Your sinuses drain through small openings called ostia that connect each sinus cavity to your nasal passages. From there, a thin layer of mucus carrying trapped dust, bacteria, and other particles flows down the back of your throat, where you swallow it without noticing. This process runs continuously, clearing roughly 200 milliliters (about three-quarters of a cup) of nasal mucus every day.
Where Your Sinuses Are
You have four pairs of sinuses, one on each side of your face, each named after the bone it sits in. Your frontal sinuses are in your forehead, just above your eyebrows. Your ethmoid sinuses sit between your eyes, behind the bridge of your nose. Your sphenoid sinuses are deeper in the skull, behind your eyes. And your maxillary sinuses, the largest pair, fill the space in your cheekbones beneath your eyes.
Each of these cavities is lined with tissue that produces mucus and is covered in microscopic hair-like structures called cilia. Together, the mucus and cilia form the system that keeps everything draining.
How Cilia Push Mucus Out
The real engine of sinus drainage isn’t gravity. It’s the cilia. These tiny, hair-like projections beat in coordinated waves at a frequency of 8 to 25 times per second, pushing mucus steadily toward the drainage openings. Each individual cilium performs two distinct motions: a fast, strong forward stroke that shoves mucus in the right direction, and a slower recovery stroke that bends it back into position without dragging mucus backward. Hundreds of cilia on a single cell move in synchronized, wave-like patterns, creating a conveyor belt effect across the sinus lining.
The mucus itself has two layers that make this work. A thinner, watery layer sits close to the surface, letting cilia move freely during their recovery stroke. A thicker, gel-like layer sits on top, trapping particles and pathogens. When cilia reach full extension during their power stroke, they catch the gel layer and push it forward. This two-layer design is what allows the system to move debris in one direction only.
Once mucus reaches the back of the nasal cavity, it flows down into the throat (the pharynx), and you swallow it. Your stomach acid then neutralizes any bacteria or pathogens that were trapped in it. You do this all day long without being aware of it.
The Drainage Pathways
Each sinus empties through its own opening, but several sinuses share common drainage corridors. The most important one is the ostiomeatal complex, a cluster of small channels on each side of your nose. Three of your four sinus pairs drain through it: the frontal, ethmoid (specifically the front portion), and maxillary sinuses. Because so much drainage funnels through this single area, even minor swelling there can back up multiple sinuses at once.
The posterior ethmoid sinuses and the sphenoid sinuses take a different route, emptying into a space called the sphenoethmoidal recess, located higher up in the nasal cavity. This separation matters because infections or blockages in the ostiomeatal complex don’t necessarily affect sphenoid drainage, and vice versa.
The Maxillary Sinus Problem
Your maxillary sinuses have a design quirk that makes them especially prone to problems. The drainage opening sits near the top of the cavity rather than the bottom, which means mucus has to travel upward to exit. Unlike a sink that drains from its lowest point, your maxillary sinus relies almost entirely on ciliary action to push mucus up and out against gravity. When cilia slow down or the opening swells shut, mucus pools at the bottom of this cavity with no easy way out. This is one reason cheek pain and pressure are such common symptoms during sinus infections.
What Blocks Normal Drainage
The ostia openings are small, roughly 1 to 2 millimeters in diameter. It doesn’t take much swelling to seal them off. Several things can cause that swelling or otherwise obstruct the pathways.
- Inflammation from colds or allergies. Viral infections and allergic reactions cause the sinus lining to swell, narrowing or closing the ostia. This is the most common cause of acute sinus congestion.
- Nasal polyps. These soft, painless growths develop on the sinus lining, often driven by chronic sinusitis, asthma, or hay fever. When they grow large enough, they physically block the nasal passages and sinus openings, leading to repeated infections and breathing difficulty.
- Thickened mucus. Dehydration, dry air, and certain medications can make mucus too thick for cilia to move efficiently. The gel layer becomes sticky and heavy, overwhelming the cilia’s ability to push it along.
- Damaged cilia. Cigarette smoke, chronic infections, and some genetic conditions (like cystic fibrosis) impair ciliary function. When cilia slow down or stop beating effectively, mucus stagnates even if the openings are clear.
- Structural issues. A deviated septum or unusually narrow passages can crowd the ostiomeatal complex and restrict flow.
How to Help Your Sinuses Drain
Because the system depends on both open pathways and functional cilia, the most effective strategies target one or both of those factors.
Staying well hydrated keeps mucus thin enough for cilia to move it. Humid air serves the same purpose: a humidifier or steam from a hot shower can loosen thick secretions and get them flowing again. Saline nasal rinses physically flush mucus and irritants out of the nasal passages, temporarily doing the job that sluggish cilia can’t.
Positioning also plays a role, especially at night. Sleeping with your head slightly elevated, using an extra pillow or a wedge under the head of your mattress, helps prevent mucus from pooling at the back of your throat. This is particularly useful if you deal with post-nasal drip that worsens when you lie flat.
For swelling that closes off the ostia, nasal decongestant sprays can shrink the tissue and reopen drainage temporarily. Steroid nasal sprays work more gradually but address the underlying inflammation, making them a better option for ongoing congestion from allergies or chronic sinusitis. When polyps or structural problems cause persistent blockages that don’t respond to other treatments, a procedure called functional endoscopic sinus surgery can remove polyps, damaged tissue, and bone to widen the drainage pathways.
Signs Your Sinuses Aren’t Draining Well
When drainage slows or stops, mucus accumulates and creates the familiar feeling of sinus pressure: fullness or pain across the forehead, between the eyes, or in the cheeks. You might notice thick, discolored nasal discharge, a reduced sense of smell, or a persistent drip down the back of your throat. Bad breath can develop because stagnant mucus becomes a breeding ground for bacteria.
If pressure and discolored discharge last more than 10 days, or if symptoms improve and then suddenly worsen, that pattern often signals a bacterial infection that has taken hold in the stalled mucus. Congestion that recurs frequently or never fully resolves may point to polyps, allergies, or structural problems keeping the drainage pathways chronically narrowed.