The primary function of the lungs is to allow gas exchange to occur.  Oxygen gas can diffuse into the blood from the air in the lungs.  Oxygen of course is needed for the process of aerobic respiration that is happening in every cell all the time.  Aerobic respiration produces carbon dioxide as a waste product.  Carbon dioxide diffuses out the blood in the lungs into the air in the lungs.  Hence the name gas exchange – one gas (oxygen) diffuses in, another (carbon dioxide) diffuses out.

This diagram above shows the bronchial tree – the branching network of tubes that carry air into the lungs.  The trachea at the top branches into the right and left bronchi, then each in turn branch into smaller bronchi and finally into the smallest tubes called bronchioles.  Bronchioles carry air into a cluster of tiny airsacs called alveoli (not ravioli as AZB told his F division today…)

Diffusion is the passive movement of molecules of a liquid or gas from a high concentration to a low concentration.  So the first question is what ensures that there is an appropriate concentration gradient for each gas to diffuse?

In order to understand this, you have to remember that the blood going to the lungs is deoxygenated.  The right ventricle pumps deoxygenated blood to the lungs in the pulmonary arteries.  The tiny alveoli are then covered with capillaries and these join together to form the pulmonary veins.   The pulmonary veins carry the oxygenated blood back to the left atrium of the heart.  So the blood coming to the lungs will have a low oxygen concentration but a high carbon dioxide concentration.

How are the structure of alveoli adapted for efficient gas exchange?

1. The alveoli in total provide a large surface area for the diffusion of oxygen and carbon dioxide.  The total surface area of the alveoli in humans is approximately 90 m2 – the equivalent of two tennis courts…..
2. The walls of the alveoli are very thin.  The alveolus is lined with a single layer of cells, and of course the capillaries are also only one cell thick. So the distance for the diffusion of oxygen and carbon dioxide is very small (hence the rate of diffusion is very fast)
3. The alveoli have a rich blood supply.  Alveoli are lined by many capillaries.
4. The surface of the alveolus is moist.  Gas exchange surfaces are always moist as oxygen and carbon dioxide will diffuse more rapidly if they are dissolved in water.
5. Alveoli also contain a cell that secretes surfactant.  This molecule reduces the surface tension in the film of water that lines the alveolus, allowing air to move in and out more smoothly.