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BiologyHuman Physiology10 minBeginner

Alveoli and Gas Exchange

Study how thin alveolar walls, capillaries, and red blood cells support fast oxygen and carbon dioxide exchange.

Annotated teaching image of alveoli, capillaries, red blood cells, and the blood-air barrier.
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Alveolar lumen

Air-filled space where oxygen enters and carbon dioxide leaves.

Alveoli maximize exchange by placing air and blood extremely close together. Thin epithelial walls, a dense capillary network, and many red blood cells create a large surface for diffusion.

Oxygen moves from the alveolar air into blood. Carbon dioxide moves from blood into the alveolar space to be exhaled.

Key Idea

Fast exchange depends on surface area, short diffusion distance, and maintained concentration gradients.