Bronchodilators are medicines that help open the bronchial tubes (airways) of the lungs, allowing more air to flow through them. They are used by people with the condition asthma. People with asthma have trouble breathing, because their airways are inflamed and narrowed.

Normally, air moves smoothly through the airways and into the tiny air sacs of the lungs while a person is breathing in (inhaling). Breathing out (exhaling) happens automatically when the person stops inhaling. In a person with asthma, inhaling is not a problem. Incoming air can slide around the blockage, because the act of inhaling makes the airways expand. The problem comes when the person with asthma tries to exhale. The air can no longer get past the blockage, and it remains trapped in the lungs. The person can then only take shallow breaths. Bronchodilators work by relaxing the smooth muscles that line the airways. This...