Inhalation increases thoracic volume via muscle contraction

Steps:

• Identify the process: Breathing in (inhalation) requires expanding the thoracic cavity to lower pressure and draw air into lungs.
• Recall diaphragm role: Contraction flattens the diaphragm, increasing vertical thoracic dimension.
• Recall intercostal muscles: External intercostals contract to lift ribs, expanding the thorax laterally and anteriorly.
• Combine actions: Both diaphragm and external intercostals working together achieve overall volume increase.

Why A is correct:

• Diaphragm contraction pulls it downward, and external intercostal contraction elevates ribs, directly increasing thoracic volume per respiratory physiology.

Why the others are wrong:

• B: Diaphragm relaxation decreases volume; used in exhalation.
• C: Internal intercostals aid exhalation by depressing ribs, not inhalation.
• D: Both relaxation and internal contraction promote exhalation, reducing volume.

Final answer: A