Mucus is roughly 95% water. The remaining 5% is a mix of mucins (the proteins that give mucus its gel-like texture), other proteins, salts, lipids, DNA, and cellular debris. That small non-water fraction is what makes mucus sticky, stretchy, and surprisingly effective as a biological shield.
Water: The Base of the Gel
Healthy mucus is about 95% water by weight, with only 1 to 2% solid content in the airways. That ratio matters more than it sounds. When mucus dries out even slightly, doubling or tripling its solid concentration, it becomes thick and difficult to clear. This is exactly what happens in conditions like chronic bronchitis and cystic fibrosis, where dehydrated mucus clogs the airways and becomes a breeding ground for infection.
Your body actively regulates mucus hydration by moving water and salt ions across the cells lining your airways, gut, and other surfaces. When that regulation breaks down, the physical properties of mucus change dramatically, even though the basic ingredients stay the same.
Mucins: The Ingredient That Makes It Slimy
Mucins are the signature component, the molecules responsible for the slippery, gel-like quality of mucus. They make up about 0.2 to 5% of mucus by volume, depending on where in the body the mucus is produced and whether you’re healthy or fighting an infection.
Each mucin molecule is rod-shaped, built around a long protein backbone studded with sugar chains that branch outward like bristles on a bottle brush. These sugar chains are critical. They make the molecule hold water, resist being broken down by digestive enzymes, and interact with pathogens. The protein core alone would be fragile, but the dense coating of sugars protects it and gives mucus its characteristic slickness.
Your body has 21 different mucin genes, but the gel-forming work is done primarily by just a few of them. In the airways, two mucin types dominate. In the gut, a different mucin forms the thick protective layer that keeps bacteria from directly contacting the intestinal lining. These regional differences explain why airway mucus feels different from the mucus in your stomach or reproductive tract.
Mucin molecules link together through chemical bonds called disulfide bridges, which connect individual mucin units into long polymer chains. Those chains then tangle with each other and with other proteins and DNA to form a crosslinked mesh. Hydrogen bonds, ionic interactions, and physical entanglement all reinforce the network. The result is a material that behaves like both a liquid and a solid: it flows when you push it (like when you blow your nose) but holds its shape at rest.
Antimicrobial Proteins: Built-In Defense
Mucus isn’t just a passive barrier. It’s loaded with proteins that actively kill or neutralize microbes. Lysozyme, one of the most abundant, breaks apart bacterial cell walls. It’s found in mucus throughout the airways and gut, and it’s also present in tears and saliva. Lactoferrin works differently: it binds iron, starving bacteria of a nutrient they need to grow. A single lactoferrin molecule can grab up to two iron atoms, making it an effective antimicrobial even at low concentrations.
Beyond these, mucus contains antibodies (particularly a type called secretory IgA) that tag pathogens for destruction and prevent them from attaching to the cells underneath. Together, these proteins turn the mucus layer into an active immune checkpoint, trapping and disabling threats before they ever reach the vulnerable tissue below.
Salts and Electrolytes
Dissolved salts make up about 0.5 to 1% of mucus. Sodium, chloride, and potassium are the main ions, generally present at concentrations lower than what’s found in blood plasma (with the exception of sodium, which can run higher in nasal and airway secretions). Calcium ions play a particularly interesting role: airway mucus contains around 3 millimoles of calcium per kilogram, and these calcium ions help stabilize the crosslinked mucin network. They act as ionic bridges between negatively charged mucin molecules, reinforcing the gel structure.
The electrolyte balance in mucus isn’t fixed. It shifts with your body’s overall hydration, salt intake, and kidney function. Studies of intensive care patients have shown that changes in the body’s water and salt balance directly alter the sodium, potassium, and chloride levels in nasal secretions.
Lipids: Lubrication and Surface Tension
Lipids account for 1 to 2% of mucus by weight. These fats and fat-like molecules contribute to lubrication, helping mucus glide smoothly over tissue surfaces. In the lungs specifically, phospholipids play a vital role in reducing surface tension in the small air sacs where oxygen exchange happens. Without this surfactant effect, the tiny airways would collapse on themselves with each breath.
Lipids in mucus also help form a hydrophobic (water-repelling) surface layer that influences which molecules can pass through and which get trapped. This selectivity is part of what makes mucus an effective filter.
DNA and Cellular Debris
Healthy mucus contains small amounts of free-floating DNA, shed from dead epithelial cells (the surface cells that line your airways, gut, and other mucus-producing tissues) and from white blood cells that have completed their immune work. Under normal conditions, this DNA is a minor component that doesn’t significantly affect mucus behavior.
During infection, though, the picture changes. White blood cells called neutrophils respond to invaders by releasing web-like structures made of DNA and antimicrobial proteins. These webs trap bacteria effectively, but they also tangle with the mucin mesh, shrinking the pore size of the gel and making mucus dramatically thicker and harder to clear. This is one reason why mucus during a bad cold or lung infection feels so much heavier and stickier than normal. In cystic fibrosis, where lung infections are chronic, the buildup of DNA from these immune responses is a major contributor to the dangerously thick mucus that characterizes the disease. Treatments that break down this excess DNA are a standard part of managing the condition.
How These Ingredients Work Together
What makes mucus remarkable isn’t any single ingredient. It’s the way the components interact. The mucin mesh provides structure. Water keeps it fluid enough to move. Salts and calcium ions stabilize the gel. Lipids lubricate it. Antimicrobial proteins patrol for threats. And the whole system is continuously produced, pushed along by tiny hair-like cilia on cell surfaces, and either swallowed or expelled.
A healthy person produces about a liter of mucus per day in the airways alone, most of it swallowed without noticing. You only become aware of mucus when something goes wrong with the balance: too little water, too many mucins, an overload of DNA from immune activity, or a failure of the cilia to keep things moving. The composition stays largely the same across the body’s mucus-producing surfaces, but the ratios shift depending on location and what the mucus needs to do, whether that’s protecting stomach lining from acid, trapping inhaled dust particles, or creating a selective barrier in the reproductive tract.