Gas Exchange in the Lungs

The alveoli are tiny air sacs in the lungs where gas exchange takes place. Oxygen moves into the blood, to be used for respiration, and carbon dioxide is removed from the blood. Alveoli are surrounded by a network of capillaries, which are small blood vessels that transport these gases.

In the diagram of an alveolus below, we can see that the capillary is close to the wall of the alveolus. The close distance makes the exchange of gases between the alveoli and the blood more efficient.

So remember:

  • When we breathe in – Oxygen diffuses out of the alveoli and into the blood
  • When we breathe out – Carbon dioxide diffuses out of the blood and into the alveoli

Features of Alveoli

The alveoli are specialised structures in the lungs that are adapted for efficient and effective gas exchange. These features help the alveoli to maximise gas exchange:

  • The moist, thin walls of the alveoli allow gases to diffuse easily across them
  • The large surface area of the alveoli means that they can diffuse large quantities of gases at once, helping to support the body’s oxygen needs.
  • The alveoli are surrounded by a network of capillaries, so the blood can take gases to and from the alveoli quickly. This helps to maintain a concentration gradient, which is essential for diffusion