Identify the correct sequence of following events.
A) Air is inspired
B) Increase in thoracic cavity
C) Increase in pulmonary cavity
D) Contraction of intercostal and phrenic muscles

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B $\to$ C $\to$ B $\to$ A

A $\to$ B $\to$ C $\to$ D

D $\to$ B $\to$ C $\to$ A

D $\to$ A $\to$ B $\to$ C

answer is D.

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Detailed Solution

Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero-posterior axis. The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in the thoracic volume causes a similar increase in pulmonary volume. An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs, i.e., inspiration.

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