Your body produces mucus continuously, coating nearly every internal surface exposed to the outside world. In the airways alone, roughly 30 milliliters of mucus is cleared daily, swallowed without you ever noticing. Across the entire respiratory and digestive tracts, production is far greater. The process starts inside specialized cells that manufacture, package, and release gel-forming proteins in a tightly regulated system.
The Cells That Make Mucus
Two main sources produce mucus: goblet cells and submucosal glands. Goblet cells are scattered throughout the lining of your airways, intestines, and other mucosal surfaces. They get their name from their shape, which resembles a drinking goblet, with a narrow base and a wide top packed with stored mucus granules ready for release.
Submucosal glands sit deeper in the tissue, beneath the surface lining. In the large airways of your lungs, these glands are the dominant source. Their total volume in the main bronchi is about 50 times greater than that of surface goblet cells. They shrink in size and become less frequent as you move into smaller airways. The advantage of submucosal glands is that they massively increase the number of mucus-producing cells without replacing the ciliated cells on the surface that are responsible for sweeping mucus along.
Different locations use slightly different mucus recipes. In the stomach, surface cells produce one type of gel-forming mucin, while glands deeper in the stomach and upper small intestine produce another. In the colon, goblet cells at the surface build and renew the inner mucus layer, while goblet cells in the crypts (small pits in the intestinal lining) can rapidly restore the mucus barrier after injury.
How Mucus Is Built Inside the Cell
The core ingredient of mucus is a family of large proteins called mucins. These proteins are assembled inside the cell’s internal factory in stages. First, the protein backbone is built. Then it gets coated with sugar molecules in a process called glycosylation, which is what gives mucus its slippery, water-attracting properties. A single fully assembled mucin building block has a molecular mass of about 2.5 million daltons, making it one of the largest molecules your body produces.
These individual mucin units don’t stay separate. They link together end to end and side to side, forming enormous net-like sheets. One end of the molecule pairs up in groups of two, the other end in groups of three, creating a structure that functions like an expandable mesh. Once assembled, these polymer sheets are compressed and stored in tightly packed granules inside the goblet cell. The granules maintain a low pH and high calcium concentration, which keeps the mucin folded into a dense, compact state.
How Mucus Gets Released
When the cell receives the right signal, the stored granules fuse with the cell’s outer membrane and empty their contents. This happens in two ways. In regulated vesicle secretion, individual granules fuse one at a time with the surface, providing a steady, controlled release. In compound exocytosis, most of the granules inside the cell fuse and dump their contents all at once, a more dramatic response typically triggered by irritation or infection.
What happens next is remarkable. As the compressed mucin hits the surface environment, it encounters bicarbonate, a substance secreted by surrounding cells. Bicarbonate raises the pH and strips away the calcium ions that were holding the mucin in its compact form. Freed from those constraints, the mucin unfolds and expands more than 1,000-fold in volume, like a compressed sponge hitting water. This rapid expansion transforms the dense granule contents into the wet, gel-like substance you recognize as mucus.
This bicarbonate-dependent unfolding is so critical that when it fails, mucus becomes sticky and dysfunctional. In cystic fibrosis, a defective ion channel reduces bicarbonate secretion onto airway and gut surfaces. The result is mucus that stays dense, adheres to the tissue, and can’t be properly cleared. In animal studies, exposing this abnormal mucus to bicarbonate-rich solutions restored it to a normal consistency, confirming bicarbonate’s central role.
What Triggers Mucus Production
Your nervous system is one of the primary regulators. The parasympathetic nervous system, the branch responsible for “rest and digest” functions, releases acetylcholine, which activates receptors on airway cells and glands to stimulate mucus secretion. This is why your nose runs when you eat spicy food or step into cold air: your parasympathetic nerves are firing.
Inflammation is the other major driver. When your immune system detects a threat, immune cells release signaling molecules that ramp up mucus output. One key player, IL-13, not only increases mucus secretion from existing goblet cells but also drives the conversion of other airway cells into new goblet cells, expanding the mucus-producing workforce. Another molecule called substance P, released by sensory nerves, promotes both airway muscle contraction and mucus secretion, contributing to the congestion you feel during a respiratory infection.
These neural and immune pathways interact with each other. Airway nerve cells can release signals that activate immune cells, which in turn release IL-13 and other molecules that feed back to the nerves. This loop amplifies the response, which is useful for clearing parasites or pathogens but can become problematic when it stays switched on chronically.
How Mucus Moves After Production
Once secreted onto airway surfaces, mucus doesn’t just sit there. Tiny hair-like structures called cilia beat in coordinated waves, propelling the mucus layer upward from the deepest parts of the lungs toward the throat. This system, often called the mucociliary escalator, moves mucus at speeds of about 4 to 20 millimeters per minute in healthy airways, with some measurements reaching up to 30 millimeters per minute in the trachea.
The efficiency of this system depends on mucus staying properly hydrated. Healthy airways actively monitor and regulate the water content of the mucus layer. When mucus is at a normal concentration (roughly 0.5 to 2 percent solids), clearance works well. As the concentration of solids rises above about 3 percent, the thicker mucus begins to compress the thin liquid layer that cilia need to beat freely. Above 10 percent solids, mucociliary clearance essentially stops. This is not primarily about how much water you drink. Your airways regulate surface hydration through local ion transport, moving salt and water in and out of the surface liquid to keep mucus at the right consistency.
Mucus in the Stomach and Gut
The digestive tract faces a different challenge than the airways. In the stomach, mucus must protect the lining from hydrochloric acid strong enough to break down food. Surface cells produce a mucus layer and also secrete bicarbonate into it, creating a pH gradient. The side facing the acidic stomach contents is highly acidic, while the side touching the cell surface is nearly neutral. This gradient keeps the cells safe even as digestion proceeds inches away.
The colon uses a two-layer mucus system. The inner layer, closest to the cells, is dense and largely free of bacteria. It forms a physical barrier that keeps the trillions of gut microbes at a safe distance from the intestinal lining. The outer layer is looser, and bacteria colonize it freely. This arrangement allows you to host a diverse microbiome while keeping it from directly contacting and inflaming your tissue.
When Mucus Production Goes Wrong
In chronic lung diseases like COPD and asthma, mucus production becomes excessive and difficult to clear. The normal airway lining contains a balanced mix of goblet cells, ciliated cells, and basal cells. Prolonged exposure to cigarette smoke, pollutants, or chronic infection shifts that balance. Goblet cells multiply (a process called hyperplasia), ciliated cells decrease in number, and submucosal glands enlarge. The result is more mucus being produced while the system for clearing it deteriorates.
Inflammation drives much of this remodeling. Neutrophils, a type of white blood cell, flood into inflamed airways and release enzymes that damage cilia, stimulate mucus secretion, and promote the conversion of normal airway cells into goblet cells. Inflammatory signals activate a chain of molecular switches that upregulate mucus gene expression while suppressing the genes that normally keep goblet cell numbers in check. In asthma, the neurotransmitter GABA contributes to long-term goblet cell overgrowth and excess mucus. Another inflammatory molecule, TNF-alpha, triggers a growth factor pathway in airway cells that has been directly linked to increased mucin production in COPD.
The mucus itself also changes in these conditions. It becomes more concentrated, stickier, and harder for cilia to propel. This creates a vicious cycle: thick mucus slows clearance, stagnant mucus attracts bacteria, bacterial infection triggers more inflammation, and inflammation drives further mucus overproduction. Breaking this cycle is a central goal in managing chronic airway disease.