Mucus in the Human Body - Functions and Health Problems
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. Mucus 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. 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. There is also a mucous membrane over the white part of the eye and on the inside of the eyelid.
An alternate name for a mucous membrane is a mucosa. A mucosa is made of a thin surface layer and a supporting layer of connective tissue underneath. Many mucous membranes contain glands that produce mucus, but some - including the membrane lining the urinary tract - make no mucus or only a tiny amount.
We produce between one and two litres of mucus in our respiratory tract each day. If we have a respiratory infection we produce even more.
Composition of Mucus
Mucus contains water, proteins such as mucins, antibodies, and antiseptics, and salts. Mucins are large 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.
Mucus 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 to try to remove the blockage.
Mucus in the Digestive or Gastrointestinal Tract
Saliva in the mouth contains mucus, which is an excellent lubricant and makes swallowing food easier. The mucus in saliva has a thin consistency.
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. Ulcers caused by a bacterial infection may develop in the first part of the small intestine as well as in the stomach.
There is generally some mucus in stool, but usually not enough to be noticed. Noticeable amounts of mucus 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.
Mucus in the Reproductive Tract
The cervix is the lower end of the uterus. Glands in this area produce cervical mucus. The consistency of the 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, there is usually very little cervical mucus made. As the cycle progresses, the amount of mucus produced increases. The mucus is white or cream in colour and is sticky. Around the time of ovulation, when an egg is released from an ovary and a woman is most fertile, the cervical mucus is most abundant and is colourless, 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. The conjunctiva 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 puncta, or a puncta lacrimalia. 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 paranasal sinuses are hollow spaces in the facial bones. They are lined with a mucous membrane that produces mucus. The function of the sinuses is unknown, but there are several theories about their job. 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, acts 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 excess mucus production. The mucus may be yellow or green in colour and is discharged through the nose or into the throat through post-nasal drip.
The Paranasal Sinuses
Mucus may be colourless, white, grey, yellow, green, brown, black, orange, pink, red or a rust colour.
Mucus cleared from the nose is often grey in colour due to the trapped dust and dirt that it contains.
Yellow or Green Mucus
Infections can produce yellow or green mucus. These colours don't necessarily indicate that an infection is present, but if the colours are persistent, they may. People suffering from bronchitis caused by a viral infection may produce coloured mucus in their airways. Mucus coughed up from the respiratory system is sometimes called phlegm. Sputum is phlegm mixed with saliva.
Brown or Black Mucus
Brown mucus can be produced by an infection. 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 Mucus
People with pneumonia may produce orange mucus in their respiratory systems. Pink, red or rust coloured mucus indicates the presence of blood.
Cystic fibrosis results from a mutation in both copies of the gene for a protein called cystic fibrosis transmembrane conductance regulator, or CFTR. A major symptom of the disorder is excessive production of a thick mucus.
Disorders Involving Excess Mucus Production
Cystic fibrosis is a genetic disorder in which the body makes mucus that is much thicker and stickier than normal mucus. The thick mucus 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. Mucus often blocks the tube that transports digestive enzymes out of the pancreas to the small intestine, resulting in reduced digestion of foods - especially carbohydrates and fats - and therefore reduced nutrient absorption.
People with cystic fibrosis generally have a shortened lifespan. However, the good news is that the average lifespan 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. As the teenager in the video below describes, a significant part of each day in a CF patient's life is spent on treatments to clear the lungs. Hospitalizations are sometimes necessary to accomplish this process. In addition, digestive enzymes must be taken with all meals and snacks in order to obtain enough nutrients from food.
Living with Cystic Fibrosis
People with asthma have airways which are sensitive to allergens or irritants such as 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.
What Happens During an Asthma Attack?
Bronchitis involves inflammation of the airways and excess 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.
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
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