Before you can understand how to treat the things that can go wrong with your sinuses, you have to know what the sinuses are and how healthy sinuses work.

We’re about to embark on a journey through the human anatomy that will be a little like the 1960s movie Fantastic Voyage, except your destination as a miniaturized traveler is the nasal passages instead of the brain, and I’m sorry to have to say that the actress Raquel Welch is nowhere to be found.

I like to think of the sinuses as existing inside a home call it the Nasal House. The house has two big doors your nostrils and they’re always open. Like many houses, the outside doors lead to a foyer. In the Nasal House, the foyer is your nose. Let’s take a look at the interior of your nose.

Your nose is mostly made of cartilage (tough elastic tissue) and bone. The tip (the part you can wiggle if you pinch it with your fingers) is made of cartilage, and the bridge (where a person’s glasses rest) is made of bone. Your nose is divided into two roughly equal-sized halves by a thin partition called the septum.

Made of flexible cartilage in front and bone in back, the septum is typically about three to four inches long. Later, I’ll discuss the consequences of having a septum that’s crooked (a deviated septum) and how this problem can be corrected.

Next on our tour, you see that the back of the foyer opens into a long, narrow hallway the nasal cavity. About halfway down this hallway, you see a series of doors, each of which opens into a different room. The rooms are your sinuses, and there are four pairs of them.

The first doors you see on your left and right lead to the maxillary sinuses, located in your cheeks. A little farther along, you come to the doors to the ethmoid sinuses, located between your eyes. Next, you see two doors leading upward, as if to an attic; those are the doors to the frontal sinuses, in your forehead.

And the last pair of doors you see at the very far end of the hallway lead to the sphenoid sinuses, behind your nose. Figures 1 and 2 show the location of the sinuses.

Unlike most real houses, the doors in the Nasal House are not big and rectangular. Instead, they’re round, like something you’d see in a hobbit’s home in The Lord of the Rings, and very tiny about the size of a pinhole. They are called ostia.

In a healthy person, the ostia usually stay open, allowing air to move in and out freely and mucus to drain from the sinuses. But if there’s trouble such as swelling from infection or allergies the ostia close, and the rooms tend to get stale and stuffy inside.

As you know, your nose serves as a pathway for air to enter and leave the lungs. You breathe in oxygen (along with nitrogen and other gases) and breathe out carbon dioxide. You could do this exclusively through your mouth, but if you did, problems would soon arise: your mouth would get dry, and you would breathe in dust particles floating in the air, which would make you cough a lot.

An amazing internal climate control system in the Nasal House enables you to avoid these problems. The key to climate control is the turbinates, large bones that act as the Nasal House’s equivalent of radiators.

As you stroll down the hallway, you see three pairs of these radiator-like structures along the walls of the nasal cavity: a large pair near the floor (the inferior turbinates), a smaller pair above them (the middle turbinates), and an even smaller pair hanging from the ceiling (the superior turbinates). Figure 2 shows the turbinates.

The inferior turbinates are typically about three inches long, the middle turbinates are half as long, and the superior turbinates are shorter still. If you look into your nostrils with a flashlight in front of a mirror, you can actually see the tips of the inferior turbinates.

They look like pink, shiny mounds along the sides of the nose. The turbinates serve three important roles in preparing air for the lungs: they provide warmth, humidification, and filtration. How does this three-in-one climate control system work?

• Heat. The turbinates function like a household hot-water radiator system, except they use circulating blood instead of water. Each turbinate is completely covered by a spongelike membrane through which blood continuously flows.

So the warm blood heats the incoming air. Interestingly, about every six hours, blood flow increases on one side of the nose and decreases on the other as this spongy lining expands and contracts.

• Humidification. Inhaled air flows over each turbinate’s membrane, which is not only warm but moist. Microscopic droplets of watery fluid and mucus are constantly secreted along the membrane’s surface into the air. If you were to linger in this part of the Nasal House, you would probably say it feels like a sauna.

To get an idea of how much moisture is contained in nasal air, you need only hold a mirror beneath your nostrils and watch it quickly fog up as you exhale.

• Filtration. The membrane’s inherent stickiness from its thin coating of mucus enables it to snare unwanted particles you breathe in and prevent them from traveling to the lungs.

All three of these functions are enhanced by the scroll shape of the turbinates, which greatly enhances their surface area. Taken as a whole, the Nasal House’s climate control system is a model of efficiency. It ensures that the nasal cavity is not just any hallway with stale air; it’s a hallway with precisely treated air, optimally conditioned for the lungs.

The OMC

There is one final critical piece of anatomy in the Nasal House I’ve yet to mention a narrow turnstile in the hallway through which you must pass before you reach the sinus doors. This turnstile is a common point of entry in and out of the sinuses. But it can also serve as a bottleneck.

If the turnstile becomes blocked, nothing can get in or out of the sinuses not air, not mucus. This turnstile has a name: the ostiomeatal complex, or OMC for short. Ostio refers to the small doors, or ostia, of each sinus, and meatal refers to the middle meatus the area adjacent to the middle turbinate, in which the OMC is located.

In the Nasal House, there are actually three meatuses inferior, middle, and superior each located adjacent to the turbinate of the same name. Figure 3 shows the OMC.

The OMC (ostiomeatal complex), shown on the left side of this diagram, serves as a common drainage pathway for the frontal, ethmoid, and maxillary sinuses. The location of the turbinates, which attach to the side walls of the nose, is also shown. Note the proximity of the OMC to the middle turbinate, which can cause sinus obstruction if it becomes enlarged.

It sounds technical, but it’s worth remembering because the OMC plays a critical role in healthy sinus function. Many sinus problems can be traced to obstructions in the OMC, and you’ll be hearing more about it. Now that you’re familiar with the major anatomical structures in the nasal cavity, let’s take a look inside each of the four pairs of sinuses.

• Ethmoid - We’ll start with the ethmoid sinuses which are located behind the bridge of your nose, between your eyes because they are in many ways the most important. You might think of the ethmoid sinuses as the gatekeepers of the Nasal House.

That’s because the frontal and maxillary sinuses first drain through the ethmoid sinuses before reaching the nose. So if your ethmoid sinuses are not draining properly, then your other sinuses are likely to get clogged as well.

The ethmoid sinuses differ from the maxillary, frontal, and sphenoid sinuses in one key respect: they are not single, large chambers.

Instead, each ethmoid sinus comprises five to ten little chambers, separated by very thin-walled bones and lined with mucous membranes (see Figure 2).

Each of these chambers has its own separate small drainage opening into the nasal cavity. Even with so many chambers, the ethmoid sinuses are usually the smallest sinuses. They are about the shape and size of a matchbox.

• Maxillary - The maxillary sinuses are the cheek sinuses. They are located behind your cheekbones, extending from just beneath your eyes to just above your upper teeth.

In fact, the roots of the teeth of the upper jaw often protrude into the floor of the maxillary sinuses, which is why many people with sinus infections have tooth pain.

The maxillary sinuses are usually the first sinuses to develop in the womb. They’re usually triangular in shape and about the size of a large walnut.

• Frontal - The frontal sinuses are the forehead sinuses. They’re located within the frontal bone of your forehead. The back wall of the frontal sinuses actually forms the bone overlying the brain. The size of the frontal sinuses can vary greatly from one person to another.

People with large frontal sinuses generally have large brows. Interestingly, about 10 percent of the population never develops frontal sinuses. We have no idea why, and people without frontal sinuses don’t seem to miss them.

• Sphenoid - The sphenoid sinuses might be considered the deep sinuses. They’re located in the back part of the nose, deep within the skull, positioned in a delicate location where the eyes and brain meet.

Each sphenoid sinus is about the size of a large grape. The carotid artery, a major artery that carries blood to the brain, runs through the outer walls of the sphenoid sinus.

In fact, a surgeon looking inside your sphenoid sinuses sometimes can see the carotid artery pulsating when the overlying bone is thin.

Mucus, Cilia, and Glands

Now it’s time for the final phase of our journey through the doors and right into your sinuses. It’s a jarring sight. Once inside a sinus, you immediately notice a strange, textured wallpaper on all the walls.

It is rough and sticky to the touch, with hundreds of small bumps on the surface that look like chocolate chips breaking through the crust of a cookie. On close inspection, you also observe thousands of tiny hairs that look like fine fiber threads completely covering the wallpaper surface.

The bumps are mucus-secreting glands, and they do exactly what their name suggests. The mucus produced by these glands coats the lining of the entire sinus interior, creating a sticky blanket that traps bacteria and other foreign particles.

If you look closely, you notice that this mucus blanket is actually moving. The movement is due to the tiny hairs, which are called cilia. They beat very quickly, about six times a second, in a genetically predetermined direction designed to sweep the mucus and entrapped debris through the ostia and into the nose.

Cilia normally are effective sweepers. In fact, cilia are so efficient that they can overcome the downward pull of gravity. The ostia in the maxillary sinuses are close to the top of the sinus, so without the cilia sweeping mucus upward, the maxillary sinuses would never drain.

Figure below shows the cilia.

This magnified view of the sinus lining shows microscopic hairlike structures (called cilia) on the membrane surface. The beating motion of these cilia sweeps a blanket of mucus that contains entrapped bacteria and debris out of the sinuses.

The continuous cycle of cilia-driven secretion of mucus is called mucociliary clearance, and it is essential for healthy sinus function. Because of mucociliary clearance, you are constantly swallowing small bits of mucus; if you’re healthy, you usually don’t even notice it.

Healthy sinuses produce about eight ounces of mucus a day. That’s quite a bit. It may be unpleasant to contemplate, but it’s a necessary part of healthy sinus function.

When you’re healthy and hydrated, mucus is thin and watery. When you’re dehydrated, mucus gets thicker. The important thing is to get the right balance of mucus not too little or too much, not too thick or too thin.

Why We Have Sinuses?

In a sense, your sinuses do not even exist. After all, they are just empty spaces in your skull created by the intersection of bony walls surrounding your eyes. Think about it. No medical examiner conducting an autopsy could extract the sinuses from a human body and place them in a jar.

This idiosyncrasy has led anthropologists and physicians to ponder why humans developed sinuses in the first place. The truth is we don’t really know. But there are at least seven theories:

1. Sinuses lighten your load. Having air pockets inside your skull makes your head weigh less than if it were solid bone and tissue. A lighter head may have made it easier for early humans to evolve from walking on all fours to becoming a two-legged, upright creature.

2. Sinuses ease pressure. Sinuses act as a safety valve of sorts when you experience dramatic changes in air pressure within the nasal cavity, such as during sneezing or nose blowing.

Without sinuses, you might not be able to equalize pressure so well. Sneezing might be painful, and you might be more prone to breaking blood vessels, causing nosebleeds.

3. Sinuses improve your ability to taste and smell. Having an expanded surface area where odor molecules in the air can linger may help smell receptors in your nose do a better job. An enhanced sense of smell could have helped alert primitive humans to dangerous situations, such as smoke and fire or predators.

And because an intact sense of smell is critical for perceiving flavor ( just think how you lose smell and taste during a cold), sinuses could have allowed our ancestors to make smarter choices about which foods to eat and which to avoid.

4. Sinuses protect your eyes and brain. The sinuses serve as a trapdoor release mechanism inside your skull, easing the impact of blows to the head and thereby lessening potential damage to the eyes and brain.

A recent illustration came during the 2000 baseball season, when Boston Red Sox pitcher Bryce Florie was hit in the eye by a line drive. Although the bones around his eyes fractured, Florie did not lose his sight.

The thin bone of the roof of the maxillary sinus allowed pressure to be transmitted into the sinus instead of the eyeball. He returned to pitch the next season.

From an evolutionary standpoint, you can see how this function might have benefited a caveman who encountered a club-wielding enemy.

5. Sinuses help your voice. As children learn from talking while pinching their noses, the human voice sounds different and less pleasant if you block the sinuses.

Having sinuses gives your voice tremendous resonance as well as a specific sound different from your neighbor’s (call it a vocal signature). This enhanced vocal quality may have helped early humans communicate better as they gained the power of speech.

6. Sinuses assist in air-conditioning. As we’ve seen, the turbinates act as a treatment center for air as it passes through the nose on the way to the lungs. The mucus they produce filters unwanted particles, and their large surface area helps warm and moisturize cold, dry air.

Because the sinuses provide a large additional supply of warm, moist mucous membranes, their role may be to enhance the airconditioning process.

7. Sinuses enable efficient facial growth. The sinuses may play an important role in the development of our facial skeletons from birth through adolescence. The facial bones must grow in proper proportion to the skull as the brain and the cranial cavity enlarge.

It’s easy to imagine how the creation of hollow sinuses allows the body to expend less energy and fewer calories than it would to grow a facial skeleton of solid bone. The development of sinuses thus allows for more rapid and efficient facial growth.

It’s likely that aspects of several of these theories combine to explain why we have sinuses. In my opinion, the final theory, on efficient facial growth, is one of the most plausible. We’ve now completed our tour of the Nasal House.

With all the strange sights we’ve seen, you’re probably relieved you live in a more conventional residence. But I hope you now have a solid understanding of where your sinuses are and how they work.

So far I’ve mostly focused on what’s happening when everything’s working well. Next, we’ll look at the many ways in which sinuses can malfunction.