In a healthy adult at rest, what is the most common form in which carbon dioxide is transported in the blood?

Explanation

## Carbon Dioxide Transport Forms in Blood **Key Point:** Approximately 70% of CO₂ is transported as bicarbonate ions (HCO₃⁻), making it the most common form of CO₂ transport. ### Quantitative Distribution of CO₂ Transport | Transport Form | Percentage | Mechanism | |---|---|---| | As HCO₃⁻ (bicarbonate) | 70% | CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺ (via carbonic anhydrase) | | As carbaminohemoglobin | 20–23% | CO₂ binds to amino groups on Hb (not heme iron) | | Dissolved in plasma | 7–10% | Direct dissolution; contributes to PaCO₂ | | Bound to plasma proteins | <1% | Negligible contribution | ### The Bicarbonate Buffer System **High-Yield:** The Henderson–Hasselbalch equation governs CO₂–HCO₃⁻ equilibrium: $$pH = 6.1 + \log \frac{[HCO_3^-]}{0.03 \times PaCO_2}$$ This system is the body's primary pH buffer and is essential for acid–base homeostasis. ### Step-by-Step CO₂ Transport in RBCs 1. CO₂ diffuses into RBCs from tissue capillaries 2. Carbonic anhydrase catalyzes: CO₂ + H₂O → H₂CO₃ 3. H₂CO₃ rapidly dissociates: H₂CO₃ → HCO₃⁻ + H⁺ 4. HCO₃⁻ exits RBC via chloride shift (Cl⁻ enters to maintain electroneutrality) 5. H⁺ is buffered by hemoglobin **Mnemonic:** **CHAP** — Carbonic anhydrase, HCO₃⁻ formation, Anion exchange (chloride shift), pH buffering by Hb. **Clinical Pearl:** In respiratory acidosis (elevated PaCO₂), HCO₃⁻ accumulates and pH drops. In respiratory alkalosis (low PaCO₂), HCO₃⁻ decreases and pH rises. This is why PaCO₂ is the primary driver of acute pH changes. **Warning:** Do not confuse oxygen transport (primarily hemoglobin) with CO₂ transport (primarily bicarbonate). This is a frequent source of error in NEET PG exams. [cite:Guyton & Hall Textbook of Medical Physiology Ch 40]