Complex fluid composed of water, glycoprotein, carbohydrates, lipids, DNA, and cellular debris. Gel phase – the relatively viscous and elastic layer at the luminal surface of the airways. Sol phase – the thinner, watery layer that contacts the beating cilia of the epithelial lining cells and carries the gel phase. Mucus captures dust debris, bacteria, chemicals by trapping them like fly paper to be swallowed, sneezed, or coughed out for removal. Also provides moisture and physical protection for all mucosal tissues.