Final answer:
The two forces that resist lung expansion are the surface tension of the alveolar fluid and the elastic recoil of the lung tissues. Surface tension tends to collapse the alveoli, while elastic recoil pulls the lungs inward. Pulmonary surfactant reduces surface tension, allowing for better lung expansion.
Step-by-step explanation:
The two forces that tend to resist the expansion of the lungs are the surface tension of the alveolar fluid and the elastic recoil of the lung tissues. The surface tension within the alveoli is primarily due to the water present in the alveolar lining which tends to pull the alveoli inwards, resisting expansion. This surface tension would cause the alveoli to collapse during expiration if not for the presence of pulmonary surfactant, which is secreted by type II alveolar cells and reduces this tension.
The elastic recoil of the lungs is another force that resists their expansion. This is a property of the elastic connective tissues within the pulmonary structure that inherently pulls the lungs inwards, away from the thoracic wall. This force is useful during exhalation, as it helps push air out of the lungs. However, during inhalation, it opposes the effort to expand the lungs and draw air in.