The majority of carbon dioxide is transported in the blood as:

 # The majority of carbon dioxide is transported in the blood as:

A. Carbaminohemoglobin

B. Dissolved CO2 in plasma

C. Bicarbonate ions (HCO3−)

 D. Carbonic acid (H2CO3​)

The correct answer is C. Bicarbonate ions (HCO3−).

The majority of carbon dioxide (CO₂) in the blood, approximately 70–80%, is transported as bicarbonate ions (HCO3−). CO₂ produced in tissues diffuses into the blood, where it reacts with water in red blood cells, catalyzed by the enzyme carbonic anhydrase, to form carbonic acid (H2CO3). This quickly dissociates into bicarbonate ions (HCO3−) and hydrogen ions (H+). The bicarbonate ions are then transported in the plasma to the lungs, where they are converted back to CO₂ for exhalation.

• A. Carbaminohemoglobin: Accounts for about 10–20% of CO₂ transport, where CO₂ binds to hemoglobin.
• B. Dissolved CO2 in plasma: Only about 5–10% of CO₂ is transported as dissolved gas in plasma.
• D. Carbonic acid (H2CO3): Exists briefly as an intermediate but is not a major transport form due to its rapid dissociation.