Mucus in the Human Body: Functions and Health Problems
Mucus in the Body
Mucus has a reputation as a gross substance and is one that many people would like to avoid. It's actually a very useful material with important functions. It destroys bacteria and viruses, traps particles such as dirt, prevents water loss, humidifies passageways, lubricates the movement of materials, and protects surfaces from damage.
Mucus is a slippery liquid made by mucous membranes, or mucosas. The membranes line the passageways in our bodies that connect to the outside environment. These passageways include the nose, the mouth, the airways, the digestive tract, and the reproductive tract. A mucous membrane is also located over the white part of the eye and on the inside of the eyelid.
Mucosas consist of thin surface layers on top of a supporting layer of connective tissue. Many contain glands that produce mucus, but some-including those in the urinary tract-—make no mucus or only a tiny amount.
Composition and State of Mucus
Mucus contains water, proteins such as mucins, antibodies, antiseptics, and salts. Mucins are glycoproteins, which are proteins with attached carbohydrates. The carbohydrate coating gives mucin molecules a large ability to absorb water. Antibodies help the immune system to attack pathogens (organisms that cause disease), while antiseptics kill pathogens directly.
Although mucus is important in many places in the body, for most of us its presence is most obvious in the nose and the respiratory tract. People may not deliberately look at their nasal mucus, or snot, but they may accidentally notice that it occasionally has an unusual appearance. The change is usually temporary and harmless, but sometimes it indicates a problem, as described below.
We produce between one and two liters of mucus in our respiratory tract each day. If we have a respiratory infection we produce even more.
The Role of Cilia in the Respiratory Tract
In the airways of the lungs, mucus traps inhaled particles and humidifies the air. Tiny hair-like structures called cilia line the airways. The cilia beat in a coordinated pattern, sweeping the mucus up to the back of the throat, where it's swallowed or released into the nose.
Smoking can damage the cilia, resulting in a buildup of mucus in the respiratory tract. The collection of mucus can make breathing difficult and cause a person to cough frequently in an effort to remove the blockage.
The Paranasal Sinuses
The paranasal sinuses are hollow spaces in the facial bones. They are lined with a mucous membrane that produces mucus. The function or functions of the sinuses are unknown. They may humidify the air that we breathe, protect sensitive areas such as the roots of the teeth from rapid temperature changes, lighten the skull, act as crumple zones during an impact, or increase the resonance of our voice.
Sinuses aren't isolated but are connected to other structures in the face, including the nose. This means that bacteria can travel into a sinus and cause an infection. A sinus infection is called sinusitis. It involves inflammation of the sinus lining and excessive mucus production. The mucus may be yellow or green in color and is discharged through the nose or into the throat through post-nasal drip.
Colored Nasal Mucus or Snot
Mucus freshly produced by the respiratory system is colorless. When it's removed from the nose, it may be grey, white, yellow, green, brown, black, orange, pink, red, or a rust color. Colored snot sometimes indicates a health problem, but this isn't always the case.
Some common reasons for changes in mucus color are listed below. There are other reasons for some of the changes, though. A doctor should be consulted if someone is concerned about the appearance of their nasal mucus.
Grey, White, Yellow, or Green Snot
Trapped mucus that is removed from the nose is often grey in color due to the dust and dirt that it contains and the fact that it has partially dried out. The dried clump of mucus is colloquially known as a "booger".
Cloudy or White
A cloudy appearance suggests that tissues are inflamed and swollen, causing mucus to move more slowly, lose moisture, thicken, and become cloudy. The inflammation may be due to an infection or an allergy. Mucus that is white rather than cloudy may indicate an infection such as a cold.
Yellow or Green
Yellow or green mucus doesn't necessarily indicate that an infection is present. If the colors are persistent, however, a viral or bacterial infection may exist. The Cleveland Clinic recommends that people see a doctor after twelve days with green mucus. Harvard Health Publications recommends a doctor's visit after ten days. If other symptoms are present in addition to the colored mucus, however, a doctor should be visited much sooner.
The green color is due to the presence of a colored enzyme called verdoperoxidase, which is released by white blood cells, or leukocytes. Some of these cells actively fight pathogens. Others are involved in the inflammatory response, which often accompanies allergies. Yellow or green mucus may therefore appear due to allergies or infections.
Medical sources sometimes differ in their ideas about the significance of mucus color. They all agree on the following points, however. If an unusual color persists, or if the change is accompanied by other symptoms such as a fever, a doctor should be consulted.
Brown, Black, Orange, or Red Snot
Brown or Black
Brown mucus can be caused by dirt in the nose or by the presence of blood. Black mucus may be produced by a major fungal infection that requires medical treatment. Tars obtained from cigarette smoke can stain mucus in the respiratory system brown or black. Coal miners may develop black mucus as well due to the inhalation of coal dust.
Orange, Red, or Rust-Colored
People with pneumonia may produce mucus with a burnt orange color, which is produced by the presence of blood. Anyone with pneumonia should be under a doctor's care. Pink, red, or rust-colored mucus also indicates the presence of blood. In these cases the blood may be produced by a minor wound in the nostrils. If the red color persists or if a large amount of blood is released, however, a doctor's advice should be sought.
Mucus in the Mouth and Stomach
Saliva in the mouth contains mucus with a thin consistency. This mucus is an excellent lubricant and makes swallowing food easier.
The stomach lining is covered by a protective layer of mucus. Glands in the stomach produce mucus, hydrochloric acid, and an inactive enzyme called pepsinogen. In the stomach cavity, the hydrochloric acid changes the pepsinogen into an active digestive enzyme called pepsin. This enzyme digests proteins. The mucus layer acts as a barrier which prevents the stomach lining from being attacked by pepsin and acid.
If the mucus layer in the stomach is thinned or removed, which may happen during an infection by a bacterium called Helicobacter pylori, pepsin and acid may be able to attack the stomach lining. The infection can cause inflammation (gastritis) and sores called ulcers.
Glands in the lining of the small intestine and the large intestine also produce mucus. There is often some mucus in stool, but generally not enough to be noticed. Noticeable amounts may appear in intestinal problems such as the diarrhea-predominant form of irritable bowel syndrome (IBS). The mucus looks like a thin, glistening jelly. In ulcerative colitis, the lining of the large intestine is inflamed and ulcers develop. Mucus and blood are often released into the stool in this disorder.
The Reproductive Tract
The cervix is the lower end of the uterus. Glands in this area produce cervical mucus. The consistency of this mucus changes during a woman’s monthly cycle as the reproductive hormone levels change in her body. Cervical mucus helps to prevent infection and supports or inhibits sperm movement.
After menstruation, very little cervical mucus is made. As the cycle progresses, the amount increases. The mucus is white or cream in color and is sticky. Around the time of ovulation, when an egg is released from an ovary and a woman is most fertile, the mucus is most abundant and is colorless, thin, and often stretchy. This is the best texture to allow for sperm penetration. After ovulation, the mucus decreases in amount, becomes cloudy, and develops a sticky texture again.
The conjunctiva is a mucous membrane that covers the white part of the eye (the sclera) and the inside of the eyelid. It produces a thin mucus that lubricates the eye and contributes to tear formation. Tears have three layers—an inner mucus layer made by the conjunctiva, a watery middle layer made by the lacrimal gland, and an oily outer layer made by the meibomian glands in the eyelids.
Once tears and mucus have spread over the surface of the eye, they drain into the nose through small openings at the inner corner of the eye. Each opening is called a lacrimal punctum (items b and e in the illustration below). Some people wake up in the morning with dried particles of mucus in the corner of their eyes due to inadequate drainage while they are asleep and not blinking.
The illustration below shows the following parts of the tear system. A: lacrimal or tear gland; B and E: lacrimal puncta; C and F: lacrimal ducts or canals; D lacrimal or tear sac; G: nasolacrimal duct or canal
Cystic Fibrosis or CF
Cystic fibrosis is a genetic disorder in which the body makes mucus that is much thicker and stickier than normal. The thick material can't move very easily and collects in body passages. Mucus in the breathing tubes makes it difficult to breathe and frequently leads to bacterial infections. In addition, mucus often blocks the tube that transports digestive enzymes out of the pancreas to the small intestine. This results in reduced digestion of foods— especially carbohydrates and fats—and therefore reduced nutrient absorption.
People with cystic fibrosis generally have a shortened life. However, the good news is that the average lifespan for patients is increasing rapidly in many countries due to improved treatments. The goal of research is to give patients a normal lifespan and a better quality of life.
A significant part of each day in a CF patient's life is spent on treatments to clear the lungs. They take many pills a day. They periodically use oxygen inhalers and wear a vibrating vest to break up mucus in the lungs. In addition, digestive enzymes are taken with all meals and snacks in order to obtain enough nutrients from food.
Cystic fibrosis results from a mutation in both copies of the gene for a protein called cystic fibrosis transmembrane conductance regulator, or CFTR.
Asthma is another problem that involves the production of more mucus than normal. People with the disorder have airways which are sensitive to specific allergens or irritants. The irritants include dust mites, cold air, cigarette smoke, certain food components, or exercise. During an asthma attack, the airways become inflamed and swollen, restricting the passage of air. The airways produce extra mucus, which further blocks the passage of air. In addition, the muscles that surround the airways contract, constricting the air passages. A wheezing or whistling sound is often heard as the person breathes.
Asthma can be treated and managed, but it can't be cured. Many asthmatics find that with their doctor's guidance they can reduce, weaken, and even eliminate asthma attacks. Sometimes asthma is harder to manage, however, and may occasionally be a medical emergency.
Bronchitis involves inflammation of the airways and excessive mucus production. Acute bronchitis lasts for only a short time and is generally caused by a viral infection. Chronic bronchitis lasts for a long time and is often permanent. It usually results from smoking or from chronic inhalation of certain chemicals or pollutants.
People suffering from acute bronchitis may produce colored mucus in their airways. This mucus may be white at first and then change to yellow or green. The color may indicate the presence of white blood cells associated with inflammation rather than a bacterial infection. As always, if symptoms persist or get worse, a visit to a doctor is in order.
Mucus coughed up from the respiratory system is sometimes called phlegm. Sputum is phlegm mixed with saliva.
Chronic Obstructive Pulmonary Disease
Chronic bronchitis and emphysema are the two most common diseases classified as a Chronic Obstructive Pulmonary Disease or COPD. Both are caused chiefly by smoking but may be worsened by inhaling polluted air. Symptoms include wheezing, shortness of breath, and coughing up mucus. Unfortunately, the trigger that causes the disease damages the lungs and the disorder is progressive. The sooner the condition is diagnosed and the trigger avoided, the better the outcome.
People with COPD can have either chronic bronchitis or emphysema, but they often have both conditions. The alveoli are tiny air sacs in the lungs in which oxygen is absorbed into the blood. In emphysema, the walls of alveoli break down, making exhalation difficult.
Mucus: Friend and Foe
Mucus is an essential substance in the human body and is usually a friend, despite its unpleasant reputation. However, at times, or in certain people, too much mucus is made and the mucus is transformed from a useful material into a dangerous foe. This is especially sad in diseases that affect the quality of life. Hopefully medical research will continue to discover improved ways of dealing with excess mucus and its consequences.
© 2011 Linda Crampton