# 5.7: Diffusion

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Recall that the alveoli are the destinations for inspired air. Their great numbers and deeply curved surfaces provide a great deal of surface area.

The walls of the alveoli are only one cell thick, and the cells are flat and very thin. Thin-walled capillaries surround the alveoli. Only an exceedingly thin noncellular layer (basement membrane) separates the alveolar wall from the capillary wall. These structural features provide a thin surface through which gases must pass. The secretions from some alveolar cells keep their surfaces moist. Thus, the alveoli supply a large, thin, and moist surface that is ideal for the diffusion of gases. Diffusion of O2 and CO2 in the lungs proceeds as follows (Figure 5.10(c)).

The blood entering the pulmonary capillaries has a very low concentration of O2 and a high concentration of CO2. The air in the alveoli, by contrast, has a high level of O2 and a low level of CO2 because ongoing ventilation continuously refreshes the alveolar air. Therefore, O2 diffuses from the alveolar air into the blood while CO2 diffuses from the blood into the alveolar air.

Much of the CO2 that diffuses into the alveoli is eliminated with the next expiration, and the following inspiration delivers a new supply of O2 into the alveoli. Only a very small amount of the O2 that enters the blood can be carried by the plasma. Almost all the oxygen in the blood is bound to hemoglobin molecules, which are contained in the red blood cells. Each hemoglobin molecule can bind up to four molecules of oxygen. When hemoglobin binds oxygen, the result is oxyhemoglobin. Decreased CO2, increased pH, or decreased temperature of the blood increases the amount of oxygen that can be bound to each hemoglobin molecule; the converse is also true. Normally, this promotes complete oxygenation of blood in the lungs, where ventilation keeps CO2 levels and temperature low. It also promotes greater release of oxygen in other capillaries, where body cell activities keep CO2 levels and temperature high. These characteristics of hemoglobin are sometimes displayed as oxyhemoglobin dissociation curves.

Continuous perfusion, coupled with continuous ventilation, keeps diffusion occurring in an uninterrupted fashion. Furthermore, alterations in the rate of ventilation or perfusion can alter the rate of diffusion to meet bodily needs. Increasing ventilation (i.e., minute volume) or perfusion increases the differences in concentrations of O2 and CO2 between the blood and alveolar air. These changes increase the rates of diffusion and gas exchange. Reducing ventilation or perfusion has the opposite effects.

## Age Changes in Diffusion

Aging causes several changes that reduce the maximum minute volume of ventilation, increase residual volume, and cause uneven ventilation in the lungs.

Age changes in the alveoli further decrease the rate of diffusion. The alveoli become flatter and shallower, decreasing the amount of surface area. The alveolar membrane that remains becomes thicker and undergoes chemical changes which further impair diffusion (Figure 5.8).

This page titled 5.7: Diffusion is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Augustine G. DiGiovanna via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.