Governing Laws

The net diffusion rate (D_R) of a gas across a fluid membrane depends upon a temperature-dependent diffusion constant (K), and is proportional to the difference in partial pressure (P₂ – P₁), proportional to the area of the membrane (A) and inversely proportional to the thickness of the membrane (d).

Mathematically D_R = KA(P₂ – P₁)/d

The anatomy of the lung is ideally suited for the migration of gases either from the alveoli to the blood or in the reverse direction. The alveolar membrane is approximately 100m² and is only 0.5μm thin. This is especially helpful to mitigate the poor solubility of O₂ in blood.

Under normal circumstances, the combination of all the factors affecting gas transfer will enable the RBCs in the pulmonary capillaries to collect their maximum limit of oxygen in less than 0.25 seconds. As the time taken by red cells to travel along the pulmonary capillaries is about 0.75 seconds in a resting healthy adult, there is normally a great deal of diffusion reserves. However, high cardiac output states may shorten the capillary transit time sufficiently to cause hypoxia.

Henry’s law

At a constant temperature, the amount of a particular gas dissolved (C) in a given type and volume of liquid is directly proportional to the partial pressure (P) of that gas in equilibrium with that liquid.

Mathematically P = kC

At most, 2% of the transported O₂ is in solution. Henry’s law dictates this insignificant quantity of O₂ dissolved in plasma. It will be noted that increasing the partial pressure will permit greater gas quantities to be dissolved and the advent of hyperbaric therapy takes advantage of this law.

There is, however, a limit of tolerance of pressures to which a patient may be subjected.