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To understand asthma, you need to understand how you breathe. Air moves in and out of your lungs through a series of tubes or airways. These airways are like a tree - think of the trachea or windpipe as the trunk, which then divides into smaller and smaller branches (bronchi and bronchioles). These branches end with tiny "leaves" that allow the oxygen to move into the blood stream and the waste products, such as carbon dioxide, to be removed.

In a person with asthma, the airways are more sensitive than usual and are often called hypersensitive or twitchy. When exposed to an irritating "trigger" such as dust, the airways overreact and become narrower, making it more difficult to breathe. In severe cases, this is called an asthma attack (your doctor may call it an exacerbation).

What happens to your lungs?
Inflammation of the airways
What brings on asthma symptoms?
How do you recognise asthma?
What is the best way to manage asthma?