Lubricating and Protective Properties of Mucus, and Importance of Mucus in the Gastrointestinal Tract
Mucus is a thick secretion composed mainly of water, electrolytes, and a mixture of several glycoproteins, which themselves are composed of large polysaccha-rides bound with much smaller quantities of protein. Mucus is slightly different in different parts of the gas-trointestinal tract, but everywhere it has several im-portant characteristics that make it both an excellent lubricant and a protectant for the wall of the gut. First, mucus has adherent qualities that make it adhere tightly to the food or other particles and to spread as a thin film over the surfaces. Second, it has sufficient body that it coats the wall of the gut and prevents actual contact of most food particles with the mucosa. Third, mucus has a low resistance for slippage, so that the particles can slide along the epithelium with great ease. Fourth, mucus causes fecal particles to adhere to one another to form the feces that are expelled during a bowel move-ment. Fifth, mucus is strongly resistant to digestion by the gastrointestinal enzymes. And sixth, the glycoproteins of mucus have amphoteric properties, which means that they are capable of buffering small amounts of either acids or alkalies; also, mucus often contains moderate quantities of bicarbonate ions which specifi-cally neutralize acids.
In summary, mucus has the ability to allow easy slip-page of food along the gastrointestinal tract and to prevent excoriative or chemical damage to the epithelium. A person becomes acutely aware of the lubricat-ing qualities of mucus when the salivary glands fail to secrete saliva, because then it is difficult to swallow solid food even when it is eaten along with large amounts of water.