(Intro)

Olivia: The human body is a remarkable machine that can do all sorts of amazing things.  We can dive tens of meters into the ocean on a single breath or climb thousands of meters towards the sky to summit the tallest peaks, but sometimes, our bodies also do stuff that's pretty weird, gross, or just plain annoying.  

Over the years, we here at SciShow have talked about a lot of these other, everyday things that our bodies do, and apparently, our Patrons are big fans of these episodes.  When we asked them to pick a compilation topic, they voted for us to smush a bunch of 'Why do we...?' episodes into one, so here we go.  

First, you've probably laughed at some point today, and if not, well, hopefully you will in the next 15 minutes or so, at least we'll try to make it happen.  Anyhow, laughing is just one of those things all people do.  We laugh when we're amused and when we're anxious, but why?  What's the point of laughing?  Let's let Hank explain. 

Hank: You know what's like super funny? Laughter. Laughter is a physiological response that involves at least 15 facial muscles, the respiratory system, the brain's limbic system, and--if the joke is really, really good--even your tear ducts.

But laughter doesn't always indicate happy times; it can be a sign that something is seriously wrong with your brain. Gelastic seizures, or uncontrollable laughter or crying, can indicate the presence of brain tumors or other conditions like pseudobulbar affect, a neurological condition that can effect stroke and brain trauma survivors and MS patients. These conditions are sometimes called emotional incontinence because the sufferer can't control the outbursts, which often have no root in have they're felling.

And how we feel, especially in groups, seems to be what laughing is all about. Some laugh researchers--and, yes, they are things known as gelotologists--think that much of our laughter is rooted in strengthening social bonds. We're much more likely to laugh in a group than we are alone, and we tend to laugh more easily around friends and family. That shared experience brings us closer, makes us feel part of the group. We also laugh to express relief or to ease our nerves in stressful moments.

Researchers theorize that there are a few specific reasons for laughing. First, there's the incongruity theory which maintains that it's the element of surprise that triggers laughter, whether it's an unexpected punchline or your friend tripping over a throw rug. So like say you're watching people walk through a room all day; your brain registers this a predictable and boring behavior. Then, your friend walks in, trips on a rug, and drops a box of ping-pong balls. When you hear that your friend hasn't say, fallen on a bag of rusty knifes, you'll find the fall hilarious because it was sudden and unexpected and incongruous to the sting of people safely walking by, and because ping-pong balls. Babies and little kids go for this kind of laughter a lot; they think think really simple, unexpected things my ah, my face is here, it's there for five consecutive hours it's still funny. You're like using a banana as a telephone.

Now, if you're the one who tripped on the throw rug, you're much more likely to laugh in surprise if you see your friend laughing with you. This goes back to that shared laughter as a social bond thing. You're probably feeling pretty embarrassed and tense, but you're also relieved that you aren't hurt. That's where the relief theory comes in.

Laughing is like a mental mini break. Your brain is constantly working, taking in all sorts of information and ordering around the body; sometimes it just needs a happy surprise. This is particularly handy is stressful moments. The whole Hollywood wise-crack in the middle of a suspenseful scene phenomenon is predicated on this idea. When Han Solo is in mortal danger, he cracks a joke to lighten the mood; he needs it, Chewie needs it, the audience at the edge of their seats need it.

Humor helps us cope with stressful situations; it sort of recharges our brains to face the task at hand. Scientists call this releasing cognitive energy; the rest of us call it comic relief.

But now back to you tripping on that throw rug. If everyone in the room is laughing at you and none of them are your friends, they may be proving the superiority theory of laughter. It means that they're laughing at your misfortune and probably means that they're kind of a bunch of jerk faces. Superiority laughter still promotes bonding, in an us vs. them sort of way. It doesn't show much goodwill though.

Teenagers make fun of their parents, and, you know, each other, and pretty much everyone, and so do lots of adults as well. But our teen years are usually awkward and confusing, and superiority laughter might help ease some of that pain.

So, simply put, we laugh hardest at what we know best and what stresses us out the most. And maybe that's why it's great for you both physically and emotionally. It reduces the release of stress hormones that jack those flight-or-flight feelings, it lowers blood pressure and oxygenates your blood flow, it even increases your T-cell levels that help your immune response and B-cells that produce antibodies. Also laughing a hundred times is estimated to burn as many calories as a 15-minute bike ride. So, you see: laughter is not the best medicine, but it's not a bad medicine either.

Olivia: Wow, there is a lot more to laughter than I thought, but of course, laughter isn't the only way we express emotions physiologically.  For example, when we're embarrassed, our cheeks turn bright red whether we want them to or not, but other animals don't do this.  So why do we?  Here's Michael with some ideas.

Michael: In every culture and every ethnicity around the world, people blush, but animals don't. And it's not just that most animals' faces are covered in fur and feathers and stuff so we can't see them blushing -- they actually don't blush.

Charles Darwin called blushing the most peculiar and most human of all expressions, which is awesome, but why do we blush?

Physiologically, we understand it. I mean, we get the mechanics of why your face turns red when Bernice sneaks up behind you and pulls your pants down in front of the whole rest of the marching band. Bernice!

Anyway, your face turns red because your sympathetic nervous system kicks in. That's the network of nerves that controls your fight or flight response. When it's triggered, it signals the release of adrenaline. Suddenly, your heart rate picks up, and you start breathing faster, and then you're ready to run away. Your pupils dilate, blood rushes to your brain so you can take in as much information as possible, and your blood vessels dilate in a process called vasodilation to improve oxygen flow.

It's basically the same effect you get from warming up before a workout. But in the very hypothetical instance of my pants being pulled down, the blood vessels in my face were responding specifically to a chemical transmitter called adenylyl cyclase. It basically tells the blood vessels in your face to let the adrenaline in.

The weird thing is that these same superficial blood vessels in your face aren't usually affected by sympathetic responses. I mean we don't blush when we're scared. If we did, that might be a sign that blushing served some sort of survival purpose. But it only happens when we're not actually in danger. So, like, why?

Well, some scientists believe that blushing evolved as a social survival trait. When you blush, then the person who's angry at you can see that you're really sorry. The person who's laughing at you can see that you're visibly embarrassed. If that sounds weird, just think about the last time your dog chewed on the furniture, then rolled onto its back when you got mad. Lots of animals have evolved this sort of behavior so they can say, "Hey, whoa, I messed up, I feel terrible."

And it's really hard to stay mad at your dog when it does that. That's how you know it's working. We may never know for sure why blushing became a thing, but it could just be our way of saying "sorry for chewing on the furniture."

Olivia: Huh.  I never thought of it that way, but blushing as a social signal does make sense, especially when you consider how often our bodies involuntarily do things that are considered offensive, like we burp and fart a lot and while I get that we all have gas, why do we have to expel so much of it all the time?  Here is Hank to explain.

Hank: Formally referred to as belching and flatulence, these two often lampooned bodily functions actually share a common cause: swallowed air.

Every time you swallow you take in some air, but you can take in even more air than normal if you eat or drink too quickly, chew gum, smoke, drink carbonated beverages, or even wear loose fitting dentures. Now, the majority of swallowed air comes back up in the form of burps. As the air builds up in the upper portion of the stomach, it causes stretching that eventually triggers the lower esophageal sphincter to relax. The result is air escaping up the esophagus, into the mouth, where the sound and smell of the burp depends on how much Coke you just chugged.

Babies, in particular, are big burp-ers, especially for their small size. Mostly because they tend to gulp in too much air while nursing. And because young digestive systems haven't developed to the point where babies can easily burp on their own, it's up to mom and dad to pat their backs, help get those gas bubbles out. But for babies and adults, what about the air that isn't burped out?

Well, it passes from the stomach into the small intestine, and later the large intestine. Along the way, as it passes through the gastrointestinal tract that air mixes with the product of some good old bacteria fermentation that's going on in your guts. That fermentation is the result of carbohydrates, like sugars and starches, that can't be digested by enzymes in the small intestines. Some foods that include these hard to digest starches include: cabbage, cauliflower, beans, and bran which explains why you may feel excessively gassy after that big bowl of chili.

When those undigested carbs reach the colon and the lower intestines, bacteria take over. The by-product is a combination of gases that include: carbon dioxide, hydrogen, methane, even hydrogen sulfide, the stinky one. That mix of gases, plus the air that you've swallowed is what makes its way through the large intestine, to the anus, where it is expelled.

Of course, everyone has different amounts of various species of bacteria and yeasts working away in their guts, but I'm sure you're dying to know, so I'll just tell you. The average human toot contains roughly 59% nitrogen, 21% hydrogen, 9% CO2, 7% methane, and 4% oxygen. It's that last 1% in the form of hydrogen sulfide and other sulfur compounds that is the most potent in terms of producing those notorious, unpleasant, rotten egg smells.

While some of these chemicals are produced by bacteria in your gut, eating sulfur-rich foods, like eggs and onions and beans... That doesn't help on the smelly front.

Olivia: Well, now I know more than I ever needed to about the composition of farts.  Thanks, hank.  While we're on the subject of expelling air though, sneezing is another kind of annoying thing our bodies do, and the reason might seem obvious, but expelling pathogens from our noses is only a part of the answer to why we sneeze, as Michael explains.

Michael:  Everyone, everywhere occasionally sneezes. It’s something humans—and even some animals like cats, and dogs, and chickens —just do. But why? Why do we sneeze? If your answer is that it’s our nose’s way of getting rid of irritants and excess mucus, you’re right -- but only partly.

See, scientists have long thought that sneezing -- technically known as sternutation -- is a reflex. When irritants -- like dust, dander, germs, or pollen -- get into your nose, your brain sends out a signal to get rid of it. That same signal goes out if an excessive amount of mucus is hanging out in there too -- say, if you have a cold. This signal triggers a deep breath, which you hold in your lungs for a moment. While you’re holding your breath, your chest muscles clench and pressure builds. Your tongue is forced to the roof of your mouth... and you breathe out -- fast -- through your nose in the form of a sneeze. That’s the old, short, and still correct answer to the question “Why do we sneeze?”

But according to a paper published in 2012 in the journal FASEB, there’s a little more to it than that. And it all has to do with cilia, the tiny, hairlike paddles that line our noses and sinuses. In the study, a group of researchers from the University of Pennsylvania basically grew a tiny nose. They took cells from the nostrils of several healthy adults and grew them in an incubator for a few weeks, until the cells formed the same type of lining that's in your sinus, complete with cilia.

Then, to mimic a sneeze, the scientists puffed air on the lining. They noticed that the burst of air triggered the cilia—which look sort of like shaggy dog hair under a microscope—to kick into high gear, moving back and forth repeatedly for up to several minutes after the trick sneeze. So, why were they so active for so long? Any potential irritants would have been cleared out already. Well, the triggered cilia were acting as a broom, basically resetting the entire nasal environment, not just the parts where there’d been irritating gunk.

Just like computers do, biologists think that our nose needs a reboot every once in a while... and its kinda-furry restart button is made of all those cilia. But it turns out that not everybody’s sneezes actually reboot their nose. In addition to looking at healthy folks’ cells, this same group of scientists took a peek at the cells of people suffering from sinusitis, which causes inflamed sinuses and general nasal discomfort—runny nose, nasal congestion, all that fun stuff. They discovered that when they puffed air onto the tissue of the sinusitis sufferers, the cilia didn’t beat faster. Meaning that chronic sinusitis might have to do with cilia that can’t properly reset post-sneeze -- and that knowledge might help researchers developing treatments for it.

Olivia: Well, I hope my cilia stays healthy so my nose can stay clean with some occasional cellular sweeping, though all this talk about irritants and nasal cells wriggling is making my nose itch and actually, that's another odd thing our bodies do: itch.  Even when we just think about things.  Here's Michael again to explain why that is.

Michael: Itching. Just thinking about it is making me kinda itchy. An itch can be caused by all kinds of things. The usual culprits are things like bug bites, poison ivy, or strands of hair brushing against your skin. But it's also brought on by skin conditions like eczema or psoriasis as well as seemingly unrelated things like brain tumors, diabetes, and chemotherapy. With such a wide range of causes the itch seems to be kind of mysterious. And it is.

But in a 2013 study published in the journal Science, two researchers from the National Institutes of Health discovered the physiology of how itching works, which may eventually help chronic itchers find relief. Scientists used to think that an itch was basically a low-level pain signal, meaning that one kind of nerve cell detected both itching and pain. But the study found that itching is actually linked to a set of special neurons that produce a neuropeptide called NPPB.

The scientists genetically engineered mice so that they wouldn't produce NPPB, then exposed the mice to chemicals known to produce itching. These mice could still feel their bodies, heat, pain, and touch, but they didn't scratch. No NPPB, no itch. That's good news for people who want to stop itching, because the more we understand about how itching works, the closer we can get to be able to control it.

Unfortunately, it's not as simple as just blocking this neuropeptide, because NPPB has another job, too. It's released by the heart to regulate the amount of sodium released by the kidneys, which helps control blood pressure. But for most people, blocking itching wouldn't be a good idea anyway. The itch gets a bad rap, but it's a useful evolutionary development. Scratching an itchy spot could remove a pesky bug, a dangerous plant, or some kind of other irritant before they do damage to your body.

With all this talk of itching, you might be starting to feel pretty itchy by now. I know I am. Just thinking about itches can make you feel like spiders are crawling all over you. Researchers gave a lecture about itching to an unsuspecting crowd to study their response, and the audience got to scratching with both verbal and visual itch-cues. And it's not just us. Researchers have also studied the contagious effect of itching on monkeys. And, like a yawn being passed back and forth, the monkeys watched another scratch and mirrored the behavior. It might be annoying, but there's also an evolutionary benefit to itching being contagious: when one member of your group is itchy, that might mean they've been exposed to a parasite or there's some kind of irritant around. Which could mean that you've been exposed, too, so it's worth getting a head start on the scratching, just in case.

Olivia: And now I'm feeling itchy all over and since he mentioned yawning, I'm strangely tired, too.  It's so weird that we yawn just because we hear or see someone else do it, but why do we yawn in the first place?  Here's Hank with some insights.

Hank: Here's a question people are always wondering about: why do people yawn? We do it every day, especially when we're in a boring meeting or we didn't get enough sleep. So it may not surprise you that we don't really understand all the reasons behind why we yawn, and it's not just humans, birds do it; jaguars do it; even fetuses in the womb do it. Oscitation, the act of yawning, happens with almost all vertebrates.

The explanation you may be familiar with is that a great big gaping yawn is the brains way of ordering you to take a deep breath. Supposedly it gets rid of extra carbon dioxide and brings in more oxygen. The problem with that is that there is absolutely no truth that yawning really affects your oxygen levels. Instead, recent studies show that yawning is your brain's natural air conditioning system, bringing cooler blood to chill out your overheated, overtired grey matter. So your brain actually heats up as it uses some 40% of your body's metabolic energy. Just like your laptop gets warm up when it's been playing video games for hours.

Body temperature also rises and falls as part of your circadian rhythm, reaching its highest point just before we fall asleep. Which is why a series of yawns at 1am is your body's way of telling you to stop going on Reddit and just go to bed. So just like your computer has a fan to cool things down, a yawn does the same thing to your brain, allowing you to continue processing information effectively.

How's that work? Well, since you asked, yawning has two parts. First, stretching your jaws when you yawn increases the rate of blood flow to your skull. Second, the inhalation sends a gulp of air into your upper nasal and oral cavities, which have mucus membranes covered with tons of blood vessels that project up into the forebrain. This is the big part of the brain responsible for receiving and processing sensory information, producing and understanding language, and controlling motor functions, many of the things involved in actually being you. So the cool, refreshing air entering your sinuses changes the temperature of the blood that's now hustling up to the brain, making you more alert, and helping you walk and talk and think for effectively.

Don't believe us? Scientist's studied the brain temperatures of mice, observing increased temperatures before yawning and measuring a dip in temperature afterward. And a more simple experiment on humans asked volunteers to hold hot or cold packets to their heads to see if changing the temperature of their brain cases triggered yawning. Turns out that the hot headed participants yawned 41% of the time, while those who chilled out yawned only 9% of the time. So next time you can't stop yawning as you're falling asleep in a sweltering classroom in the middle of the afternoon, remember that it's literally how your brain keeps its cool.

Olivia: I guess my brain really needs to cool down right now, because I (yawn) just can't seem to stop yawning, so I'm gonna head out, but before I go, I want to say thank you to all of our Patrons who voted for this compilation topic.  We hope you enjoyed the episode.  If you want to support what we do here at SciShow and help us pick some future episode topics, you can learn more about becoming one of our Patrons at Patreon.com/SciShow, and if you can't stop yawning either, well, we have a whole episode on why yawning is so contagious.  You can find it over at our sister channel, SciShow Psych, a channel which, I might add, was also picked by our Patron community.  They're just so awesome.

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