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This essay is excerpted from The Unseen Body: A Doctor’s Journey Through the Hidden Wonders of Human Anatomy by Jonathan Reisman. Copyright © 2021 by Jonathan Reisman. Reprinted with permission from Flatiron Books. All rights reserved.
Mucus holds a special place among bodily fluids. This fact became apparent to me quickly in medical school. Health care workers, who deal with bodily fluids of every sort, seemed to have a particular distaste for mucus above all the others. I’ve heard countless doctors declare their disgust for sputum’s chunky, gelatinous, sticky texture. Many nurses have told me they would prefer cleaning up a patient’s bloody stool, even C. diff, to disposing of mucous secretions any day. When friends and family members wonder about my work as a physician, they often ask if blood grosses me out—I explain that blood is not the bodily fluid that typically grosses out health care workers. Mucus is.
Mucus’s most fundamental quality is its consistency—this is what differentiates it from plain water. While water flows and drips easily and fluently, mucus oozes. While water droplets grab on to each other, easily melding edges to form larger drops, viscous mucus holds on to itself cohesively and resists all disturbances. With a tenacity that water lacks, mucus clings to surfaces, including the linings of our air passages, and it puts up a fight when we try to clear it out. The difference between water and mucus is the same as the difference between fruit juice and jelly, between simple salt water and bone broth, between gumbo before okra is added and after—mucus has body to it. In our own bodies, it appears in many forms beyond just lung phlegm, including snot, saliva, and vaginal discharge, with less noticeable amounts in stool and sometimes urine. But whatever we call its various manifestations, all are simply variations on the same theme of mucus.
Dr. John McGinniss is a pulmonologist at the University of Pennsylvania whom I met in medical school, and he describes being a pulmonologist the following way: “I do mucus all day, every day.” Though he chose a career focused on this bodily fluid, he admits that it evokes a unique visceral reaction in many people because of its consistency, its distinctive thick bubbly sound, and sometimes its smell. “When you sit next to someone on the bus or plane,” he said, “and you see them coughing and hear wet, rattling mucus, you think to yourself, Oh geez, what disease am I going to get now?” More than any other product of the human body, he said, people associate mucus in particular with illness. “And there’s an emotion attached to it.”
My own perception as a medical student quickly moved from one of disgust to one of wonder and appreciation. I realized that mucus is made by the human body for the same reason it coats the bodies of many animals, plants, and fungi throughout the natural world—for protection. A mucous covering safeguards snails and slugs, those creatures bathed in a slimy layer that leaves a silver sheen as they trudge across leaves and sidewalks. Their mucous coatings prevent them from drying out, and also fight off microbes like a shield. Rays, sharks, and tamarind seeds are bathed in a similar layer of lubricating and defensive mucus, as are several species of mushroom.
But unlike these other creatures, the human body is not coated from head to toe in mucus; instead it appears only in specific areas where an opening disrupts the body’s otherwise continuous outer veneer of tough, dry skin. There are several of these disruptions, where skin folds in on itself to form a pocket, and all of them are clustered in the body’s face, groin, and backside. Some of them, like the sinuses, have blind ends, but most, like the mouth, nose, vagina, and rectum, are passageways leading deeper into the body’s anatomical cavities and tracts. Even the branching air passages that fill our lungs are just another of the body’s pockets, though one that is more complex and manifold than most.
What all our bodily openings have in common is mucus. Unlike the desiccated crust of regular skin, the linings of these areas remain perpetually moist thanks to the steady production of mucus. Our various invaginations are like the human body’s wetlands—while most of the surface is dry land, every once in a while, you come across a soggy patch. And unlike skin’s varying colors and shades, everybody’s soggy patches are lined by a universally deep-pink, blood-rich layer called mucous membrane, a layer named after its primary product and the thing we all have in common.
Our bodies require these perforations to serve as entrances and exits, as transition zones between the body’s dry outside and its forever moist innards. But at the same time, they’re constantly in danger: Besides enticing people to put objects into them, which often leads to an ER visit when they cannot get them back out again, the primary threat is microbial invasion.
While intact skin provides a layer of keratinized armor to fend off bacteria and other invading microbes, each disruption in skin’s continuity is a vulnerable chink in the armor and a potential avenue for them to breach the body. Infectious microorganisms love nothing more than to bask in our dank openings, thriving and multiplying to their heart’s content in the humid darkness offered by the human body’s pink pockets.
Every microbe that attacks us has its predilections for certain spots. Yeast bloom in the vagina (and sometimes the mouth), while influenza and coronavirus prefer the nose, throat, or deeper into the lungs for their moistened revelry. Gonorrhea is the least picky of them all—it will take whatever mucus-lined pocket it can get into and regularly invades the urethra, rectum, and vagina. Sometimes gonorrhea climbs farther into both male and female genitals tracts, reaching all the way to the ovaries and testicles. I’ve even seen it infect my patients’ eyes and throat.
Precisely because the body’s many fenestrations are not easily guarded, mucus is essential. As a universal defense weapon and survival strategy, mucus flows outward from all of them in a steady, unending tide to keep microbes out—they’d have to swim upstream against a viscous current to get in. Keeping our membranes perpetually moist is also essential for maintaining their health and integrity, and mucus accomplishes this as lubrication with a staying power that plain water could never muster. Though mucus is often annoying and repulsive, it shouldn’t be hated—instead, in the right balance, it is the key to how we stay healthy against an onslaught of invaders. And in healthy times, we make only enough to coat our surfaces with a thin veneer, the minimum needed to carry out its protective mission unnoticed.
