MODULE SUPPLEMENT: PULMONARY SYSTEM
Component 2--Gas Exchange Between
the Alveoli and the Blood by Diffusion

So far we've been discussing the first component of respiration--pulmonary ventilation. However, there is also some change in diffusing capacity that accentuates the lowering of the PaO₂ that is discussed below. Culver and Butler (1985) estimate this change in diffusing capacity at about a decrease of 0.12-0.15ml / year in women and 0.2-0.25 ml per year in men. This is possibly related to the loss of alveolar surface area and possibly somewhat related to changes in the RBC membrane (Culver & Butler, 1985). However, more data are needed regarding the consistency of these changes and whether they plateau at a certain age or continue to decline.

There may also be some change in pulmonary vasculature that can influence diffusion, such as thickening of the intima and fibrous changes as well as some overall loss of vessels. At the same time, changes or stressors to pulmonary circulation, because it is a low pressure system, may be less than those that occur in the high pressure systemic circulation. In terms of lung nourishment itself, vessels supplying the lung are prone to the same changes as blood vessels elsewhere in the body.