First slide

Mechanism of Breathing

Question

Select the correct events that occur during expiration.
(a) Contraction of diaphragm
(b) Contraction of external inter-costal muscles
(c) Pulmonary volume decreases
(d) Intra pulmonary pressure increases

Moderate

A

(c) and (d)
B

(a), (b) and (d)
C

only (d)
D

(a) and (b)

Solution

During expiration, the pulmonary volume decreases and intra pulmonary pressure increases.
Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume. This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration.
Contraction of the diaphragm and external intercostal muscles occurs during inspiration (but not expiration).

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