How thin is the air-blood barrier in the lungs?

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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.

The air-blood barrier in the human lung is approximately 0.5 μm thick, which is about 1/100th of the diameter of a human hair.1

This extremely thin membrane is formed by the type I pneumocytes of the alveolar wall and the endothelial cells of the pulmonary capillaries. Its minimal thickness is essential for efficient gas exchange, as it allows oxygen and carbon dioxide to move rapidly between the air in the alveoli and the blood via diffusion. If this barrier thickens—for example, due to the accumulation of excess fluid in a condition known as pulmonary edema—the distance gases must travel increases, which severely impairs the lung's ability to function properly.21