Flow along a pathway results from a difference in pressure (ΔP) between one end to the other. This can occur from either an upstream rise or a downstream fall in pressure.

When breathing spontaneously, inspiratory flow occurs by creating a small negative pressure in the alveoli (~ 2-3 cm H₂O below atmospheric).

This is brought about by contraction of the diaphragm and external intercostals.

Question: What generates the positive pressure in the alveoli for expiration?

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Flow along a pathway results from a difference in pressure (ΔP) between one end to the other. This can occur from either an upstream rise or a downstream fall in pressure.

When breathing spontaneously, inspiratory flow occurs by creating a small negative pressure in the alveoli (~ 2-3 cm H₂O below atmospheric).

This is brought about by contraction of the diaphragm and external intercostals.

Question: What generates the positive pressure in the alveoli for expiration?

Answer: The elastic recoil of the lungs and chest wall, utilizing some of the energy used during inspiration.