Physiologic components of
ventilation and respiration
The lung is highly elastic. Lung inflation
results from the partial pressure
of inhaled gases and the diffusion-
pressure gradient of these
gases across the alveolar-capillary
membrane. The lungs play a passive
role in breathing, but ventilation
requires muscular effort. When
the diaphragm contracts, the thoracic
cavity enlarges, causing the
lungs to inflate. During forced inspiration
when a large volume of
air is inspired, external intercostal
muscles act as a second set of inspiratory
muscles.