## OBJECTIVES

The reader understands the diffusion of gases in the lung.

• Defines diffusion, and distinguishes it from “bulk flow.”

• States Fick’s law for diffusion.

• Distinguishes between perfusion limitation and diffusion limitation of gas transfer in the lung.

• Describes the diffusion of oxygen from the alveoli into the blood.

• Describes the diffusion of carbon dioxide from the blood to the alveoli.

• Defines the diffusing capacity and discusses its measurement.

• Interprets standard pulmonary function test (PFT) data.

Diffusion of a gas occurs when there is a net movement of molecules from an area in which that particular gas exerts a high partial pressure to an area in which it exerts a lower partial pressure. Movement of a gas by diffusion is therefore different 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, different gases move according to their own individual partial pressure gradients. Gas transfer during diffusion occurs by random molecular movement. It is therefore dependent on temperature because molecular movement increases at higher temperatures. Gases move in both directions during diffusion, but the area of higher partial pressure, because of its greater number of molecules per unit volume, has proportionately more random “departures.” Thus, the net movement of gas is dependent on the partial pressure difference between the two areas. In a static situation, diffusion continues until no partial pressure differences exist for any gases in the two areas; in the lungs, oxygen and carbon dioxide continuously enter and leave the alveoli, and so such an equilibrium does not take place.

## FICK’S LAW FOR DIFFUSION

Oxygen is brought into the alveoli by bulk flow through the conducting airways. When air flows through the conducting airways during inspiration, the linear velocity of the bulk flow decreases as the air approaches the alveoli. This is because the total cross-sectional area increases dramatically in the distal portions of the tracheobronchial tree, as was seen in Figure 1−2. The linear velocity of bulk flow through a tube is equal to the flow divided by the cross-sectional area:

$Linear velocity (cm / s)=Flow (cm3​/ s)÷Cross-sectional area (cm2​)$

By the time the air reaches the alveoli, bulk flow probably ceases, and further gas movement occurs by diffusion. Oxygen then moves through the gas phase in the alveoli according to its own partial pressure gradient. The distance from the alveolar duct to the alveolar-capillary interface is usually less than 1 mm. Diffusion in the alveolar gas phase is believed to be greatly assisted by the pulsations of the heart and blood flow, which are transmitted ...

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