The maxillary sinus drains through a small opening called the natural ostium, located high on the inner wall of the sinus, roughly 28 to 34 millimeters above the sinus floor. Because this opening sits near the top rather than the bottom, the maxillary sinus cannot rely on gravity. Instead, it depends on an active transport system: millions of tiny hair-like structures called cilia that sweep mucus upward and out through the ostium in a coordinated wave pattern.
The Two-Layer Mucus System
The inside of the maxillary sinus is lined with the same type of tissue found throughout the nasal passages. Goblet cells in this lining produce mucus that traps bacteria, dust, and other particles. This mucus exists in two layers: a thin, watery layer on the bottom where the cilia can beat freely, and a thicker, stickier gel layer on top that catches debris.
The cilia beat in a coordinated rhythm, moving the gel layer and everything trapped in it toward the ostium at a rate of about 6 millimeters per minute. That’s slow, but it’s continuous. Every part of the sinus lining directs its cilia toward the same exit point, creating a conveyor belt effect that gradually pushes all mucus toward the drainage opening.
The Path From Sinus to Throat
Once mucus reaches the ostium, it follows a specific route through a series of narrow channels on the side wall of the nasal cavity. First, it passes into a groove called the ethmoid infundibulum. From there, it moves through a crescent-shaped gap called the hiatus semilunaris and empties into the middle meatus, a passage beneath one of the bony shelves inside the nose. These structures together form what’s known as the ostiomeatal unit, the shared drainage corridor for the maxillary, frontal, and front ethmoid sinuses.
From the middle meatus, the mucus travels backward toward the nasopharynx (the space behind the nose and above the throat) and is eventually swallowed. Most people swallow about a liter of nasal and sinus mucus per day without noticing it.
Why the Drainage Opening Is Poorly Placed
The natural ostium averages about 6 millimeters wide and 4.6 millimeters tall. It sits near the roof of the sinus rather than the floor, which seems like a design flaw. Researchers believe this reflects an evolutionary holdover: in four-legged ancestors, the ostium would have been positioned at the front and bottom of the sinus, allowing gravity to assist drainage. When humans became upright, the same opening ended up high on the wall.
Studies comparing drainage in upright versus forward-tilted head positions confirm this. Human maxillary sinuses drain significantly better when the head is tilted forward to mimic a quadrupedal posture. In normal upright position, the sinus relies almost entirely on ciliary transport rather than passive flow. This is one reason the maxillary sinus is so prone to infection compared to other sinuses.
What Blocks the Drainage Pathway
Because the entire drainage system funnels through a single small opening and a narrow corridor, even minor swelling or structural variations can cause problems. Several factors commonly obstruct the pathway:
- Swelling from colds or allergies. The ostium is small enough that even modest inflammation of the surrounding tissue can seal it shut, trapping mucus inside the sinus.
- The uncinate process. This thin, curved piece of bone sits just in front of the ostium. If it’s angled too far toward the side wall of the nose, it can physically narrow or block the opening. This anatomical variation is a recognized risk factor for recurrent sinus infections.
- Extra bone air cells. Some people have air-filled pockets in the uncinate process or in the surrounding bones (sometimes called Haller cells or concha bullosa). These take up space in the drainage corridor and crowd the ostium. When one type of extra pneumatization is present, others tend to be as well, compounding the problem.
Accessory Openings and Mucus Recycling
About 10 to 20 percent of healthy people have a second, naturally occurring opening in the maxillary sinus wall called an accessory ostium. In people with chronic sinusitis, that number rises to around 30 percent. Some imaging studies have found accessory openings in as many as 65 percent of sinuses examined, though this varies with the population and imaging method used.
A second opening might seem helpful, but it typically makes drainage worse. Mucus exiting through the natural ostium can loop back into the sinus through the accessory opening, or vice versa, creating a recycling pattern. Instead of clearing, the mucus circulates between the two holes without ever reaching the nasal passage. This recycling effect is thought to contribute to chronic inflammation and persistent infection.
How Sinus Surgery Restores Drainage
When medications can’t resolve a persistent blockage, the most common surgical approach is functional endoscopic sinus surgery. The procedure focuses on restoring the natural drainage pathway rather than creating an artificial one. The surgeon first removes or trims the uncinate process to expose the natural ostium, then widens that opening using small cutting instruments. The enlarged opening is called a maxillary antrostomy.
The goal is to make the natural ostium large enough that it resists closure from swelling and allows topical treatments like nasal sprays to reach inside the sinus. Surgeons are careful to distinguish the natural ostium from any accessory openings. If both exist and aren’t connected, mucus can continue recycling between them. Joining the two into a single larger opening eliminates that loop and restores one-directional flow out of the sinus, through the infundibulum, and into the nasal cavity.