Pulmonary Oxygenation: Blood Is Oxygenated in the Lungs, Not "Filled"

Blood is oxygenated in the lungs, not "filled with oxygen." The distinction is physiologically precise:

• Deoxygenated blood (mixed venous, ~75% saturated) enters pulmonary capillaries.
• Diffusion: O₂ crosses alveolar-capillary membrane (driven by P_AO₂ – P_cO₂ gradient).
• Binding: O₂ binds to hemoglobin (Hb) → oxyhemoglobin. Each Hb binds 4 O₂.
• Oxygenated blood (~97–98% saturated) leaves via pulmonary veins.

Key Points

• O₂ is carried bound to hemoglobin, not dissolved in plasma (minimal).
• Pulmonary capillaries provide massive surface area (~70 m²) and minimal diffusion distance (~0.5 µm).
• Transit time ~0.75 sec; equilibration occurs in ~0.25 sec (reserve for exercise).
• Ventilation-perfusion (V/Q) matching optimizes efficiency.

Pathology (shunt, V/Q mismatch, diffusion limitation) impairs oxygenation → hypoxemia.