A peak flow meter is commonly used by a person with asthma to measure the amount of air that can be expelled from the lungs. If the airways become narrow or blocked due to asthma, peak flow values will drop because the person cannot blow air out of the lungs as well. A peak flow meter can be a useful aid in monitoring a person's asthma over time and can also be used to help determine how well a patient's medications are working.

The major features of the lungs include the bronchi, the bronchioles and the alveoli. The alveoli are the microscopic blood vessel-lined sacks in which oxygen and carbon dioxide gas are exchanged.

During an asthma attack smooth muscles located in the bronchioles of the lung constrict and decrease the flow of air in the airways. The amount of air flow can further be decreased by inflammation or excess mucus secretion.