Answer: Bound to haemoglobin within red blood cells.
- A Bound to plasma albumin proteins according to standard texts
- B Dissolved directly in blood plasma in most observed cases
- C As bicarbonate ions in plasma under typical physiological conditions
- D Bound to haemoglobin within red blood cells
Correct answer: D. Bound to haemoglobin within red blood cells
Explanation: About 97% of oxygen in blood is transported bound to haemoglobin as oxyhaemoglobin within red blood cells; only a small fraction dissolves directly in plasma.
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.