Questions: Carbon Dioxide Transport and the Bicarbonate Buffer System

Question 1

A patient has a genetic condition that eliminates carbonic anhydrase activity in their red blood cells. What would be the most significant physiological consequence?

• A. Severely impaired oxygen delivery to tissues, since carbonic anhydrase is needed for hemoglobin to bind O₂
• B. Massively reduced CO₂ transport capacity and inability to maintain blood pH, since the bicarbonate buffer system would be crippled
• C. Increased CO₂ accumulation in tissues, but blood pH would remain stable through plasma buffering
• D. No significant consequence, since dissolved CO₂ and carbaminohemoglobin can compensate for loss of bicarbonate

Answer: (B)

Carbonic anhydrase catalyzes CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ in red blood cells. Without it, the bicarbonate buffer system — which carries ~70% of CO₂ — would be severely compromised (the reaction still occurs but far too slowly without the enzyme). The result would be CO₂ retention, respiratory acidosis, and pH instability. Dissolved CO₂ (~7–10%) and carbaminohemoglobin (~20%) cannot compensate. Oxygen binding to hemoglobin is not primarily dependent on carbonic anhydrase.

Question 2

At the tissues, hemoglobin releases oxygen and simultaneously becomes a better transporter of carbon dioxide. At the lungs, hemoglobin binds oxygen and simultaneously releases carbon dioxide. What term describes this reciprocal coupling?

• A. The Bohr effect
• B. The chloride shift
• C. The Haldane effect
• D. The Hamburger equilibrium

Answer: (C)

The Haldane effect describes the coupling between O₂ and CO₂ transport through hemoglobin: deoxygenated hemoglobin binds more CO₂ as carbaminohemoglobin AND is a better H⁺ buffer (reducing the pH drop from bicarbonate formation). At the tissues, O₂ release makes Hb a better CO₂ carrier; at the lungs, O₂ binding makes Hb release CO₂ and H⁺, facilitating elimination. The Bohr effect is the reverse phenomenon — CO₂ and H⁺ reducing Hb's O₂ affinity. The chloride shift is the HCO₃⁻/Cl⁻ exchange across the RBC membrane.

Question 3

Most carbon dioxide produced by tissues is transported in the blood as dissolved CO₂ gas in plasma, since CO₂ is far more soluble in water than oxygen.

• T. True
• F. False

Answer: False

False. Despite CO₂'s greater solubility than O₂, dissolved CO₂ accounts for only about 7–10% of total CO₂ transport. The dominant mechanism (~70%) is the bicarbonate buffer system: CO₂ enters red blood cells, carbonic anhydrase converts it to carbonic acid, which dissociates into H⁺ and HCO₃⁻, and the bicarbonate is exported to plasma via the chloride shift. Carbaminohemoglobin carries an additional ~20–23%. The misconception arises from confusing solubility with transport capacity — while dissolved CO₂ drives the partial pressure gradients for gas exchange, it is not the primary transport vehicle.

Question 4

The chloride shift — the exchange of bicarbonate ions for chloride ions across the red blood cell membrane — serves to maintain electrical neutrality as HCO₃⁻ is exported from red blood cells into plasma.

• T. True
• F. False

Answer: True

True. As carbonic anhydrase generates HCO₃⁻ inside red blood cells, the negatively charged bicarbonate ions exit into plasma via an anion antiporter. If this outward movement of negative charge were not balanced, the inside of the red blood cell would become electrically positive. The antiporter simultaneously imports Cl⁻ to balance the charge. At the lungs, the process reverses: Cl⁻ exits and HCO₃⁻ re-enters so it can be reconverted to CO₂ for exhalation.

Question 5

Why does the Haldane effect mean that hemoglobin's oxygen-carrying and CO₂-carrying functions are not independent but actively coordinated in a single circuit?

Model answer: Deoxygenated hemoglobin binds CO₂ more readily as carbaminohemoglobin and buffers H⁺ more effectively than oxygenated hemoglobin. At the tissues, as Hb releases O₂, it becomes a better CO₂ carrier — capturing the CO₂ produced by metabolism. At the lungs, as Hb binds O₂, it releases CO₂ and H⁺, which drive CO₂ out into the alveoli. The same conformational change that allows oxygen loading also promotes CO₂ unloading, so the two gases are transferred in opposite directions in coordinated, mutually facilitating fashion.

This coupling is elegant because it means the body uses one molecule — hemoglobin — to optimize both gas transport simultaneously. The Haldane effect ensures that wherever O₂ is being released (the tissues), CO₂ can be captured, and wherever O₂ is being loaded (the lungs), CO₂ is expelled. If the two processes were independent, the system would require separate mechanisms and would be less efficient. The coordination also means that conditions affecting hemoglobin's O₂ affinity (pH, CO₂ concentration via the Bohr effect) also directly affect its CO₂-carrying capacity.