The gas exchange system and the alveoli

Last time

Before we start: last time, in The cardiac cycle and blood cells, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Air in, oxygen across

Every breath carries air down a branching tree of tubes. The tubes must stay open, clean the air, and finally deliver it to millions of thin walled sacs. Each part of the tree has its own tissues, and Cambridge asks you to recognise them on photographs. Then we follow the oxygen across the alveolus wall.

The airway tree

Air passes down the trachea, which divides into two bronchi, one for each lung. Each bronchus divides repeatedly into smaller bronchioles. The bronchioles end in clusters of tiny air sacs called alveoli. Each alveolus is surrounded by a network of capillaries. In the lungs there are around three hundred million alveoli, giving a huge surface area for gas exchange.

What each tube contains

The trachea has C shaped rings of cartilage, which keep it open but allow food to pass behind in the oesophagus. It has ciliated epithelium, goblet cells, smooth muscle, and elastic fibres. A bronchus has the same tissues, but its cartilage is in smaller curved plates. As the tubes divide, the cartilage becomes less. A bronchiole has no cartilage. Its wall has smooth muscle and elastic fibres, with a thin layer of epithelium, and the cilia and goblet cells disappear as it gets smaller. An alveolus has only a single layer of flat squamous epithelium, with elastic fibres in between.