Partial Pressures and Diffusion of Gases Through Liquids

Partial Pressure

At sea level the atmospheric pressure is close to 760 mm Hg (i.e., the mixture of gases that comprise atmospheric air exerts a total pressure of 760 mm Hg). The major components of dry air are nitrogen (approximately 79%) and oxygen (approximately 21%). According to Dalton’s law, in a mixture of gases the portion of the total pressure resulting from each type of gas is determined by the percentage of the total volume represented by each gas type. The pressure exerted by each type of gas in a mixture is referred to as the partial pressure of that gas. Because nitrogen comprises 79% of the volume of atmospheric air, the partial pressure resulting from nitrogen is 0.79 times 760 mm Hg, which equals 600.2 mm Hg. Because oxygen comprises approximately 21% of the volume of atmospheric air, the partial pressure resulting from oxygen is 0.21 times 760 mm Hg, which equals 159.5 mm Hg. It is traditional to designate the partial pressure of individual gases in a mixture as PN₂, Po₂, or Pco₂, for example.

When air comes into contact with water, some of the water turns into a gas and evaporates into the air. Water molecules in the gaseous form also exert a partial pressure. This partial pressure (PH₂o) is sometimes referred to as the vapor pressure of water. The composition of dry, humidified, alveolar, and expired air is presented in Table 1. The composition of alveolar air and of expired air is not identical to the composition of dry atmospheric air for several reasons. First, air entering the respiratory system during inspiration is humidified; second, oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the alveolar capillaries into the alveoli; and third, the air within the alveoli is only partially replaced with atmospheric air during each inspiration.

 Diffusion of Gases Through Liquids

 When a gas comes into contact with a liquid such as water, there is a tendency for the gas to dissolve in the liquid. At equilibrium the concentration of a gas in the liquid is determined by its partial pressure in the gas and by its solubility in the liquid.  This relationship is described by Henry’s law.

 Concentration of dissolved gas = Partial pressure of gas x Solubility coefficient

 The solubility coefficient is a measure of how easily the gas dissolves in the liquid. In water the solubility coefficient for oxygen is 0.024, and for carbon dioxide it is 0.57. Thus carbon dioxide is approximately 24 times as soluble in water as oxygen. Gases do not actually produce partial pressure in a liquid as they do when in the gaseous state. However, knowing the concentration of the gas in liquid, it is possible to determine mathematically (general gas law) its partial pressure as if it were in a gaseous state. Because the partial pressure thus calculated is a measure of concentration, it can be used to determine the direction of diffusion of gas through a liquid: gases move from areas of higher to areas of lower partial pressure.

Partial pressures of gases at sea level

Table 1

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