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Changes in altitude have a profound effect on the human
body. The body attempts to maintain a state of homeostasis or balance to ensure the optimal operating environment for its complex chemical systems. Any change from this homeostasis is a change away from the optimal operating environment. The body attempts to correct this imbalance. One such imbalance is the effect of increasing altitude on the body's ability to provide adequate oxygen to be utilized in cellular respiration. With an increase in elevation, a typical occurrence when climbing mountains, the body is forced to respond in various ways to the changes in external environment. Foremost of these changes is the diminished ability to obtain oxygen from the atmosphere. If the adaptive responses to this stressor are inadequate the performance of body systems may decline dramatically. If prolonged the results can be serious or even fatal. In discussing altitude change and its effect on the body, mountaineers generally define altitude according to the scale of high (8,000 - 12,000 feet), very high (12,000 - 18,000 feet), and extremely high (18,000+ feet), (Hubble, 1995). A common misperception of the change in external environment with increased altitude is that there is decreased oxygen. This is not correct, as the concentration of oxygen at sea level is about 21% and stays relatively unchanged until over 50,000 feet (Johnson, 1988). What is really happening is that the atmospheric pressure is decreasing and subsequently the amount of oxygen available in a single breath of air is significantly less. At sea leve,l the barometric pressure averages 760 mmHg while at 12,000 feet it is only 483 mmHg. This decrease in total atmospheric pressure means that there are 40% fewer oxygen molecules per breath at this altitude compared to sea level (Princeton, 1995). The human respiratory system is responsible for bringing oxygen into the body and transferring it to the cells where it can be utilized for cellular activities. It also removes carbon dioxide from the body. The respiratory system draws air initially either through the mouth or nasal passages. Both of these passages join behind the hard palate to form the pharynx. At the base of the pharynx are two openings. One, the esophagus, leads to the digestive system while the other, the glottis, leads to the lungs. The epiglottis covers the glottis when swallowing so that food does not enter the lungs. When the epiglottis is not covering the opening to the lungs, air may pass freely into and out of the trachea. The trachea sometimes called the "windpipe" branches into two bronchi which in turn lead to a lung. Once in the lung the bronchi branch many times into smaller bronchioles which eventually terminate in small sacs called alveoli. It is in the alveoli that the actual transfer of oxygen to the blood takes place. The alveoli are shaped like inflated sacs and exchange gas through a membrane. The passage of oxygen into the blood and carbon dioxide out of the blood is dependent on three major factors: 1) the partial pressure of the gases, 2) the area of the pulmonary surface, and 3) the thickness of the membrane (Gerking, 1969). The membranes in the alveoli provide a large surface area for the free exchange of gases. The typical thickness of the pulmonary membrane is less than the thickness of a red blood cell. The pulmonary surface and the thickness of the alveolar membranes are not directly affected by a change in altitude. The partial pressure of oxygen, however, is directly related to altitude and affects gas transfer in the alveoli. To understand gas transfer it is important to first understand something about the behavior of gases. Each gas in our atmosphere exerts its own pressure and acts independently of the others. Hence the term partial pressure refers to the contribution of each gas to the entire pressure of the atmosphere. The average pressure of the atmosphere at sea level is approximately 760 mmHg. This means that the pressure is great enough to support a column of mercury (Hg) 760 mm high. To figure the partial pressure of oxygen you start with the percentage of oxygen present in the atmosphere which is about 20%. Thus oxygen will constitute 20% of the total atmospheric pressure at any given level. At sea level the total atmospheric pressure is 760 mmHg so the partial pressure of O2 would be approximately 152 mmHg. 760 mmHg x 0.20 = 152 mmHg A similar computation can be made for CO2 if we know that the concentration is approximately 4%. The partial pressure of CO2 would then be about 0.304 mmHg at sea level. Gas transfer at the alveoli follows the rule of simple diffusion. Diffusion is movement of molecules along a concentration gradient from an area of high concentration to an area of lower concentration. Diffusion is the result of collisions between molecules. In areas of higher concentration there are more collisions. The net effect of this greater number of collisions is a movement toward an area of lower concentration. In Table 1 it is apparent that the concentration gradient favors the diffusion of oxygen into and carbon dioxide out of the blood (Gerking, 1969). Table 2 shows the decrease in partial pressure of oxygen at increasing altitudes (Guyton, 1979). Table 1 ATMOSPHERIC AIR ALVEOLUS VENOUS BLOOD OXYGEN 152 mmHg (20%) 104 mmHg (13.6%) 40 mmHg CARBON DIOXIDE 0.304 mmHg (0.04%) 40 mmHg (5.3%) 45 mmHg Table 2 ALTITUDE (ft.) BAROMETRIC PRESSURE (mmHg) Po2 IN AIR (mmHg) Po2 IN ALVEOLI (mmHg) ARTERIAL OXYGEN SATURATION (%) 0 760 159* 104 97 10,000 523 110 67 90 20,000 349 73 40 70 30,000 226 47 21 20 40,000 141 29 8 5 50,000 87 18 1 1 *this value differs from table 1 because the author used the value for the concentration of O2 as 21%. The author of table 1 chose to use the value as 20%. In a normal, non-stressed state, the respiratory system transports oxygen from the lungs to the cells of the body where it is used in the process of cellular respiration. Under normal conditions this transport of oxygen is sufficient for the needs of cellular respiration. Cellular respiration converts the energy in chemical bonds into energy that can be used to power body processes. Glucose is the molecule most often used to fuel this process although the body is capable of using other organic molecules for energy. The transfer of oxygen to the body tissues is often called internal respiration (Grollman, 1978). The process of cellular respiration is a complex series of chemical steps that ultimately allow for the breakdown of glucose into usable energy in the form of ATP (adenosine triphosphate). The three main steps in the process are: 1) glycolysis, 2) Krebs cycle, and 3) electron transport system. Oxygen is required for these processes to function at an efficient level. Without the presence of oxygen the pathway for energy production must proceed anaerobically. Anaerobic respiration sometimes called lactic acid fermentation produces significantly less ATP (2 instead of 36/38) and due to this great inefficiency will quickly exhaust the available supply of glucose. Thus the anaerobic pathway is not a permanent solution for the provision of energy to the body in the absence of sufficient oxygen. The supply of oxygen to the tissues is dependent on: 1) the efficiency with which blood is oxygenated in the lungs, 2) the efficiency of the blood in delivering oxygen to the tissues, 3) the efficiency of the respiratory enzymes within the cells to transfer hydrogen to molecular oxygen (Grollman, 1978). A deficiency in any of these areas can result in the body cells not having an adequate supply of oxygen. It is this inadequate supply of oxygen that results in difficulties for the body at higher elevations. A lack of sufficient oxygen in the cells is called anoxia. Sometimes the term hypoxia, meaning less oxygen, is used to indicate an oxygen debt. While anoxia literally means "no oxygen" it is often used interchangeably with hypoxia. There are different types of anoxia based on the cause of the oxygen deficiency. Anoxic anoxia refers to defective oxygenation of the blood in the lungs. This is the type of oxygen deficiency that is of concern when ascending to greater altitudes with a subsequent decreased partial pressure of O2. Other types of oxygen deficiencies include: anemic anoxia (failure of the blood to transport adequate quantities of oxygen), stagnant anoxia (the slowing of the circulatory system), and histotoxic anoxia (the failure of respiratory enzymes to adequately function). Anoxia can occur temporarily during normal respiratory system regulation of changing cellular needs. An example of this would be climbing a flight of stairs. The increased oxygen demand of the cells in providing the mechanical energy required to climb ultimately produces a local hypoxia in the muscle cell. The first noticeable response to this external stress is usually an increase in breathing rate. This is called increased alveolar ventilation. The rate of our breathing is determined by the need for O2 in the cells and is the first response to hypoxic conditions.


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