Figure 12 - Membrane diffusion.

Diffusion of gases is mediated by the random movements of molecules, which tend to proceed from regions with higher concentrations to regions of lower concentration. Diffusion is directly proportionate to the concentration difference between the alveoli and capillaries, and the permeability and surface areas of the barrier separating these compartments, and is inversely related to the thickness of the barrier. The diffusing capacity measured with CO (DLCO) is influenced by the hemoglobin concentration of the blood, the distribution of the gas to the alveoli, and the capillary surface area, but is relatively insensitive to the thickness and permeability of the alveolar-capillary membranes. The amount of gas that diffuses through the barrier is directly proportionate to the partial pressure gradient of the gas (p), surface area of the membrane (A), the exposure time (t), and the permeability of the membrane (P), and is inversely proportionate to the thickness of the membrane (x).