Mechanism of Breathing · NCERT Biology

During expiration, the volume of the thoracic cavity decreases in the dorso-ventral axis due to:

  1. Contraction of the diaphragm, which pushes air upward
  2. Relaxation of external intercostal muscles
  3. Passive recoil of the sternum toward the spine
  4. Outward expansion of ribs driven by muscle pull
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Correct answer: Relaxation of external intercostal muscles

As the external intercostal muscles relax, the ribs and sternum return to their original positions (moving downward and inward). This reduces the thoracic volume in the dorso-ventral axis.

Difficulty: Medium Question 15 of 20

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