## Oxygen Transport Forms in Blood **Key Point:** Approximately 98.5% of oxygen in blood is transported bound to hemoglobin; only 1.5% is dissolved in plasma. ### Quantitative Breakdown | Transport Form | Amount (%) | Clinical Significance | |---|---|---| | Bound to hemoglobin | 98.5 | Primary O₂ carrier; reversible binding | | Dissolved in plasma | 1.5 | Contributes to PaO₂; important in hyperbaric O₂ therapy | | Bound to myoglobin | Negligible | Intracellular storage in muscle; not a blood transport form | | As bicarbonate | 0 | CO₂ transport form, not O₂ | ### Hemoglobin-Oxygen Binding **High-Yield:** Each hemoglobin molecule (Hb) contains 4 heme groups, each capable of binding one O₂ molecule reversibly. This forms oxyhemoglobin (HbO₂). **Mnemonic:** **CHOP** — Cooperative binding, Hill coefficient, Oxygen affinity, Partial pressure. Hemoglobin exhibits cooperative (allosteric) binding, meaning oxygen binding to one subunit increases affinity of remaining subunits. ### Why Hemoglobin is the Ideal Carrier 1. High oxygen-carrying capacity: 1 g Hb carries ~1.34 mL O₂ 2. Reversible binding: allows loading in lungs and unloading in tissues 3. Sigmoidal dissociation curve: facilitates efficient gas exchange 4. Responsive to physiologic factors (pH, PCO₂, temperature, 2,3-DPG) **Clinical Pearl:** In severe anemia, dissolved O₂ becomes relatively more important but remains insufficient to meet metabolic demands, explaining why patients with Hb <7 g/dL develop tissue hypoxia despite normal PaO₂. [cite:Guyton & Hall Textbook of Medical Physiology Ch 40]
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