The two factors that need to be overcome for gas to reach the alveoli are resistance and compliance.

Resistance is the barrier presented to air passage through the conducting airways, and has elements relating to:

• Gas flow through the airways
• Tissues moving against each other

An increase in resistance is seen in obstructive airways disease.

Compliance describes how easily expansion takes place for a given inflation pressure, and involves both the lungs and chest wall. It varies as the lung inflates. A decrease (worsening) in compliance occurs with restrictive lung disease.