What do your Sinuses do

The sinuses are usually written off as “four pairs of air-filled chambers in the facial bones that connect to the nose”, but for the 35 million who suffer from chronic sinus problems in the U.S. alone, or the 500,000 who will have sinus surgery in the next year, a better understanding of these cavities is surely needed.

The sinuses are not simple “chambers in your face” that cause some people grief; they’re actually important producers of something your nose needs in great quantities; mucous.


 Everything in your respiratory system (nose and sinuses included) is covered with mucous membrane, the internal skin that also lines your stomach, intestines, and urinary tract. In your nasal system (also called your “upper respiratory tract” or “UR” for short) these tissues have the important job of producing the quart-a-day of mucous your nose uses.

You nose’s main job (besides smelling) is to condition the air you breathe by warming, humidifying, and cleansing it. Mucous is crucial for these tasks, because it traps impurities and washes them away, absorbs and radiates heat from the blood vessels in the tissue below, and it’s moisture is the very thing that humidifies. Mucous is also rich in antibodies, immune cells, and antibacterial proteins which help defend against infections and allergens.

Your sinuses produce a lot of the mucous your nose needs. These “air filled chambers” are lined with very productive membranes that continuously into your nasal cavities with this essential fluid.

 The importance of these chambers for nasal function was clearly demonstrated by the side effects of sinus operations performed in the early and mid 1900’s called the “Caldwell-Luc procedure”. At that time doctors sought to improve sinus drainage by creating new drainage paths, and sometimes stripped the mucous membrane from the sinuses to reduce fluid production.

 Sometimes it worked, but more often patients were left with chronically dry noses, “nasal crusting” (meaning their mucous dried in their noses), irritated nasal linings, and caused more frequent colds and infections. Disrupting the mucous flow from the sinuses disrupted the function of the entire nose.

The nose is a complicated place, and throwing one element out of whack can cause a lot of problems. To understand how the sinuses fit into the picture, you need to get…


Your nasal passages are like a complicated cave system that occupies a big space in your head. The main chambers just inside your nostrils rise up to your eyes and slope back to your throat (almost back to your ears), and is roughly as wide as the outline of your nose.

 Branching off from these cavities are four sinus chambers on each side;

 –         one in each cheekbone (called the “maxillary” sinuses, the biggest with a whopping capacity of 15 ml – three teaspoons – each);

–          another behind each eyebrow (called the “frontal” sinuses, with a capacity of a teaspoon-and-a-half or so);

–         a collection of sub-chambers behind the bridge of the nose called the “ethmoids” (a teaspoon and a half each);

–         a pair behind the ethmoids called the “sphenoids” (almost between your ears, each with around a teaspoon-and-a-half capacity).

These small chambers are well-protected by arches of tough facial bone, but the tough exterior belies a softer backside; the bone of the back walls ranges from cracker-thin (in the maxillaries) to paper-thin (the walls separating your sphenoids from your brain).


Your sinus chambers connect to your nasal cavities through “ostia” (“ostium” in the singular), which are 3-4 mm openings in the bony walls (roughly the size of  a grape seed). Each sinus has only one ostium, and anything that gets in or out of the sinus has to pass through it.

You’d think these drain holes would be at the bottom, like bathtubs and sinks, and your frontal sinuses (behind the eyebrows) and the ethmoid sinuses (between your eyes) are arranged this way. But the pair deeper in your head (the sphenoids) drain forward toward your face, while your biggest sinuses, the maxillaries in your cheekbones, drain from the TOP!

How does a sinus “drain” from the top? You might as well ask how a sinus drains at all. Normal mucous is sticky, and if you put a teaspoon in a cup with a grape-seed sized hole at the bottom it would barely drain at all. The fluid would dry to a crust before a half-dozen drops dripped out.

Your sinuses, however, do not simply “drain”. Rather, they have hundreds of thousands of tiny structures that act like oars to paddle the fluid from them.


Your mucous membranes bristle with microscopic hairlike “cilia” that wave like whips 10-15 times a second, working together to sweep mucous in a single direction. The cilia number 50-200 per CELL, and in your nasal passages they sweep toward your throat where the goo usually dribbles away unnoticed. Your sinuses are richly lined with cilia that were programmed at birth to sweep mucous toward the ostium and have been doing so 24/7 ever since.

The importance of the cilia was unknown for a very long time, and for good reason; there’s no way to observe what happens in living sinuses because they’re sealed in bone. But in the 1950’s a German doctor named Walter Messerklinger discovered the function of cilia in a somewhat ghoulish manner.

Messerklinger had an arrangement with a medical school to lend him the heads of the recently deceased, and he would pick up these “packages” and take them to his lab. There, he would dissect the sinuses and examine them with state-of-the-art optics (Germany was considered the king of cameras and lenses at the time). One day he noticed a foreign substance-some legends say it was a bit of cigarette ash, other say it was a drop of blood- moving in a sinus, and he took a closer look.

Messerklinger discovered that cilia were active up to 48 hours after death, but more amazingly he made time-lapse photographs that show how the cilia only move sinus mucous toward the birth ostium.

This discovery didn’t shake the medical community, and for years it was considered just another piece in the puzzle. But it led to development of new surgical treatments for chronic sinus problems that enlarged the existing sinus openings to improve drainage without creating new holes or removing mucous membrane. These techniques came to be called Functional Endoscopic Sinus Surgery (“FESS” for short). FESS has been shown to be far more effective than older, more destructive techniques, with far fewer unpleasant side effects.


Many sources say the sinuses serve to “warm, moisten, and filter the air you breathe”, but this is like saying your faucet “washes and hydrates your body”. Your sinuses produce a lot of the mucous that does the warming/filtering/humidifying, but they are only a source of the liquid and don’t do the job themselves. (The tiny ostia allow so little air to enter that the sinuses can help sense changes in air pressure and air flow in the nose, but that’s about it.)

Some have suggested that sinuses are weak spots that will crunch in to absorb severe impacts and reduce trauma to the brain. This “crumple zone” theory is ridiculous because the sinuses are behind arches of facial bone, and the arch is one of the strongest structures. The sinuses are clearly intended to withstand impacts, and if they fail only a very fragile layer is left protecting your brain. An impact that “crumples” the sinus will very likely destroy the underlying barrier and damage the brain.

Others suggest they serve to lighten the skull and help humans to walk upright. But almost everything that ever had a nose (even the dinosaurs) had sinuses. Pigs, cows, and horses (which all spend much of their life with their face to the ground) have well-developed sinus cavities. They don’t need them to lighten their faces, and yet… there they are with sinuses.

Some suggest sinuses provide a unique timbre and tone to the human voice so other humans can recognize it, and it’s true your facial bones create many of your voice’s qualities. Singers and radio personalities are often cautioned about sinus and plastic surgery because it may change their voices a bit. But these structures are not that important to voice recognition. Small children and pets can recognize even lousy recordings of a family member’s voice.

Still others claim the sinuses are vestigial organs like the appendix, remnants of bygone eras when they served some purpose we no longer need. But the dramatic failure of early sinus surgery shows they still play an important function in the nose today.

And then again, these may all be true. Sinuses in birds and some dinosaurs tend to be large open cavities that play an enormous roll in the quality of their voices. Early humans were small and light creatures who may not have had the musculature to hold their heads upright for long periods of time. And animals like hogs and horses may have needed structures in their faces to deflect or absorb impacts while running through wooded areas.

And we may have needed these traits in the past, but are slowly evolving away from them. For now, however, we know we need the mucous they produce, and this function is clear.

This is Part 1 of a series of articles on sinus issues. Please click on my name at the top of the article and check out the rest of the series, “What is Sinusitis” and “Tips on Relieving Sinus Pressure”, and others for more information.