Inspiration & Expiration


Inspiration (inhalation)
When the inspiratory muscles, the diaphragm, and external intercostals, contract, the size of the thoracic cavity increases. As intrapulmonary volume (the volume within the lungs) increases, the gases within the lungs spread out to fill the larger space. The resulting decrease in the gas pressure in the lungs produces a partial vacuum which sucks air into the lungs until the intrapulmonary pressure equals atmospheric pressure.

Expiration (exhalation)
As the inspiratory muscles relax and resume their initial resting length, the rib cage descends and the lungs recoil. Thus, both the thoracic and intrapulmonary volumes decrease. As the intrapulmonary volume decreases, the gases and the intrapulmonary pressure rises to a point higher than atmospheric pressure. This causes the gases to flow out to equalize the pressure inside and outside the lungs. 

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