Mucus

The slippery, viscous, water-based fluid produced by specialized mucous membranes. Mucous membranes line the respiratory, digestive, and reproductive tracts. Mucus contains hydrophilic proteins, immunoglobulins, enzymes, and shed cells, and serves a variety of functions in different circumstances. For example, mucus in the nose and respiratory tract serves to trap dust and other contaminants before they reach the lungs; in the gut it lubricates the lining; in the female reproductive tract, cervical mucus changes its properties during the menstrual cycle so as to promote or retard the passage of sperm.