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How does blood carry oxygen from the lungs to body tissues?

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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.

Blood transports oxygen from the lungs to body tissues primarily through the action of red blood cells, which contain the protein hemoglobin.12

Because oxygen is poorly soluble in plasma, less than 2 percent is transported in a dissolved state. Instead, the vast majority of oxygen binds to the iron-containing heme groups within hemoglobin molecules. In the lungs, the partial pressure of oxygen is high enough to saturate these binding sites. Once oxygenated, the blood is pumped by the heart through the vascular system to reach the tissues.23

At the tissue level, oxygen is released from hemoglobin. This process is facilitated by factors such as the local partial pressure of oxygen, temperature, and the presence of substances like carbon dioxide and hydrogen ions, which decrease hemoglobin's affinity for oxygen and promote its release where metabolic demand is highest.2