Illustration of the lining of the trachea (wind pipe). The trachea links the larynx to the lungs. The epithelial lining consists of cells that have hair like cilia (blue) on their surface and mucus-secreting glands. Mucus secreted by the glands traps debris, such as dust particles or bacteria, in the inhaled air, while the beating of the cilia moves the mucus and particles upwards out of the respiratory tract. This helps to keep the lungs and airways clear and prevent infection.