mucus

Mucus is the airway's flypaper: a clear, slippery, sticky film that coats the inside of the breathing tubes and grabs whatever you inhale — dust, pollen, smoke particles, bacteria, viruses — before they can reach the delicate gas-exchange surfaces deep in the lung.

Chemically, mucus is mostly water (about 95 percent) held together by long, thread-like sugar-coated proteins called mucins, which give it its gel-like, stretchy character. It also carries antimicrobial molecules, antibodies, and salts. The body produces a surprising amount each day, most of which is swept up by cilia and swallowed without notice.

Healthy mucus is thin enough to be moved easily. Disease changes both its amount and its quality: asthma and chronic bronchitis make excess mucus, infection makes it thick and discolored, and in cystic fibrosis defective salt transport dehydrates the mucus into a sticky paste that clogs airways. Too little mucus is also harmful, leaving the airway dry and vulnerable.