A 62-year-old man with COPD presents with dyspnea. Arterial blood gas analysis shows PaCO₂ 55 mmHg, pH 7.32, and HCO₃⁻ 28 mEq/L. All of the following mechanisms contribute to CO₂ transport in blood EXCEPT:

Explanation

## CO₂ Transport in Blood — Quantifying the Pathways ### The Three Routes of CO₂ Transport **Key Point:** CO₂ is transported via three mechanisms, but their **relative contributions are NOT equal**. The percentages are crucial for NEET PG. ### Breakdown of CO₂ Transport | Mechanism | % of Total CO₂ | Pathway | Notes | |-----------|---|---------|-------| | **Bicarbonate (HCO₃⁻)** | 70–80% | CO₂ → H₂CO₃ → HCO₃⁻ + H⁺ | **Dominant pathway** | | **Carbaminohemoglobin** | 5–10% | CO₂ + Hb → HbCO₂ | Binds to amino groups on globin chains | | **Dissolved CO₂** | 5–10% | CO₂ dissolved in plasma | **NOT 85–90%** | **High-Yield:** The correct answer reverses the actual percentages. Dissolved CO₂ contributes only **5–10%**, NOT 85–90%. The bicarbonate pathway dominates at **70–80%**. ### Why Each Correct Mechanism Works #### 1. Bicarbonate Formation & Chloride Shift ```mermaid flowchart LR A["CO₂ in RBC"] --> B["H₂CO₃<br/>(via carbonic anhydrase)"] B --> C["H⁺ + HCO₃⁻"] C --> D["HCO₃⁻ exits to plasma"] D --> E["Cl⁻ enters RBC<br/>(chloride shift)"] F["H⁺ buffered by Hb"] -.-> C style A fill:#e1f5ff style E fill:#c8e6c9 style F fill:#fff9c4 ``` - **Chloride shift** (Hamburger phenomenon) maintains electroneutrality - HCO₃⁻ is the **major form** in which CO₂ is transported in venous blood #### 2. Carbaminohemoglobin Formation - CO₂ binds directly to the **amino groups** (–NH₂) on the globin chains - Does **NOT** compete with O₂ binding (O₂ binds to heme iron) - Accounts for **5–10%** of total CO₂ transport - **Haldane effect**: deoxygenated hemoglobin has greater affinity for CO₂, promoting CO₂ loading in tissues #### 3. Dissolved CO₂ in Plasma - CO₂ is **20× more soluble** in plasma than O₂ - Still contributes only **5–10%** of total CO₂ transport - Follows **Henry's Law**: dissolved gas ∝ partial pressure ### Clinical Relevance to the Case This COPD patient has **respiratory acidosis** (pH 7.32, elevated PaCO₂): - Impaired CO₂ elimination → ↑ PaCO₂ - Metabolic compensation: ↑ HCO₃⁻ (28 mEq/L, slightly elevated) - All three CO₂ transport mechanisms are overwhelmed by the ventilatory failure **Clinical Pearl:** In acute respiratory failure, the bicarbonate pathway becomes the bottleneck—if ventilation cannot clear CO₂, HCO₃⁻ accumulates and pH drops despite the body's buffering efforts. **Warning:** ~~Dissolved CO₂ is the major transport form~~ — this is a common NEET PG trap. Bicarbonate is dominant; dissolved CO₂ is minor.