Mucus is a protein infused fluid produced by the mucous membranes that coats the membrane to prevent damage to it. It is found in the lungs, nasal cavities, sinuses and stomach. In the stomach, it protects the stomach wall from stomach acid while still allowing water and nutrients to be absorbed into the tissue. In the respiratory tract, it protects the tissues from particles in the atmosphere, such as dust or even soot, and also prevents infection from penetrating into the delicate tissues. However, it can still allow the passage of gases. It is constantly being produced and expelled by the body, requiring fluids from the body to replenish it. If a patient starts to dehydrate, the mucous in the respiratory tract may dry out, exposing the membranes to damage and infection.
In response to infection or inflammation, the respiratory tract tends to produce more mucus, resulting in common symptoms such as respiratory congestion, coughing, runny nose and post-nasal drip.
In a healthy person, mucus is clear and liquid. However, as it is exposed to air, it becomes thicker. The mucus itself can become infected, and will then become much thicker and turns yellow or green. When infected mucus is spotted, immediate medical attention is advised as such an infection can easily develop into bronchitis or even pneumonia.
Mucus in the stomach reacts differently. When fluid is absent, in addition to not producing mucus, the stomach also tends not to produce stomach acid, resulting in a loss of appetite. However, the patient will become thirsty instead.
In cystic fibrosis, the mucus is much thicker than normal, interfering with breathing, digestion of food, and providing a fertile breeding ground for infection. As a result, fibrosis patients rarely live to the age of thirty and require huge amounts of medication to keep the mucous from solidifying and to assist with digestion.
House is noted for his interest in mucus, and several characters have noted that House would always take a case where the patient's mucus was an interesting color.