The conducting airways do not participate in gas exchange. Water vapor pressure, however, has reduced the partial pressures of O 2 and N 2. Note that the total pressure has remained 760 mm Hg (150 + 563 + 47 mm Hg). Thus, in the conducting airways the partial pressure of O 2 is To calculate the partial pressure of a gas in a humidified mixture, the water vapor partial pressure must be subtracted from the total barometric pressure. Water vapor pressure at body temperature is 47 mm Hg. Inspired gases become saturated with water vapor, which exerts a partial pressure and dilutes the total pressure of the other gases. Thus, ambient O 2 tension can be increased through the administration of supplemental O 2 and is decreased at high altitude.Īs inspiration begins, the ambient air is brought into the airways, where it becomes warmed to body temperature and humidified. The O 2 tension at the mouth can be altered in one of two ways-by changing the fraction of O 2 or by changing barometric (atmospheric) pressure. This is the O 2 tension (i.e., the partial pressure of O 2) of ambient air at the mouth at the start of inspiration. Therefore, the partial pressure of O 2 in ambient air (P O 2) is The second important principle is that the partial pressure of a gas (P gas) is equal to the fraction of that gas in the gas mixture (F gas) times the total or ambient (barometric) pressure.Īmbient air is composed of approximately 21% O 2 and 79% N 2. Thus, at sea level, where atmospheric pressure is 760 mm Hg, the partial pressures of the gases in air (also known as barometric pressure (P b) are It follows, then, that the sum of the partial pressures (in mm Hg) or the tensions (in torr) of a gas must be equal to the total pressure. Henry’s law states that the concentration of a gas dissolved in a liquid is proportional to its partial pressure. Boyle’s law states that when temperature is constant, pressure (P) and volume (V) are inversely related, that is,ĭalton’s law states that the partial pressure of a gas in a gas mixture is the pressure that the gas would exert if it occupied the total volume of the mixture in the absence of the other components. Three important gas laws govern ambient air and alveolar ventilation: Boyle’s law, Dalton’s law, and Henry’s law.
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