each component gas, which, in the case of air, includes nitrogen, oxygen, and water vapor. Functionally, the partial pressure of a gas is determined by the product of a gas's fractional concentration and the total pressure. Oxygen composes 21% of air. At sea level (760 mm Hg), the partial pressure of oxygen is 160 mm Hg (760 mm Hg × 0.21). Since nitrogen is 79% of air, the partial pressure of nitrogen is 600 mm Hg (760 mm Hg × 0.79). The water vapor in air is quite variable and dependent upon the temperature and relative humidity of air. In the conductingzone of the lungs, air is 100% humidified, which, at 37°C, is 47 mm Hg, which decreases the partial pressure of the other gases (760 - 47 = 713 mm Hg total pressure). The amount of carbon dioxide in air is quite small, making up only 0.03% of air, which correlates to a PCO2of less than 1 mm Hg at sea level. While partial pressure is the driving force for the diffusion of a gas, the movement of a gas from air to water is affected by the solubility of that gas. Carbon dioxide is more soluble in water than oxygen. At the alveolus, the interface between air and water, the PO2and PCO2of alveolar air and blood have equilibrated such that the blood leaving the lungs has the same partial pressure as alveolar air. However, the difference in gas solubility means that a greater partial pressure gradient is required for oxygen to diffuse into the blood than carbon dioxide. At the same time, this means that, at equal partial pressures, there is 20 times more carbon dioxide in water than oxygen. The solubility of a gas is described by Henry's Law, which states that c = k × P, where c is molar concentration and k is a constant, which varies by temperature and the solubility of the gas. The greater the solubility of the gas, the greater the concentration of that gas in solution.
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