Which of the following is the primary way that carbon dioxide (CO\(_2\)) is transported in the blood from body tissues to the lungs?

• A. Dissolved directly as CO\(_2\) gas in plasma
• B. As carbonic acid (H\(_2\)CO\(_3\)) in red blood cells
• C. As bicarbonate ions (HCO\(_3^-\)) dissolved in plasma
• D. Bound to the heme group of hemoglobin

Answer: (C)

Verified step by step guidance

1. Understand the context: Carbon dioxide (CO\(_2\)) is a waste product of cellular respiration and must be transported from body tissues to the lungs for exhalation. The question asks about the primary mechanism of CO\(_2\) transport in the blood.
2. Recall the three main ways CO\(_2\) is transported in the blood: (1) dissolved directly as CO\(_2\) gas in plasma, (2) bound to hemoglobin (carbaminohemoglobin), and (3) converted to bicarbonate ions (HCO\(_3^-\)) in plasma.
3. Focus on the primary mechanism: The majority of CO\(_2\) is transported as bicarbonate ions (HCO\(_3^-\)). This occurs through a chemical reaction catalyzed by the enzyme carbonic anhydrase in red blood cells. The reaction is: CO\(_2\) + H\(_2\)O ⇌ H\(_2\)CO\(_3\) ⇌ H\(^+\) + HCO\(_3^-\).
4. Explain the process: Inside red blood cells, CO\(_2\) reacts with water (H\(_2\)O) to form carbonic acid (H\(_2\)CO\(_3\)), which quickly dissociates into hydrogen ions (H\(^+\)) and bicarbonate ions (HCO\(_3^-\)). The bicarbonate ions then diffuse into the plasma, where they are transported to the lungs.
5. Summarize why this is the primary mechanism: While some CO\(_2\) is transported dissolved in plasma or bound to hemoglobin, the majority (approximately 70%) is transported as bicarbonate ions in plasma, making this the primary method of CO\(_2\) transport in the blood.