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.
Accessory muscles in the neck
and chest are the last group of inspiratory
muscles, used only for
deep and heavy breathing, such as
during intense exercise or respiratory
failure. During expiration, the diaphragm
relaxes, decreasing thoracic
cavity size and causing the