Inspiration mechanics in breathing

Steps:

• Inspiration requires increasing thoracic volume to lower pressure and draw air in.
• Diaphragm contracts to descend and enlarge the vertical thorax dimension.
• External intercostal muscles contract to elevate ribs, expanding the transverse dimension.
• Internal intercostals relax; thoracic pressure falls steadily during inspiration.

Why A is correct:

• Matches physiology: contracting diaphragm and external intercostals increase volume, relaxing internals allow expansion, and pressure falls (no rise during inspiration).

Why the others are wrong:

• B: Describes expiration, with relaxing inspiratory muscles and rising pressure.
• C: Wrongly states external intercostals relax and internals contract, which aids expiration.
• D: Identical to C; incorrect muscle actions for inspiration.

Final answer: A