Answer: Lowers surface tension in alveoli, preventing their collapse.
- A Lowers surface tension in alveoli, preventing their collapse
- B Raises surface tension in alveoli to keep them rigid under typical physiological conditions
- C Pumps oxygen directly into alveolar capillaries according to standard texts
- D Converts CO<sub>2</sub> into bicarbonate inside the alveoli in general clinical practice
Correct answer: A. Lowers surface tension in alveoli, preventing their collapse
Explanation: Pulmonary surfactant, a phospholipid film, lowers surface tension at the air-water interface in alveoli, preventing their collapse, especially during exhalation when alveolar volume decreases.
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.