Explainer: The benefits of phlegm, mucus and snot | Science News for Students

Explainer: The benefits of phlegm, mucus and snot

We cough it up and spit it out, but mucus deserves our appreciation
Feb 20, 2019 — 6:35 am EST
a close up photo of a baby wiping his nose

Mucus, snot and phlegm are made from the same thing and serve the same purpose. They’re a first line of defense against germs.


Mucus. You hack it up. Spit it out. Blow it into tissues and throw it away. But while it’s gross once it leaves the body, mucus, phlegm and snot play important roles inside us.

Part of the immune system, the role of this sticky goop is to help, explains Brian Button. He studies biophysics — the physics of living things — at the University of North Carolina in Chapel Hill. Mucus covers every part of our bodies that is exposed to the air but unprotected by skin. That includes our noses, mouths, lungs, reproductive areas, eyes and rectum. “All are lined with mucus to trap and clear the stuff we are exposed to,” he notes.

The sticky substance is made of long molecules called mucins (MEW-sins). Mixed with water, mucins link up to form a gluey gel. That gel traps bacteria, viruses, dirt and dust in its sticky embrace. In fact, mucus is the lung’s first line of defense against germs, which explains why the lung makes so much of it. Our lungs produce about 100 milliliters of mucus per day, enough to fill about a quarter of a 12-ounce soda can.

Lung mucus is known as phlegm. It’s thicker and stickier than the mucus in our noses or reproductive areas. But all of our mucus is made from mucins, which Button says come in “different flavors.” Button says. Those flavors are isoforms, proteins that get instructions from the same genes to form but end up with slightly different sequences. Various isoforms will produce mucus that can be thicker or thinner.

“They say doctors pick their specialties by what they find least gross,” notes Stephanie Christenson. “I can’t take poop, but my doctor friends [in other specialties] hate what I do because they think mucus is gross.” Christenson is a pulmonologist — someone who studies the lungs — at the University of California, San Francisco.          

Mucus, she explains, is natural. “Lungs are exposed to the environment,” she notes. Each inhaled breath can bring in bacteria, viruses and more. The body needs a way to expel them and has turned to mucus. That’s why, she argues, “Mucus is our friend.” 

To get invaders out of the lungs, phlegm has to keep flowing. The cells that line the lungs are covered in cilia — tiny hairlike structures. They wave back and forth, shoving the mucus up and out of our airways. When it reaches the throat, we will hack it up. Then, most of the time, we swallow it without a second thought. The stomach will later break down whatever germs it picked up along the way. Delicious!

After a cold or flu, “our bodies produce more mucus to trap and clear the [germs],” Button explains. If there’s too much phlegm in the lungs for the cilia to wave it all away, we cough. The rushing air rips the mucus off the lungs so we can hack it up.

In other areas of the body, mucus plays other roles. It keeps the surface of our eyes moist. Snot coats our mouths and noses to keep us safe from germs and soothe our irritated membranes. In the rectum, mucus helps to determine how quickly mammals expel their poop. And in a woman’s reproductive tract, mucus can control whether a sperm cell gets to an egg.

No matter how disgusting or gloppy it may seem, mucus is with us every moment of our lives. “If you think about what it’s doing,” Christenson says. “It’s a little bit less gross.”

Power Words

(more about Power Words)

bacteria     (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

biophysics     The study of physical forces as they relate to living things. People who work in this field are known as biophysicists.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

cilia     (singular cilium) Small hairlike features that occur on the surface of some cells and larger tissue structures. They can move and their wavelike motion can propel liquids to move in a particular direction. Cilia play an important role in many biological functions throughout the body.

defense     (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)

egg     The unfertilized reproductive cell made by females.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

gel     A gooey or viscous material that can flow like a thick liquid.

gene     (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

germ     Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

immune     (adj.) Having to do with the immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.

immune system     The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.

influenza  (also known as flu) A highly contagious viral infection of the respiratory passages causing fever and severe aching. It often occurs as an epidemic.

mammal     A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.

membrane     A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

mucus     A slimy substance produced in the lungs, nose, digestive system and other parts of the body to protect against infection. Mucus is made mainly of water but also includes salt and proteins such as mucins. Some animals use mucus for other purposes, such as to move across the ground or to defend themselves against predators.

phlegm     A thick, sticky mucus produced by the lungs.

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

sequence     (in genetics) n. The precise order of the nucleotides within a gene. (v.) To figure out the precise order of the nucleotides making up a gene.

sperm     The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.

tissue     Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.