Oxyhemoglobin Formation: How Oxygen Binding Changes Blood Color

In pulmonary capillaries, hemoglobin (Hb) binds O₂ to form oxyhemoglobin (HbO₂)—a bright red molecule. Each Hb tetramer carries up to 4 O₂ molecules cooperatively.

At tissues, HbO₂ releases O₂ (driven by lower pO₂, higher CO₂, H⁺, temperature, 2,3-BPG) becoming deoxyhemoglobin (deoxy-Hb)—dark red/bluish. Deoxy-Hb binds CO₂ and H⁺ (Haldane effect) for transport back to lungs.

This reversible oxygenation cycle is the basis of pulse oximetry (SpO₂), which measures HbO₂/(HbO₂+deoxy-Hb) ratio via differential light absorption at 660 nm (red) and 940 nm (infrared).