Subtitles section Play video Print subtitles You know how it feels — that crushing headache, a building pressure in your face, and thick mucus dripping down the back of your throat. Sinusitis, or sinus infections, are incredibly common and no fun whatsoever. Anywhere from thirty to thirty-five million sinus infection diagnoses are made every year in the US. They're one of those sicknesses that usually go away on their own, but they're a nuisance nonetheless. And in fact, our tendency to get sinus infections might actually be a quirk of human evolution. There's dust, pollution, and pathogens like viruses and bacteria in every breath we take, and one tool that our body uses to keep us healthy is our nasal cavities. Within the fleshy nose itself, hair and mucus catch big dust particles, which is a good first line of defense for keeping our lungs clean. From there, our breath enters a series of air-filled chambers behind the facial bones called the paranasal sinuses. Paranasal for around the nose, sinus for empty space. Their first job is to warm and humidify air, making it easier to breathe when it gets down to the lungs. The next line of defense is mucus — and it's totally normal. We're all secreting at least a little mucus all the time. Each of our sinuses are lined with cells that can make more or less mucus depending on the situation. They're also coated in cells that have little hair-like structures called cilia. These things wiggle back and forth to keep this mucus moving down collection ducts in each of our sinuses where they drain into the nasal cavity, then into the throat. This becomes more complicated when you get a sinus infection. Which is exactly what it sounds like: infection from pathogens or allergens that stick around in your sinuses, like from a cold or flu, which then causes inflammation of these mucus membranes. Most of the time, these pipes and chambers flow freely, and we can take big clear breaths. But when sinuses have to deal with germs, it results in clogged up noses and difficulty breathing. Viruses cause the majority of sinus infections, but bacteria can trigger them as well. When you do get a sinus infection, that wet mucus becomes thicker and more viscous, which causes the typical symptoms: congestion, headache, and difficulty breathing. And those symptoms might be made worse by the plumbing of our sinuses. Each sinus has a mucus collection duct, or what are called ostia, to shuttle its discarded mucous to the nose and throat. It's basically plumbing for snot. Well, for most of our sinuses like the frontal sinuses behind the forehead or the sphenoid sinuses a few centimeters behind our eyes, the collection duct is towards the bottom of the sinus. The ethmoid sinus has a bunch of tiny cavities that it drains out of, but it still drains downward. The big exception then is the maxillary sinuses: the ones behind our maxillas, or upper lip and cheeks area. These are our biggest sinuses, at about 15 milliliters, and the ones that most often get infected. And for most humans, we have one collection duct towards the top innermost wall of the maxillary sinus. Having the collection duct at the top of the sinus might keep us from draining all that collected snot more efficiently. Which, in turn, might predispose humans to sinus infections. On one hand, if you have a sinus infection, your cilia have to wick a bunch of thick, viscous mucus towards the collection duct against gravity, and that mechanism tends not to work as well when we have an infection. So we end up with more and more bacteria which creates infinite snot and makes symptoms worse. On the other hand, that duct might act like an overflow drain. That's why it sometimes helps to lie down or bend forward when you have a sinus infection: it helps drain mucus towards the ducts. Obviously, that doesn't remove the infection, but it might help for a little bit. Ultimately, the location of the maxillary ostia is a consequence of evolving from ancestors that walked around on four legs. Asking whether our bodies would work better if it had different anatomy is a hard question to test scientifically, so researchers might compare our anatomy to a different animal's. Like in the past decade, scientists published a paper where they filled human sinuses and goat sinuses with a saline solution and measured how quickly they drained. In an upright position, the human sinuses didn't drain that well, but when they were tilted forward, they drained as quickly as the goats' did. On a practical level, different scientists also showed that moving into the quadrupedal head position, essentially just leaning forward, does relieve sinusitis symptoms. That makes them think that the location of the sinus ostia was appropriate for our four-legged ancestors, but not for modern humans on two legs. But, it's also worth remembering that everyone has slightly different nasal anatomy which might predispose some people to sinus infections more than others. While our sinuses might be designed less than ideally, having a big brain was more important, so we're left with the nasty, snotty consequences of evolution. While we can't go back in time to say if this was a good or bad design, our sinuses are something that make us human. Thanks for watching this episode of Seeker Human. We're so excited to get more episodes of Human to you just a little bit, so make sure you're subscribed on YouTube and following us on social media.