Primary CO2 Transport via Bicarbonate Ions

Steps:

• CO2 enters blood from tissues and diffuses into red blood cells.
• Inside RBCs, CO2 reacts with water to form carbonic acid, catalyzed by carbonic anhydrase.
• Carbonic acid dissociates into H+ and HCO3- (hydrogencarbonate) ions.
• HCO3- ions diffuse out of RBCs into plasma, forming the majority of transported CO2.

Why C is correct:

• Over 70% of CO2 is transported as hydrogencarbonate ions (HCO3-) via the reaction CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-, the dominant reversible buffering system in blood.

Why the others are wrong:

• A: Carbaminohemoglobin accounts for only about 5-10% of CO2 transport, binding directly to hemoglobin.
• B: Carbonic acid (H2CO3) is a short-lived intermediate, not a stable transport form.
• D: Dissolved CO2 in plasma (not cytoplasm) is only about 7% of total transport.

Final answer: C