What are the two forces that oppose inflation of the lungs?

• A. Tidal Volume
• B. PEEP
• C. Elastance & Resistance
• D. Compliance

Answer: (C)

Inflating the lungs requires overcoming two opposing forces: elastic recoil of the lung-tissue/chest-wall system and resistance to airflow in the airways. Elastic recoil, quantified as elastance when high, is the lungs’ tendency to spring back and collapse inward, so more pressure is needed to achieve the same expansion. Airway resistance is the friction to airflow along the conducting airways, so higher resistance means more pressure is required to push air in at a given flow. Together, these two forces determine the pressure needed to achieve a given tidal volume.

Tidal volume is the amount of air delivered, not a force. PEEP is a pressure applied at end expiration to keep alveoli open, not a force opposing inflation. Compliance describes how easily the lungs expand (the inverse of elastance) but is not a separate opposing force itself.