Question

In pathologic conditions where the alveolar membrane is thickened there is little effect on the exchange of carbon dioxide, but oxygen diffusion is impaired. Based on your understanding of the respiratory membrane function, propose an explanation for this phenomenon.

(2pts) Why is alveolar ventilation rate a more important phyisological measure of respiratory function than total air moved into the respiratory passages?

Answer 1

Fick's Law gives us a number of factors that determines the diffusion rate of a gas through a fluid:

• pressure difference across the diffusion barrier
• solubility of gas
• cross-sectional area of fluid
• distance molecules need to diffuse
• molecular weight of gas
• temperature of fluid (not relevant)

Solubility of carbon dioxide is higher compared to oxygen. Precisely speaking, the difference in solubility is such that carbon dioxide diffuses 20 times faster than oxygen. Despite this fact, oxygen diffusion is more compared to carbon dioxide because of greater pressure difference across alveolar membrane and lesser molecular weight. When the alveolar membrane thickness is increased, distance molecules need to diffuse is increased and hence oxygen diffusion is impaired because of lesser solubility while carbon dioxide diffusion, because of greater solubility, remains almost unaffected.

Alveolar ventilation rate is a more important physiological measure of respiratory function than total air moved into the respiratory passages because only ventilated oxygen can perform the required physiological functions to satisfy the body needs.