Human body

Alveoli & gas exchange

Anatomy · Respiratory system

How does oxygen get from the air you breathe into your blood in less than a second?

Your airways branch again and again until they end in about 480 million tiny air bubbles called alveoli. Each one is wrapped in a net of very thin blood vessels, and the wall between air and blood is about 100 times thinner than a sheet of paper. Oxygen simply drifts across that thin wall into the blood, and carbon dioxide drifts out.

Beyond the terminal bronchiole the airways end in hundreds of millions of thin-walled alveoli, each wrapped in a sheet of capillary blood. Oxygen and carbon dioxide cross the half-micrometre blood–air barrier by passive diffusion, driven by partial-pressure differences that ventilation maintains.

Key facts

Number of alveoli (adult)

≈ 480 million (range ≈ 270–790 million)

Alveolar surface area

≈ 70–130 m²

Blood–air barrier

≈ 0.2–0.6 µm at its thinnest

Alveolar PO₂ / PCO₂

≈ 100 / 40 mmHg

Mixed venous PO₂ / PCO₂

≈ 40 / 46 mmHg

Capillary transit time (rest)

≈ 0.75 s; O₂ equilibrates in ≈ 0.25 s

Parts you can explore (14)

• Terminal bronchiole
• Respiratory bronchioles
• Alveolar ducts
• Alveolar sacs
• Pulmonary capillaries
• Pulmonary arteriole
• Pulmonary venule
• Type I pneumocytes
• Type II pneumocytes
• Alveolar macrophage
• Surfactant layer
• Red blood cells
• Smooth muscle & elastic fibres
• Respiratory membrane (inset)

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