Approximately what fraction of the total work is required to overcome the static elastic forces of the lungs and chest wall?

• A. One-half
• B. Two-thirds
• C. One-third
• D. All of it

Answer: (B)

The key idea is that the work of breathing can be split into two parts: overcoming the elastic recoil of the lungs and chest wall (static elastic forces) and overcoming airway/ tissue resistance to flow. The static elastic forces require the energy to expand the lungs and chest wall from their resting state to the tidal volume, and this energy is stored as elastic potential energy that the system must supply during inspiration. In normal quiet breathing, most of the energy goes into this elastic expansion, so roughly two-thirds of the total work of breathing is devoted to overcoming these static elastic forces. The remaining one-third is needed to push air through the airways and tissues against resistance.