Question

how is the majority of carbon dioxide carried in the blood?
o in the cytoplasm of red blood cells
o as co₂ dissolved in the plasma
o as bicarbonate ion in the blood plasma
o combined with hemoglobin

Explanation:

When carbon dioxide ($CO_2$) enters the blood, about 70% of it is converted into bicarbonate ions ($HCO_3^-$) in red blood cells through the action of the enzyme carbonic anhydrase. The bicarbonate ions then diffuse into the plasma. This is the major way carbon dioxide is transported in the blood.

• Option 1: While some processes related to $CO_2$ transport occur in red - blood - cell cytoplasm (like the initial conversion to bicarbonate), the majority of the transported form (bicarbonate) is in the plasma, not just confined to the cytoplasm of red blood cells.
• Option 2: Only about 7 - 10% of $CO_2$ is dissolved in plasma as free $CO_2$ molecules.
• Option 4: Only about 20 - 30% of $CO_2$ combines with hemoglobin (as carbamino - hemoglobin).

Answer:

as bicarbonate ion in the blood plasma