Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Alveolus
What is Alveolus?
A microscopic air sac at the end of a bronchiole, and the site of gas exchange in the human lung. An alveolus is not a single cell but a small sac, roughly a quarter of a millimetre across, whose wall is made of extremely flattened cells and is only one cell thick. Alveoli occur in clusters that open off a shared alveolar duct, so neighbouring sacs share their walls, and each shared wall carries a dense network of blood capillaries. The inner surface of every alveolus carries a thin film of moisture in which gases dissolve. Because the capillary wall is also one cell thick, a gas crossing between the alveolar air and the blood passes through only two cells, a total distance of well under a thousandth of a millimetre. There are very many alveoli in each lung, and together they provide a very large total surface area for diffusion.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.
Common mistakes with Alveolus
- An alveolus is not a cell. It is a small sac, and its wall is made of cells. Writing “the alveolus is one cell thick” is acceptable shorthand only if you mean its wall; writing “an alveolus is a single cell” is wrong, and it makes the rest of your answer hard to mark. Say “the alveolar wall is one cell thick”.
Questions students ask about Alveolus
Does expired air contain any oxygen?
Yes, about 16 %. Only about a fifth of the oxygen breathed in is actually absorbed: a large fraction of each breath never reaches an alveolus at all, and even in the alveoli the blood does not take up all the oxygen available in a single pass. This is why one person’s expired air can still supply another person during rescue breathing.