Answer: Decreases as the diaphragm and intercostal muscles relax.
- A Decreases as the diaphragm and intercostal muscles relax
- B Increases as the diaphragm contracts downward
- C Increases due to outward movement of the rib cage
- D Stays roughly constant while internal pressure rises
Correct answer: A. Decreases as the diaphragm and intercostal muscles relax
Explanation: During expiration, the diaphragm and external intercostal muscles relax, reducing thoracic volume and increasing pulmonary pressure, pushing air out.
The S-shaped (sigmoid) curve reflects cooperative binding between Hb's four O₂ sites; the Bohr effect shifts the whole curve to the right under conditions found in actively respiring tissue (more CO₂, more acid, more heat), making haemoglobin release oxygen more readily exactly where it's needed.
Concept context
The mechanics of breathing, lung volumes, gas transport in blood, the Bohr effect, and respiratory regulation.