During a physiology practical, a student measures oxygen transport in a healthy volunteer. Which is the most common form in which oxygen is transported in arterial blood?

Explanation

## Oxygen Transport in Arterial Blood ### Quantitative Distribution of O₂ Transport **Key Point:** Approximately **98.5% of oxygen** in arterial blood is carried as **oxyhemoglobin (HbO₂)**. Only 1.5% is dissolved in plasma. ### Breakdown of O₂ Transport Mechanisms | Transport Form | Amount (mL O₂/100 mL blood) | Percentage | Notes | |---|---|---|---| | **Oxyhemoglobin (HbO₂)** | ~19.5 | **98.5%** | Reversible binding to Hb iron | | Dissolved in plasma | ~0.3 | 1.5% | Follows Henry's Law; PaO₂ = 100 mmHg | | Bound to albumin | Negligible | <0.1% | Not physiologically significant | | Other forms | Negligible | <0.1% | Carboxyhemoglobin, methemoglobin | **High-Yield:** The dissolved oxygen (0.3 mL/100 mL) is **critical for tissues** because it is the dissolved O₂ that exerts the partial pressure (PaO₂) and drives diffusion into cells. However, the **bulk of O₂ transport** is via hemoglobin. ### Why Hemoglobin Dominates O₂ Transport 1. **Oxygen-carrying capacity**: Each hemoglobin molecule has 4 heme groups, each binding one O₂ molecule 2. **High concentration**: Normal Hb = 15 g/dL = ~5 mmol/L 3. **Cooperative binding**: Sigmoidal oxygen-hemoglobin dissociation curve allows efficient loading in lungs and unloading in tissues 4. **Solubility limit**: Dissolved O₂ in plasma follows Henry's Law: only 0.3 mL/100 mL at normal PaO₂ ### Oxygen-Hemoglobin Dissociation Curve ```mermaid flowchart TD A["Arterial Blood<br/>PaO₂ = 100 mmHg<br/>SaO₂ = 98%"]:::outcome --> B["Oxyhemoglobin<br/>HbO₂ carries 19.5 mL O₂/100 mL"]:::action C["Tissue Capillary<br/>PvO₂ = 40 mmHg<br/>SvO₂ = 75%"]:::outcome --> D["O₂ unloads from Hb<br/>Dissolved O₂ drives diffusion"]:::action E["Bohr Effect<br/>↓ pH, ↑ CO₂, ↑ temp"]:::decision --> F["Right shift of curve<br/>Facilitates tissue O₂ release"]:::action ``` **Clinical Pearl:** Patients with severe anemia (low Hb) or carbon monoxide poisoning (Hb unavailable) cannot compensate by increasing dissolved O₂ because the solubility of O₂ in plasma is fixed. This is why they develop tissue hypoxia despite normal PaO₂. ### Why Other Options Are Wrong - **Dissolved O₂**: Only 1.5% of total O₂ transport, though it is the physiologically active form that determines PaO₂ - **Bound to albumin**: Albumin does not bind O₂ in any physiologically significant amount; it carries fatty acids and bilirubin - **HCO₃⁻**: This is the form in which **CO₂** is transported (70%), not O₂