Answer: Bicarbonate ions (HCO 3 - ) in plasma (~70%).
- A Dissolved CO<sub>2</sub> in plasma, accounting for the majority of transported gas
- B Carbaminohemoglobin formed by binding to globin chains exclusively
- C Bicarbonate ions (HCO<sub>3</sub><sup>-</sup>) in plasma (~70%)
- D Carbonic acid accumulating freely in plasma without further dissociation
Correct answer: C. Bicarbonate ions (HCO<sub>3</sub><sup>-</sup>) in plasma (~70%)
Explanation: About 70% of CO<sub>2</sub> is transported as bicarbonate (HCO<sub>3</sub><sup>-</sup>) in plasma. About 20% bound to Hb (as carbaminoHb). 10% dissolved.
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.