The Effects Of Altitude On Human Physiology — страница 2

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directly related to altitude and affects gas transfer in the alveoli. GAS TRANSFER 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 choose to use the value as 20%. CELLULAR RESPIRATION 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. ANOXIA 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