In what form is the majority of carbon dioxide transported in the blood?

• A. Carbonic acid
• B. Bicarbonate ions
• C. Carbon monoxide
• D. Dissolved in plasma

Answer: (B)

The majority of carbon dioxide in the blood is transported primarily as bicarbonate ions. When carbon dioxide enters the blood from the tissues, it reacts with water to form carbonic acid, which is a reversible reaction catalyzed by the enzyme carbonic anhydrase. This carbonic acid then dissociates into bicarbonate ions and hydrogen ions.

Bicarbonate plays a crucial role in maintaining acid-base balance within the body and facilitates the transport of carbon dioxide from tissues to the lungs, where it can be expelled. About 70% of carbon dioxide is transported in this form, which allows for efficient transport and regulation of pH levels.

In comparison, a smaller portion of carbon dioxide is indeed transported as carbonic acid or dissolved in plasma, but these amounts are significantly lesser than that of bicarbonate ions. Carbon monoxide, while a gas that can bind to hemoglobin, does not play a significant role in carbon dioxide transport and is instead a toxic byproduct of combustion and not a natural constituent of carbon dioxide transport mechanisms.