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How does gas exchange occur between alveoli and pulmonary capillaries?

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Illustration of a human heart cross-section showing four chambers-left and right atria and ventricles-with labeled major blood vessels. An inset highlights cardiac muscle tissue, showing myocytes and intercalated disks. — human cardiovascular system The human heart is both a pump and a muscle. Its four chambers move blood efficiently, while cardiac muscle cells joined by intercalated disks synchronize each contraction.

Gas exchange between the alveoli and pulmonary capillaries occurs primarily through the process of diffusion. This process is driven by differences in the partial pressures of oxygen and carbon dioxide across the thin air–blood barrier, which is approximately 0.5 μm thick.21

Gases move from areas of higher partial pressure to areas of lower partial pressure. Specifically:2

Oxygen: Because the concentration of oxygen is higher in the alveolar air than in the deoxygenated blood within the pulmonary capillaries, oxygen diffuses across the alveolar-capillary membrane into the blood.21
Carbon Dioxide: Conversely, carbon dioxide, which is a byproduct of cellular metabolism, is present at a higher partial pressure in the blood arriving at the lungs. Consequently, it diffuses from the blood into the alveoli to be expelled during exhalation.21

This efficient exchange is facilitated by the immense internal surface area of the lungs and the close proximity of the capillaries to the alveolar epithelial cells.23