Sinus drainage comes from four pairs of hollow cavities in the bones of your face and skull, each lined with tissue that constantly produces mucus. This mucus exits through small openings called ostia, flows into your nasal passages, and typically slides down the back of your throat without you noticing. You produce and swallow roughly 30 milliliters of airway mucus every day. When that system gets disrupted by allergies, infection, or inflammation, the drainage becomes noticeable.
The Four Sinus Cavities
Your sinuses are air-filled pockets carved into the bones surrounding your nose. Each pair sits in a different location and drains through its own pathway:
- Maxillary sinuses are the largest, sitting beneath your eyes in your cheekbones. Their drainage opening is located high on the inner wall, which means mucus has to travel upward before it can exit. That’s why these sinuses are especially prone to congestion.
- Frontal sinuses sit above your eyes in your forehead bone. They drain downward through a narrow funnel-shaped passage in the sinus floor.
- Ethmoid sinuses are a cluster of small air cells between your nose and eyes. The front cells drain into the middle part of the nasal passage, while the rear cells drain higher up.
- Sphenoid sinuses are the deepest, tucked behind your nose near the center of your skull. They drain into a small groove high in the back of the nasal cavity.
How Mucus Is Made
The entire inner surface of your sinuses is lined with a moist membrane containing specialized cells called goblet cells. These cells continuously secrete mucus, forming a sticky barrier that traps dust, bacteria, viruses, and other particles before they can reach deeper into your airways. The mucus itself is about 95% water. The remaining fraction contains proteins that actively fight infection, including antibodies, enzymes, and antimicrobial peptides. It also contains salt, lipids, and a type of large protein called mucin that gives mucus its gel-like consistency.
Goblet cells can ramp up production in response to threats. They carry receptors that detect pathogens, and when triggered, they increase mucus output to help flush invaders out. This is why your nose runs when you’re sick or exposed to an allergen.
How Mucus Moves Through Your Sinuses
Mucus doesn’t just drip out of your sinuses by gravity. Millions of microscopic hair-like structures called cilia line the sinus walls and beat in coordinated waves, pushing mucus toward the drainage openings. These cilia beat roughly 12 times per second, creating a slow but steady conveyor belt that moves mucus at about 4 to 5 millimeters per minute.
This system, called mucociliary clearance, is remarkably organized. In the maxillary sinuses, for example, cilia become more densely packed near the drainage opening to help push mucus through the bottleneck. Once mucus exits through the ostia, it enters the nasal cavity and merges with drainage from the other sinuses.
Where It All Converges
Inside your nose, three curved shelves of bone called turbinates create channels along the nasal wall. Most of your sinus drainage converges in the channel beneath the middle turbinate, known as the middle meatus. This is where the maxillary, frontal, and front ethmoid sinuses all empty. The area where these pathways meet is called the osteomeatal complex, and it’s the most common site of blockage during sinus infections.
The sphenoid and rear ethmoid sinuses take a different route, draining into a groove higher up in the nasal cavity near the top of the nose. From there, all the mucus travels backward toward the throat.
The Trip Down Your Throat
Under normal conditions, most sinus drainage flows from the nasal cavity into the nasopharynx, the space behind your nose and above your soft palate. From there, you swallow it unconsciously throughout the day. Your stomach acid neutralizes the bacteria and debris the mucus collected.
This process is called postnasal drip, and everyone has it all the time. It only becomes a noticeable symptom when the volume increases or the mucus thickens. When drainage becomes excessive, you may feel it pooling or trickling at the back of your throat, sometimes triggering a cough, a sensation of something stuck in your throat, or a need to constantly clear your throat.
What Makes Drainage Increase
Several things can shift your sinuses from quiet background drainage to the kind you actually feel:
Infections. A cold virus or bacterial sinus infection inflames the sinus lining, causing it to swell and produce more mucus. The mucus also thickens as your body floods it with white blood cells to fight the infection. That thicker mucus is harder for cilia to move, creating a cycle where congestion builds and triggers even more mucus production.
Allergies. When your immune system overreacts to pollen, dust, or pet dander, it releases histamine. Histamine causes the nasal lining to swell and the glands to pump out extra mucus. Allergic drainage tends to stay thin and watery, unlike the thicker mucus of an infection.
Blocked ostia. If inflammation, a polyp, or swelling narrows or blocks one of those small drainage openings, mucus backs up inside the sinus. This creates pressure and pain and can set the stage for a bacterial infection in the trapped fluid.
What the Color Tells You
Clear drainage is normal. Your sinuses produce clear mucus around the clock, and it stays clear during allergic reactions too.
Yellow mucus typically appears when your body is fighting something. The color comes from white blood cells that have arrived at the site of infection and been swept into the mucus stream after doing their work.
Green mucus means your immune response has intensified. The green tint comes from a high concentration of dead white blood cells and the enzymes they release. If green drainage persists beyond 10 to 12 days, it may indicate a bacterial sinus infection rather than a simple cold.
Color alone isn’t a reliable way to distinguish a viral cold from a bacterial infection. Yellow and green mucus appear in both. The more useful signal is how long symptoms have lasted and whether they’re getting worse after initially improving.