CO2 Transport in Blood via Bicarbonate

Steps:

• CO2 from tissues diffuses into blood plasma and RBCs.
• In RBCs, CO2 reacts with H2O to form H2CO3, catalyzed by carbonic anhydrase.
• H2CO3 dissociates into H+ and HCO3-; H+ binds to Hb to buffer pH, while HCO3- exits RBCs via chloride shift.
• Separate process: Some CO2 binds directly to Hb amino groups forming carbaminohemoglobin, without H+ involvement.

Why B is correct:

• Statements 1 and 2 accurately describe carbonic anhydrase catalyzing CO2 + H2O → H2CO3 in RBCs for bicarbonate formation, per standard physiology.

Why the others are wrong:

• A includes 3, but H+ from H2CO3 binds Hb as HHb, not forming carbaminohemoglobin (which is direct CO2-Hb binding).
• C includes 3, which misattributes carbaminohemoglobin formation to H+.
• D relies only on incorrect statement 3.

Final answer: B