A sticky substance produced by specialized (goblet) cells within the respiratory tract. After synthesis it is stored in mucous droplets in goblet cells until it is needed, at which time it is expelled from the cell and sits atop the pericellular fluid layer that coats the airway epithelium. The sticky material "catches" inhaled particulates and is then eliminated from the body by the mucociliary escalator. Mucous is the major component of phlegm that is expectorated from the lungs in individuals with a "productive" cough. Lung diseases associated with airway inflammation (acute and chronic bronchitis, asthma, COPD) often lead to increased production of mucous.