Phlegm is a form of respiratory mucus produced by the lungs. The airways between the larynx and the respiratory bronchioles are lined by columnar epithelium over which an airway surface liquid (ASL) ...

Clear, bubbly phlegm does not typically indicate a problem. However, coughing up large amounts could indicate an underlying health issue, such as fluid in the lungs. Phlegm, or sputum, is a type of ...