Why/How Reasoning Chain — CO2 Transport and the Chloride Shift — Notes

Why Does 70% of CO2 Travel as HCO3–?

Step 1 — WHY can't CO2 just dissolve in plasma? CO2 is relatively insoluble in plasma (only 7% dissolves). Plasma cannot carry enough CO2 to meet the body's metabolic CO2 production rate. A more efficient transport mechanism is needed.

Step 2 — HOW does CO2 enter RBCs? CO2 is a small, non-polar molecule → freely diffuses across the RBC membrane down its concentration gradient from plasma into the cell.

Step 3 — WHY is the carbonic anhydrase reaction inside RBCs? Carbonic anhydrase speeds up the reaction CO2 + H2O → H2CO3 by ~5000-fold. This rapid conversion prevents CO2 from building up in RBCs. Without the enzyme, this reaction would be too slow to handle metabolic CO2 production.

Step 4 — WHY does HCO3– leave the RBC? H2CO3 immediately dissociates to H+ + HCO3–. The HCO3– concentration inside RBCs rises sharply → concentration gradient drives HCO3– out through the anion exchanger (Band 3 protein) into plasma. This is the exit mechanism.

Step 5 — WHY is the chloride shift necessary? When HCO3– (a negative ion) leaves the RBC, the interior becomes electropositive. To maintain electrical neutrality, Cl– (also a negative ion) moves in from plasma through the same anion exchanger. This is the Hamburger's phenomenon.

Step 6 — HOW is H+ handled inside the RBC? H+ (the other dissociation product) cannot leave the RBC easily. Instead, deoxyhaemoglobin (which is a better buffer than oxyhaemoglobin — Haldane effect) absorbs the H+ ions, preventing acidification of RBCs.

Step 7 — HOW is CO2 released at the alveoli? At the lungs, O2 loads onto Hb (→ oxyHb). OxyHb has lower affinity for H+ (Haldane effect) → releases H+. H+ + HCO3– → H2CO3 → CO2 + H2O (reverse of Step 3, catalysed by carbonic anhydrase). CO2 diffuses from blood (pCO2 45) into alveoli (pCO2 40). Exhaled.

Step 8 — WHY is this system physiologically elegant? The entire CO2 transport system is self-regulating: CO2 produced by tissues automatically drives its own conversion and transport; O2 loading in lungs automatically drives CO2 release. The Bohr and Haldane effects operate simultaneously to optimise both O2 delivery and CO2 removal.