, hyperventilation just means breathing faster. Therefore, at high altitudes, hyperventilation is an adaptive response because increasing your breathing rate increases air intake. In contrast, dead space can't be decreased because it's a fixed volume. So ultimately, this means that increasing tidal volume is a more efficient response to hypoxia than increasing breathing rate. Increased tidal volume is the primary response to hypoxia. However, the amount we can increase tidal volume is limited by our maximum lung volume. Once this limit is reached, the only way to further increase oxygen uptake is to increase our breathing rate. But increasing ventilation doesn't fully restore the oxygen concentration gradient to the same degree as sea level. So other mechanisms are required to help get enough oxygen to where it is needed. In addition to breathing faster and more deeply, increased heart rate is a relatively rapid response to low oxygen conditions. When heart rate is increased, more blood is pumped through the body in a given amount of time. The increased amount of blood passing by the alveoli in the lungs means that more oxygen can be picked up. Even if less oxygen can be picked up on a given red blood cell, more red blood cells can pass through the lung to pick up oxygen over a given amount of time. Similarly, the increased amount of blood passing by the tissues allows more oxygen to be delivered. In this way, an increase in blood flow can compensate for lower amount of oxygen in the blood, so that oxygen demands of the tissues can still be met. In spite of these various ways that our bodies respond to help compensate for low oxygen levels at high altitudes, our capacity to perform work invariably declines. Reduced oxygen levels simply make physical tasks and exercise more difficult. For example, if you drove to the lookout at top Mount Evans in Colorado, at 4,300 meters above sea level, your capacity to perform work would be reduced by almost 25%, compared to sea level, even if you're healthy. The capacity to perform work at high altitudes may be suppressed even more in individuals who have breathing conditions, such as asthma, that limit how much they can increase their breathing rate or tidal volume. The ability to perform work becomes increasingly more difficult at higher altitudes. At extreme altitudes, even walking takes monumental effort. Beyond 8,000 meters above sea level, also referred to as the death zone by mountain climbers, the amount of oxygen is generally considered insufficient for humans to acclimatize and survive. The immediate and obvious effects of hypoxia on the body are just the beginning. Low oxygen levels also influence the autonomic nervous system. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions such as heart rate, digestion, respiratory rate, pupillary response, and urination. The serious health affects associated with ascending to high altitudes very rapidly have been known for centuries. The first accounts of the dangerous affects of altitude were recorded in Chinese texts in the first century, where they documented people becoming feverish and pale and suffering from headaches and vomiting. These so-called headache mountains were also known to Marco Polo during his travels in the far east at the end of the thirteenth century. He wrote of lofty mountains, which no one may visit at any price because the air in summer is so unwholesome and pestilent and it's a death to any foreigner. Other early accounts of human responses to hypoxia at high altitudes come from the 1800s, when the advent of ballooning meant that people were able to rapidly ascend to high altitudes for the first time. Along with adventurers, a number of scientists took up ballooning to explore the affects of altitude on the human body. Two scientists, James Glaisher and Henry Tracey Coxwell, ascended higher than 8,000 meters above sea level in less than an hour. To put this in context, there are only 14 mountains in the world with an altitude over 8,000 meters. It's no wonder that at the apex of their journey, Glaisher became incapacitated with the loss of movement and vision. Luckily, the pair was able to release the valve of the balloon and descend to safety. Unfortunately, other crews of scientists did not always return safely. In 1875, three scientists boarded The Zenith, and ill-fated French balloon, and ascended to 8,600 meters above seal level in just an hour. Two of the scientists did not survive, while the third was left partially deaf. The rapid ascent of these scientists to high altitudes produced examples of some of the extreme affects of hypoxia. However, more moderate increases in altitude can lead to more subtle affects of hypoxia. Most people can ascend to 2,500 meters without being affected by the lower oxygen levels. However, one in four people who travel to altitudes higher than 2,500 meters experience an illness called Acute Mountain Sickness, or AMS. The most common symptom of AMS is a headache, but it can also be accompanied by fatigue, loss of appetite, nausea, dizziness, an inability to sleep, and vomiting. These symptoms usually begin a few hours after ascent. Luckily, they usually fade in a couple of days as your body acclimatizes to the new environment. There are a number of factors that increase the likelihood that someone will get AMS. Here is University of Alberta physiologist, Craig Steinbach. >> Perhaps surprisingly, physical fitness and age do not seem to be influential factors. Young, healthy people are no less susceptible to AMS than older or less fit individuals. In fact, it's remarkably hard to predict who will be affected by AMS. It appears that some people are more inherently susceptible. Perhaps because they have limited ability to physiologically acclimatize. However, we do know that the likelihood that you'll be affected by AMS increases with your rate of ascent because faster ascents mean your body has less time to acclimate. Similarly, climbing to higher altitudes increases the likelihood of AMS because oxygen deprivation is more severe and acclimatization is less likely. At altitudes higher than 3,000 meters, at least 75% of people experience at least a mild form of AMS. Vigorous exercise also increases the likelihood of AMS, because higher levels of physical activity require higher levels of oxygen intake. >> The best way to prevent AMS is to ascend slowly and take it easy. A good rule of thumb is to increase the altitude at which you sleep by no more than 300 to 500 meters per day. Mountain climbers often use the old adage, climb high, sleep low, because your breathing rate decreases while you sleep, so the risk of AMS also increases. It's also recommended to take a rest day without any altitude gain for every 1,000 meters of ascent, or approximately every third day. It's also important to avoid the use of depressants, such as alcohol, that suppress your breathing rate. Only one drug, called acetazolamide, has been proven effective at preventing and treating AMS. It works by speeding up your breathing rate, so it can be especially helpful while sleeping. Remember, when you increase your breathing rate, you increase the amount of air available for oxygen exchange. Acetazolamide does not seem to mask the symptoms of AMS. However, like all prescription drugs, there can be mild to serious side effects. Common side effects of acetazolamide include a tingling sensation or pins and needles in your feet and hands, a greater tendency to sunburn more easily, and because it's a diuretic, it can also contribute to dehydration. People who occasionally travel to high altitude, including mountain climbers, tourists and trekkers, and rescue workers, are at risk of developing high altitude illnesses. If you do become affected by AMS and you find that your symptoms are not improving, you should stay at the same altitude and delay further ascent. If your symptoms worsen or do not go away in a couple of days, you should seek medical attention and descend to a lower altitude as quickly as possible. Severe AMS symptoms indicate that your body is not acclimatizing to the low levels of oxygen and can lead to more serious medical conditions, including high altitude cerebral edema, often abbreviated as HACE, or high altitude pulmonary edema, often abbreviated as HAPE. Edema is a condition in which excess fluid accumulates in body tissues. In HACE, the fluid accumulates in the brain, while in HAPE, the fluid accumulates in the lungs. Both conditions can be life threatening as fluid leaks from blood vessels into the tissues, causing them to swell. HAPE, in particular, can progress rapidly and is often fatal because when fluid collects in the lungs, it interferes with the diffusion of oxygen into the blood, causing oxygen levels in the blood to plummet further in a downward spiral. HACE is not well understood because it generally occurs in remote areas far from hospitals and doctors. Increased education and helicopter rescue capabilities have reduced the number of deaths attributed to HACE in recent years, but the symptoms of HACE have been reported in several cases of mountaineering accidents. Here's one such report from the world's highest mountain, Mount Everest. >> Dale Kruse was having an incredibly difficult time simply trying to dress himself. He put his climbing harness on inside out, threaded it through the fly of his wind suit, and failed to fasten the buckle. It was like I was very drunk, Kruse recollects. I couldn't walk without stumbling, and completely lost the ability to think or speak. It was a really strange feeling. I'd have some word in my mind but I couldn't figure out how to bring it to my lips. By the time Kruse arrived in base camp, he says, it was still another three or four days before I could walk from my tent to the mess tent without stumbling all over the place.
