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Diffusion of a gas occurs when there is a net movement of molecules from a region in which that particular gas exerts a high partial pressure to a region in which it exerts a lower partial pressure. Movement by diffusion therefore differs from the movement of gases through the conducting airways, which occurs by bulk flow (mass movement or convection). During bulk flow, gas movement results from differences in total pressure, and molecules of different gases move together along the total pressure gradient. During diffusion, gas movement occurs in both directions, but because of its greater number of molecules per unit volume, the area of higher partial pressure has proportionately more random departures. The net movement of gas therefore depends on the partial pressure difference between the two areas. The rate of diffusion is temperature-dependent because random molecular movement increases at higher temperatures. In a static situation, diffusion continues until no partial pressure differences exist. In the lungs, oxygen and carbon dioxide continuously enter and leave the alveoli, so such an equilibrium is never reached.


By the time inspired air reaches the alveoli, the linear velocity of bulk flow decreases to zero. This decrease results mainly from the tremendous increase in the total cross-sectional area of the branching conducting airways, respiratory bronchioles, and alveolar ducts. As the total cross-sectional area increases, the linear velocity decreases:


In the alveoli, oxygen then moves through the gas phase according to its own partial pressure gradient. The distance from the alveolar duct to the alveolar-capillary interface is usually less than 1 mm. In the alveolar gas phase, diffusion occurs very rapidly and is believed to be assisted by the pulsations of the heart.

Oxygen then diffuses through the alveolar-capillary interface. First, it must move from the gas phase to the liquid phase, according to Henry’s law, which states that the amount of a gas absorbed by a liquid with which it does not combine chemically is directly proportional to the partial pressure of the gas and the solubility of the gas in the liquid. Oxygen must dissolve in and diffuse through the thin layer of pulmonary surfactant, the alveolar epithelium, the interstitium, and the capillary endothelium. It must then diffuse through the plasma, where some remains dissolved but most diffuses through the erythrocyte cell membrane and combines with hemoglobin. The thickness of this alveolar-capillary diffusion barrier is normally only about 0.5 μm, but this barrier thickness can increase in interstitial fibrosis or interstitial edema.

The blood then carries the oxygen out of the lung and distributes it to the other tissues of the body. At the tissues, oxygen diffuses from the red blood cell ...

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