Following are the statements deduced from the curves above and or based on the knowledge about CO₂ transport, which may or may not be correct:
A. The deoxygenated haemoglobin has greater affinity for CO₂, than oxygenated haemoglobin.
B. The deoxygenated haemoglobin does not bind with free H⁺ ions released during the formation of HCO₃^– from CO₂.
C. The haemoglobin saturation with O₂ has no effect on CO₂, dissociation curve.
D. O₂ and CO₂ bind to haemoglobin at different sites.

Which one of the following options represents a combination of all correct statements?
(1) A and B (2) B and C
(3) C and D (4) A and D

Introduction

Carbon dioxide (CO2) transport in blood involves complex interactions with hemoglobin, influenced by oxygenation status. The CO2 dissociation curves of oxygenated and deoxygenated blood differ due to hemoglobin’s changing affinity for CO2 and H+ ions. Understanding these differences is crucial for grasping the Haldane effect and overall acid-base balance in the body. This article analyzes common statements regarding CO2 binding and identifies which are correct.

Evaluation of the Statements

A. The deoxygenated hemoglobin has greater affinity for CO2 than oxygenated hemoglobin.

• True.
  Deoxygenated hemoglobin binds CO2 more readily than oxygenated hemoglobin, facilitating CO2 uptake in tissues. This phenomenon is the Haldane effect, allowing improved CO2 transport from tissues to lungs.

B. The deoxygenated hemoglobin does not bind with free H+ ions released during the formation of HCO3- from CO2.

• False.
  Deoxygenated hemoglobin binds free H+ ions efficiently, helping buffer blood pH. This binding helps maintain acid-base stability during CO2 transport.

C. The hemoglobin saturation with O2 has no effect on CO2 dissociation curve.

• False.
  Oxygenation state of hemoglobin significantly affects the CO2 dissociation curve. Oxygen binding reduces CO2 affinity of hemoglobin (Haldane effect), so saturation does influence CO2 transport.

D. O2 and CO2 bind to hemoglobin at different sites.

• True.
  Oxygen binds to the iron atoms in the heme groups, while CO2 binds covalently to the amino groups of globin chains, forming carbamino compounds. Thus, their binding sites are distinct.

Summary Table

Correct Combination of All Correct Statements

Accordingly, the combination with all correct statements is:

(4) A and D

Physiological Context

• The Haldane effect describes enhanced CO2 binding by deoxygenated Hb helping CO2 loading in tissues and unloading in lungs.

• Binding of H+ ions by Hb buffers blood acid-base status during tissue metabolism.

• Separate binding sites for O2 and CO2 on Hb allow these gases to be transported efficiently and simultaneously.

Conclusion

The correct combination of statements explaining CO2 binding differences and transport in blood is:

(4) A and D