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Hemoglobin's oxygen-carrying capacity depends on cooperative binding: oxygen binding to one subunit increases affinity of others, creating a sigmoidal binding curve. This cooperativity allows high oxygen loading in lungs and substantial oxygen unloading in tissues. The oxygen content of blood includes both dissolved oxygen and hemoglobin-bound oxygen; dissolved oxygen contributes minimally but becomes significant at high altitude or hyperbaric conditions.
From your study of respiratory mechanics and gas exchange, you know that oxygen moves from alveoli into capillary blood down a partial pressure gradient. But simply dissolving in plasma would be hopelessly inadequate — at normal arterial PO₂ of 100 mmHg, only about 0.3 mL of O₂ dissolves per 100 mL of blood. Resting tissues need roughly 5 mL/100 mL. Hemoglobin solves this problem by binding oxygen reversibly and carrying it in a chemically bound form: a single gram of hemoglobin can carry 1.34 mL O₂ when fully saturated, so blood with 15 g/dL hemoglobin can carry nearly 20 mL O₂ per 100 mL — a 66-fold amplification over dissolved oxygen alone. The total oxygen content formula captures both contributions: CaO₂ = (1.34 × Hgb × SaO₂) + (0.003 × PaO₂).
The sigmoidal shape of the oxyhemoglobin dissociation curve is not an accident — it is a direct consequence of cooperative binding, which you studied in the prerequisite on hemoglobin cooperativity. Hemoglobin's four subunits communicate through conformational change: when the first O₂ binds, it shifts the protein toward a high-affinity "R state," making subsequent binding easier. This cooperativity produces the S-shaped curve. The steep middle portion (PO₂ 20–60 mmHg) is where most O₂ unloading to tissues occurs. The flat upper portion (PO₂ 70–100 mmHg) means hemoglobin stays highly saturated across a wide range of alveolar conditions — a safety margin that lets you breathe comfortably at altitude without dramatic drops in oxygen delivery.
Three physiological variables shift the curve and fine-tune O₂ delivery. The Bohr effect: rising CO₂ and falling pH in metabolically active tissues shift the curve rightward, reducing hemoglobin's affinity for O₂ and promoting unloading exactly where it is needed. 2,3-bisphosphoglycerate (2,3-BPG), a glycolytic intermediate that accumulates in red blood cells under hypoxia, binds the central cavity of deoxyhemoglobin and stabilizes the T (low-affinity) state — another rightward shift. Temperature also shifts the curve rightward in hot, active muscle. All three effects converge to ensure that working tissues, which are acidic, CO₂-rich, warm, and hypoxic, extract more oxygen from each hemoglobin molecule that passes through.
Understanding oxygen content (not just saturation) prevents a common clinical error. A patient with severe anemia may have normal SaO₂ of 99% yet critically low oxygen delivery — because the hemoglobin concentration term dominates the content equation. Conversely, hyperbaric oxygen therapy raises PaO₂ high enough that the dissolved oxygen term alone can sustain metabolism even without hemoglobin, which is why it can treat carbon monoxide poisoning where hemoglobin is blocked. The interplay between the two forms of oxygen transport — dissolved and hemoglobin-bound — defines the physiological and clinical picture across a wide range of conditions.