However, this means that in disease states which impair the ability of the lungs to adequately ventilate with oxygen, oxygen exchange is often compromised before that of carbon dioxide. This difference between the rate of diffusion of the individual molecules is compensated for by the large difference in partial pressures of oxygen, creating a larger diffusion gradient than that of carbon dioxide.
In the lungs, whilst oxygen is smaller than carbon dioxide, the difference in solubility means that carbon dioxide diffuses roughly 20 times faster than oxygen. The temperature of the fluid – not important within the lungs and can be assumed to be 37 oC.The partial pressure difference across the diffusion barrier.Fick’s Lawįick’s law gives us a number of factors that affect the rate of diffusion of a gas through fluid: If we assume that the conditions of temperature and pressure for all gases remain fixed (as they roughly do in the alveoli) then it is the inherent differences between different gases that determine their solubility.Ĭarbon dioxide is inherently more soluble than oxygen, and thus diffuses much faster than oxygen into liquid. “The amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid”. The solubility of a gas is defined by Henry’s law, which states that: The more soluble a gas is, the faster it will diffuse. When gases are diffusing through liquids, for example across the alveolar membrane and into capillary blood, the solubility of the gases is important. In other words, the smaller the mass of a gas, the more rapidly it will diffuse. “The rate of diffusion is inversely proportional to the square root of its molar mass at identical pressure and temperature” When gases are diffusing through other gases (such as in the alveoli), their rate of diffusion can be defined by Graham’s Law: Fig 1 – Equation to calculate pressure of a gas in a container Diffusion of Gases Through Gases
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