Introduction

When a person breathes, air enters the lungs through progressively smaller airways called bronchioles. There are millions of these bronchioles that lead to microscopic sacs called alveoli. It is in the alveoli that oxygen and carbon dioxide are exchanged.

Asthma is a chronic inflammatory condition in which the airways undergo changes when stimulated by allergens or other environmental triggers that causes an inability to breathe properly. The word "asthma" was derived from an ancient Greek word meaning panting or breathlessness. Other terms used are reactive airways disease and reversible airways disease. There is inflammation, hyperresponsiveness and at least partial obstruction of the airways that cause both short and long-term symptoms. There is also excessive production of mucous.

The heightened responsiveness of asthmatic airways causes reactions to stimuli that trigger little or no change in the airways of normal subjects. The cause of this hyperresponsiveness is not clearly understood. There is some evidence that chronic inflammation may play a role.

Premature closing of small airways causes reduced ability to expel air, eventually leading to air trapping and lung over-inflation. This can cause permanent changes in the anatomy of the chest wall (the so-called "barrel chest"). Young children are especially at risk for these long-term consequences because of ongoing development of the rib cage.