Answer: Bicarbonate ions.
- A Bicarbonate ions
- B Carbonic acid crystals
- C Dissolved gas in plasma
- D Carbaminohemoglobin
Correct answer: A. Bicarbonate ions
Explanation: About 70% of CO<sub>2</sub> is transported as bicarbonate ions (HCO<sub>3</sub><sup>-</sup>) formed in RBCs with the help of carbonic anhydrase; a smaller fraction binds hemoglobin as carbaminohemoglobin.
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.