Asthma is defined as a disease of the airways – passages of lungs through which air-containing oxygen passes in and carbon dioxide passes out. The disease occurs because of inflammation of the airways, a process characterized by swelling of the airway tissues due to fluid and collection of certain cells. The mucous glands of the airways secrete too much of mucous. Due to swelling and too much of mucous, the air passages become narrower.
The airways have smooth muscle in their walls. The excessive contraction of this muscle during attacks of asthma further narrows the passages making it more difficult for air to pass in and out and requiring greater effort from the patient to breathe. The narrowing of the air passages with swelling of the tissues and presence of mucous are responsible for the symptoms of asthma – breathlessness with wheezing, cough and phlegm.
Read more on What is Bronchial Asthma.