Q. What is mucus?

A. A. Mucus is chiefly composed of proteins called mucins (above), salts and water.

In the human body, most of it is secreted by widely distributed cells called goblet cells.

Large quantities are made in both the intestinal tract and in the respiratory tract, and its many roles as a lubricant and barrier make it essential to human health.

Other cells in the salivary glands also make mucus.

The most familiar form of it appears with the sneeze, which is not a mere nuisance, but just the tip of the iceberg: a large supply of mucus that protects the lining of the breathing passages all the way down into the lungs.

It also flows and carries impurities away from the respiratory system to the stomach to be excreted.

One of the most serious maladies involving mucus is cystic fibrosis, in which an abnormally sticky form is made in excessive quantities and clogs the chest.

The molecular structure of a mucin includes a surface element that is essentially a sugar; this gives the protein the ability to absorb water. It also appears to help make the mucus resistant to digestion.

The mucus coating of the stomach lining is now thought to form a one-way barrier that protects the stomach from digesting itself while still allowing corrosive hydrochloric acid to be secreted into the cavity to digest food.

[via C. Claiborne Ray and the New York Times]