Bicarbonate Ion Formation Dominates CO2 Blood Transport

Steps:

• CO2 diffuses from tissues into blood plasma and red blood cells (RBCs).
• Inside RBCs, CO2 reacts with H2O to form H2CO3, catalyzed by carbonic anhydrase.
• H2CO3 dissociates into HCO3- (hydrogencarbonate) and H+ ions.
• HCO3- diffuses out of RBCs into plasma via chloride shift, carrying most CO2.

Why D is correct:

• The bicarbonate buffer system (CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-) transports ~70% of CO2 as HCO3- ions in RBCs and plasma.

Why the others are wrong:

• A: Dissolved CO2 in plasma accounts for only ~7% of total transport.
• B: Carbaminohemoglobin formation binds ~23% of CO2 to hemoglobin.
• C: Carboxyhemoglobin results from toxic CO binding to hemoglobin, unrelated to CO2.

Final answer: D