Respiration

Each lung contains roughly 300 million alveoli, the tiny air sacs at the very end of each bronchiole. The alveoli are where oxygen actually crosses from the air into the blood, and where CO₂ goes the other way.

How the alveoli are adapted for gas exchange

Each adaptation matches one of the four general features of a good exchange surface:

• Large surface area: there are hundreds of millions of alveoli per lung. If you flattened all the alveolar walls of one human's lungs into a single sheet, the area would be around 70 m², about half a tennis court.
• Thin walls: each alveolus is just one cell thick, and so are the capillaries wrapped around it. The total diffusion distance from air to blood is therefore only about 0.5 μm.
• Steep concentration gradient: maintained by two things working together. The lungs are constantly ventilated with fresh air (rich in oxygen, low in CO₂), and the capillaries are constantly carrying blood away (taking the absorbed oxygen with it, replacing it with deoxygenated blood).
• Moist surface: each alveolus is lined with a thin film of fluid. Oxygen and CO₂ both dissolve in this fluid before crossing the cell wall.

Direction of diffusion

• Oxygen moves into the blood, from a high concentration in the alveolar air to a lower concentration in the deoxygenated blood arriving in the capillaries.
• Carbon dioxide moves out of the blood, from a high concentration in the arriving blood to a lower concentration in the alveolar air, then is breathed out.