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A mucous membrane is a layer of epithelial tissue which lines an area of the body which comes into contact with air. Mucous membranes are moist because of the presence of glands which secrete a thick fluid known as mucus, and they are important for a number of bodily functions. Mucous membranes line the urogenital tract, digestive tract, and respiratory tract, with one of the more well known mucous membranes being the lining of the interior of the nose.
The moisture found in a mucous membrane acts to protect the body by creating a barrier and preventing the inside of the body from drying out. Mucus also traps pathogens, dirt, and particulate matter so that they can be sequestered and eliminated by the body. The nose is particularly famous for this, using mucus as a barrier between many harmful substances and the respiratory tract. Some sections of mucous membrane also have small hairs known as cilia which act as traps, and can move to push things across the surface of the membrane.
Mucus can act as a lubricant, and it also facilitates gas exchange and absorption. In the lungs, for example, a thin lining of mucus is critical to healthy lung function. The absorption qualities of mucous membranes are also important in the digestive tract, where the body pulls necessary nutrients out of food as it passes along the alimentary canal. The digestive tract also has very active cilia, like those found in the nose.
People have to be careful with their mucous membranes, because the mucosa can be very delicate. Although the mucus helps to protect the body, its capacity for absorption can also become a problem, because many toxins and other harmful substances can be quickly absorbed through the mucosa. As anyone who has cut peppers and then rubbed an eye knows, mucous membranes are very adept at absorbing various compounds, and they are also very vulnerable to pain.
The mucous membrane which lines the urogenital tract are designed to prevent infection and provide lubrication, but they can also make someone vulnerable to infections passed between sexual partners. If the mucosa rips or is cut, the opening can provide a way for various infectious agents to enter the body, and the mucus will be unable to halt the infection in its tracks. As women with vaginal infections have noted, changes in the balance of beneficial organisms around the mucous membranes can result in unpleasant discharges, itching, and other symptoms as the mucous membrane struggles to do its job.