Which of the following actions does not occur during exhalation in man?

Step 1: Analyze the question We need the action that does NOT happen during exhalation (breathing out).

Step 2: Recall the concept Breathing works by changing the volume and pressure of the thoracic cavity (chest cavity).

| Process | Inhalation | Exhalation | |---|---|---| | Diaphragm | Contracts and flattens | Relaxes and becomes dome-shaped (curved upward) | | Intercostal muscles / ribs | External intercostals contract; ribs move up and out | Ribs move down and in | | Thoracic volume | Increases | Decreases | | Thoracic air pressure | Decreases (below atmospheric) | Increases (above atmospheric) |

Step 3: Check each option

• A. Thoracic cavity decreases in volume: This happens during exhalation. ✔
• B. Diaphragm flattens: The diaphragm flattens during inhalation. During exhalation it relaxes and returns to its dome shape. So this does NOT occur in exhalation. ✘
• C. Ribs move down and in: This happens during exhalation. ✔
• D. Air pressure increases in the thoracic cavity: With reduced volume, pressure rises and pushes air out. This happens during exhalation. ✔

Step 4: Answer The action that does not occur during exhalation is B. Diaphragm flattens.

Exam tip: Remember "Flat for Fresh air in." The diaphragm is flat when you breathe in and domed when you breathe out. Also, volume and pressure are inversely related (Boyle's law): when volume goes down, pressure goes up, and air is forced out.