Sputum is the result of secretions from the lower airways (trachea, bronchi, alveoli) being expelled through the mouth. Sputum is usually a sign of inflammation of the mucous membrane lining the airways. This is why it is usually expelled by a cough that is called a wet or productive cough. 

The airways are lined by the epithelium, a mucous membrane made up of cells with small vibrating cilia. The epithelium produces mucus which moistens the air and provides a barrier against irritants and infectious agents. It is essential for the proper functioning of the organs. Normally, mucus is produced in small quantities. It is expelled by the vibrating cilia which move and is usually swallowed.

In the presence of infection or inflammation, mucus production increases, thickens and its composition changes. This naturally triggers expectoration to clear the airways.