At the bottom of the trachea, or windpipe, there are two large tubes. These tubes are called the main stem bronchi, and one heads left into the left lung, while the other heads right into the right lung. Each main stem bronchus – the name for just one of the bronchi – then branches off into tubes or bronchi that get smaller and even smaller still, like branches on a big tree. The tiniest tubes are called bronchioles and there are about 30,000 of them in each lung. Each bronchiole is about the same thickness as a hair.

At the end of each bronchiole is a special area that leads into clumps of teeny tiny air sacs called alveoli. There are about 600 million alveoli in your lungs and if you stretched them out, they would cover an entire tennis court. Each alveolus – what we call just one of the alveoli – has a mesh – like covering of very small blood vessels called capillaries. These capillaries are so tiny that the cells in your blood need to line up single file just to march through them.