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 (mm...
