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Why is most oxygen in the blood bound to hemoglobin?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — human respiratory system The lungs serve as the gas-exchanging organ for the process of respiration.

Most oxygen in the blood is bound to hemoglobin because oxygen is poorly soluble in plasma. Less than 2 percent of oxygen is transported dissolved directly in the plasma, whereas hemoglobin allows for a significantly higher capacity for oxygen transport.12

Specifically, hemoglobin is a protein contained within red blood cells that features four iron-containing heme groups, each capable of binding an oxygen molecule. This mechanism is highly efficient; in normal human blood, about 50 times as much oxygen is combined with hemoglobin as is dissolved in the plasma. By binding to hemoglobin, the blood can transport approximately 0.2 millilitres of oxygen per millilitre of blood, ensuring that living cells—particularly those in the brain—receive the continuous supply of oxygen necessary to prevent unconsciousness and death.132