Asthma is a disease characterized by the swelling of the walls of the airways in the lungs due to certain factors and the resulting narrowing of these airways. Asthma is thought to be caused by a mixture of hereditary and environmental factors.

Inside the lungs are the airways, which are made of flexible tissues and branch out like a tree. The airways are surrounded by muscle bands. These airways divide and narrow as they travel through the lung.

At the far end, the smallest airways end in clusters of tiny balloon-like air sacs (alveoli). These clusters are surrounded by blood vessels. When you breathe in, air enters the lungs. It travels through the airways until it reaches the air sacs.

When we exhale, air moves through the airways and out of the lungs. The airways produce mucus that traps particles in the breath we breathe. Normally, mucus is expelled from the lungs by small hairs (cilia) in the airways.

In asthma, the walls of these airways thicken, the muscles around them contract and excess mucus is produced, making it difficult for the person to breathe.