The correct option is 0.7.
Explanation
Blood transports carbon dioxide ($CO_2$) from the tissues to the alveoli in the lungs through three distinct mechanisms. Due to the high solubility of $CO_2$, it is transported much more efficiently than oxygen.
Modes of Carbon Dioxide Transport:
- As Bicarbonate ($HCO_3^-$): Approximately 70% of carbon dioxide is transported in this form. Within red blood cells (RBCs), the enzyme carbonic anhydrase catalyzes the reaction between $CO_2$ and water to form carbonic acid ($H_2CO_3$), which rapidly dissociates into bicarbonate ($HCO_3^-$) and hydrogen ions ($H^+$). The bicarbonate ions diffuse into the plasma, while chloride ions shift into the RBCs to maintain ionic balance (Chloride Shift).
- As Carbamino-haemoglobin: About 20-25% of $CO_2$ is transported bound to haemoglobin. The $CO_2$ binds to the amine group of the globin protein to form carbamino-haemoglobin. This binding is reversible and depends primarily on the partial pressure of carbon dioxide ($pCO_2$).
- Dissolved in Plasma: Approximately 7% of $CO_2$ is transported in a dissolved state within the blood plasma.
Key Takeaway: The dominant mode of carbon dioxide transport in the blood is as bicarbonate ions (approx. 70%), facilitated by the enzyme carbonic anhydrase in erythrocytes.