[Past Michael] If you pulled out a thermometer and took your temperature right now, you'd probably get a reading around 37° Celsius. That number may vary a little from day to day or even hour to hour, but overall, your body temperature is pretty consistent.

If you get too hot, your blood vessels will expand, and you'll sweat to cool down. If you get too cold, your blood vessels will contract, and you'll shiver to warm up. And if you stray more than a couple degrees in either direction, like with a really bad fever or in freezing weather, your proteins and cells will stop working, and you could die.

So why is 37° the magic number? Why not 35 or 40? Well, one major theory suggests it's all about keeping out fungus.

Scientists have found that certain animals, like reptiles and amphibians, get a lot of different fungal diseases. And they're all ectotherms: they depend on external heat sources to stay warm. Specifically, they get way more fungal infections than their endotherm counterparts, like mammals, which generate body heat internally.

Researchers from Yeshiva University wanted to find out why. They tested the heat tolerance of different species of fungus, starting at a temperature of 30° Celsius.

They found that every one degree increase in temperature caused a 6% decrease in the number of fungal species that are able to infect an animal host.

So if you're a frog depending on sunlight to warm you up, you've got to worry about tens of thousands of fungal species that can infect you and cause life-threatening disease. But if you're a mammal hovering around 37°, only a few hundred fungi can survive long enough to mess with you.

In other words, when it comes to the risk of fungal infection, it really helps to be hot. Then again, you don't want to be too hot.

After all, it takes a whole lot of energy to maintain a high body temperature, and you don't want to spend all of your time finding food and eating it.

You want to find a perfect balance, and guess what? When these researchers ran some mathematical models, weighing the benefits of protecting against fungi versus the costs of extra food consumption, they calculated an ideal body temperature of 36.7° Celsius, which is right around our toasty 37°.

In fact, these scientists even think that maintaining this warm body temperature helped mammals thrive as the dinosaurs went extinct, even though it's energy-intensive.It's possible early mammals started to beat out reptiles simply because we were more resistant to fungus.

So, I guess... our hotness helped us survive.

[Present Michael] I stand by that line. We're hot, and that's a good thing.

Now you also need to put your human body into rest mode periodically to maintain optimal performance. Yes, I know it sounds strange, but stay with me. We really do need to sleep sometimes. The question is whether it's possible to spend too long in this powered down state.

Research suggests that might be the case. Here's Olivia to explain how.

[Olivia] When people talk about having too much of a good thing, they usually mean overindulging in something like cake or fast food––things you probably enjoy even if they aren't very good for you.

But what about something that's objectively important to your health and well-being, like sleep? Well, it turns out that you can get too much of that, too.

Most people need seven to nine hours of sleep, and oversleeping is connected to health problems like depression, heart disease, and diabetes.

A 2014 study of 894 pairs of twins, for example, showed that the genetic risk of depression was higher in subjects who got less than 7 hours of sleep, or 9 or more hours of sleep every night, meaning people who overslept or underslept were more likely to be depressed because of genetic factors as opposed to environmental ones.

And according to a study on the sleep habits of 400,000 Taiwanese adults, the risk of coronary heart disease is about the same in people who sleep less than 4 hours a night as it is in those who sleep more than 8 hours a night. Subjects who underslept had a 35% higher risk of heart disease, and people who overslept had a 34% increase.

Another study, published in 2009, followed 276 subjects for six years and found that people who slept either less than 7 hours or more than 8 hours were at least twice as likely to develop type 2 diabetes or trouble tolerating glucose.

And there's more!

A 2013 study of about 54,000 adults over the age of 44 found links between too much sleep and increased rates of heart disease, diabetes, obesity, stroke, and mental health issues. In fact, the rates of coronary heart disease, diabetes, and stroke were even higher in people who overslept than in those who slept too little. 

So the links are there. Sleep correlates with all kinds of health problems, but it's hard to say whether too much sleep actually causes these issues.

It's totally possible that oversleeping is actually a symptom of things like depression or heart disease, or that there's some other connection. Either way, consistently sleeping too much might be a bad sign.

[Present Michael] Good to know. Oversleeping might be a sign it's time for some maintenance by a physician.

It's also possible that, at times, your human body might request maintenance in the form of input of specific foodstuffs.

Cravings, I mean. Sometimes you just get a hankering for Taco Bell, and nothing but a Cheesy Gordita Crunch with extra jalapeño sauce will make it go away.

Here's me again to explain why that happens.

[Past Michael] It starts off quiet, a gentle whisper in your ear. You're working hard, so you push the thought away, but soon it comes back, louder and more urgent.

You start thinking about the bar in your backpack, how delicious it would taste, and pretty soon it's all you can think about as your brain chants louder and louder, "Chocolate!"

Until you cave in and give it exactly what it wants. Sound familiar? You're not alone.

Maybe it was bacon or pizza or a chai latte, but nearly everyone has felt food cravings at some point. But why? And is this unshakable desire for a chocolate bar my body's way of telling me I need more magnesium?

Well, before we can talk about cravings, let's look at hunger. Actual physiological hunger is how your body tells your brain it needs a resupply of calories. It isn't about specific flavors; it's about survival.

When your stomach is empty and your blood sugar levels are dropping, your body starts secreting ghrelin, otherwise known as "the hunger hormone." The ghrelin tells the hypothalamus in your brain that you need to eat and triggers a chain reaction that revs up your appetite and gets your digestive system ready to receive food.

And then, once you've gobbled down enough food to stretch your stomach a bit, the ghrelin tap turns off, and you feel satiated for a while.

But if you've ever been tempted by the dessert menu even after a big meal, you know that food cravings are different from straight up hunger.

In some cases, a food craving can be a sign that you need more of a specific nutrient. If you're very low on salt, for example, you might crave potato chips or pretzels, but if you need something more specific, say, magnesium, there's no evidence that you'll start craving chocolate, even though chocolate has magnesium in it.

Often, cravings are more about a psychological hunger than a physiological one, and they can be wrapped up in emotions like stress and anxiety.

In the past, we've talked about how humans tend to be drawn to fatty, high-carb foods in part because they're loaded with the energy we need to think and move and do stuff. Sure, that's a physiological need, but the pull toward junk food is often a psychological one as well. Eating a butter-frosted cupcake or a bag of salty French fries releases an opioid typhoon that lights up the brain's pleasure center and makes us feel awesome, at least for a while.

Celery just doesn't have the same effect.

Cravings are also tied to your brain's memory center, which explains why you might also crave a food that isn't full of fat or sugar. Your brain could be tying that food to a happy memory or a feeling of reward, and thinking about a memory associated with a food can make you crave that food. 

Another psychological cause of cravings can be a boring or restricted diet.

In studies where subjects had to drink only meal replacement shakes, they ended up with a lot more cravings than usual, especially for solid foods. And this diet provided all of the calories and nutrition that the subjects' bodies technically needed, so they probably didn't get those extra cravings because they were missing out on some nutrients.

Pregnancy hormones, on the other hand, probably don't cause cravings. Pregnant women are often portrayed as having serious urges for things like pickles and ice cream, but there isn't actually much evidence for a connection between those hormones and cravings.

Instead, these cravings probably have more to do with mood. So if you're really jones-ing for that chocolate, it's probably just because your brain remembers that it feels good to eat it.

[Present Michael] I mean, is anyone here gonna deny that Taco Bell is delicious?

Next, you might be confused by some of the seemingly useless extra parts that your human body shipped with, like your appendix.

Except, well, the appendix was dismissed as pointless for years, until researchers discovered the point. Here's Stefan with more.

[Stefan] The appendix gets a bad rap. You probably never even think about it, unless it's the reason you're doubled over in abdominal pain, and for the most part it gets written off as a useless organ left over from our evolutionary past.

But even though you can usually ignore it, you've gotta give your appendix some credit, because it might not be as useless as you thought.

The appendix has evolved in mammals at least 29 times, which is a pretty good sign that it does something.

And back in 2016, an international team of researchers set out to understand why it appears so many times. They started by looking at what kinds of things animals with appendixes have in common, but first they had to define what an appendix even is, since the thing we call an appendix comes in all shapes and sizes across mammals.

As a starting point, the researchers defined it as a section of tissue extending from the cecum, the beginning of the large intestine. They then used computer models to analyze data on hundreds of mammals. They gathered information about their habitats and social behavior and, of course, whether or not they had an appendix.

And they concluded two things. The first was that the appendix has evolved more times than it's been lost, so it must have some kind of evolutionary advantage.

And second, after finding high concentrations of lymph tissue, which protects the body against foreign invaders, they concluded that the appendix is involved in immunity across mammals.

And that seems to be the case for humans, as well.

Within the inner layers of the appendix, we find all kinds of densely packed immune cells, including T and B cells and natural killer cells, which are all important parts of your body's immune response.

But we also find a reservoir of good gut bacteria hanging out in there. That's the same type of bacteria that line the insides of the intestine, creating a protective barrier against invaders.

Given its prime position just off the colon, researchers think the appendix might dish out emergency rations of gut bacteria in times of crisis, like during cases of extreme diarrhea. Diarrhea can flush out your intestines, but the appendix may be able to provide a fresh population of the gut bacteria that keeps your digestion on track.

They say you don't know what you have until it's gone, and it's true. One way to appreciate the immune function of the appendix is to see what happens when it comes out.

A study published in 2015 found that patients who contracted a particular bacterial infection called Clostridium difficile were twice as likely to develop a severe infection if they didn't have an appendix.

That study alone doesn't prove that the appendix prevents infection, but it does raise questions about whether or not surgeons should remove the appendix when they don't need to, since that might actually cause trouble rather than prevent it.

That said, appendicitis can be deadly, so if your appendix needs to come out, it's coming out.

But otherwise, maybe we shouldn't be so quick to dismiss this organ that's usually just trying to be a pal.

[Present Michael] That's sweet. It's doing it's best.

Speaking of extra bits and bobs, our human bodies also ship with considerable variation from unit to unit, and in general, these differences are great.

Sometimes, though, there's even more going on than we realize, like the fact that we have hundreds of blood groups beyond the four that we usually learn about. Once more, let me explain what that means.

[Past Michael] At some point, like if you've donated blood, you might have been asked about your blood type.

And even if you don't know what yours is, you're probably aware that you could have A, B, AB, or O blood, and that your blood can be positive or negative.

But that's not the whole story, because there are potentially millions of blood types out there. There are so many possible blood types because of the way blood types are defined.

You see, your blood type is defined by the antigens present in your blood. Antigens are anything that can elicit a response from your body's self defense system, though your immune system normally ignores the ones that belong to you. And you can find antigens on cells throughout your body, with different cells having different combinations of antigens. 

The ones that matter for blood classification are found on the surfaces of your body's red blood cells. Simply put, your particular blood type depends on which antigens are or aren't there.

Like, if you have AB blood, that means you have both the A and B antigens in the ABO blood group. You could be just A or just B or, if you're O, you don't have either. 

But those ABO antigens are just two of over 600 blood antigens identified by the International Society of Blood Transfusion, and the list hasn't stopped growing yet. Many of these antigens fit into blood group systems, like ABO, each of which is defined by a gene at a single site or by multiple genes that are closely related.

And all of us have these genes. Your type for a particular group depends on how your genes translate into the antigens that end up on the outside of your blood cells. 

There are 36 blood group systems currently recognized, so your full blood type, if written out, would include all 36 of these groups and the variants you have or don't have for all of those 600+ antigens, which is why we can say that there are millions of potential blood types.

Of course, there are only eight common ones, and that's because many of these antigens are found in practically everyone, while others are present in only a few individuals.

For example, the SARA antigen has only ever been seen in two families, while 99.96% of people have the VEL antigen.

And blood groups can get really complicated, too. Just look at the Rh blood group. That's the group that gives you a positive or negative blood type. Like, if you're AB positive, the positive part generally means you have the Rh antigen called Rh(D).

But to make things more confusing, whether your blood is considered positive or negative may depend on how much of the antigen you have in your system, since that can impact what sorts of antibodies your immune system makes to protect you. And D is just one of over 60 known antigens in the Rh group, so positive or negative doesn't even begin to capture your overall Rh blood type.

In fact, one of the rarest blood types in the world occurs if you have none of the antigens in the Rh group. If you're one of the roughly 50 people with this blood type, which is known as the Rh null, your body will reject the blood from practically anyone else.

And the Rh group isn't the only blood group where having no antigens can be a matter of life or death.

Another example is the Diego group. Its antigens are proteins that help the lungs and kidneys perform essential functions. Like with the ABO group, there are two primary antigens, A and B, that determine a person's Diego blood type. But unlike the ABO group, where O or the lack of antigens is most common, there has only been one documented case of someone lacking both of these key Diego antigens.

Sometimes, though, having no antigens can help you out.

For example, one of the malarial parasites uses antigens in the Duffy blood group to target and infiltrate cells, so having no antigens from that group can make you more resistant to the disease.

But it also means there's a chance that your body will attack another person's blood if you're given a transfusion, since the cells you receive could have Duffy antigens on them that your body sees as foreign.

The good news is that, despite all the potential blood types you could have, for the most part, the ABO-Rh blood typing we're using does a pretty good job of matching people's blood. With this system, if you receive blood from someone with the same ABO-Rh type, there's a 99.8% chance your blood will be compatible with your donor's.

For some reason, your body's immune system doesn't go after every antigen equally, so you don't usually need to know what version of every single known antigen you have.

And if you do want to be extra sure, there are ways doctors can tell if you have a rare blood type.

For example, they can screen for unexpected antibodies that could potentially target donated red blood cells, or perform cross-matching, where your blood is mixed with a donor's to see how the two react. Doing these two steps racks your safety margin up to 99.95%.

And if you do happen to have a rare blood type like Rh null, don't fret.

Efforts like the International Rare Donor Panel work hard to make sure you can get the blood you need no matter where in the world you are.

One person needing a transfusion had a rare blood type delivered from the UK to Cameroon. That's about 4000 miles away. So even if your blood is literally one in a million, you can be pretty confident that you'll be able to find someone whose blood matches yours.

[Present Michael] And finally, it's important to realize that our bodies sometimes require the use of carefully designed chemical to maintain optimal function.

I'm talking about medicine. But the dose of these chemicals can vary a lot depending on the unit's age.

In other words, you can't give the same amount of medication as you do to an adult, but how the dose varies might surprise you. Stefan can tell us more.

[Stefan] If you've ever strolled through a pharmacy in search of over-the-counter meds, you might have noticed that lots of drugs have special children's formulas.

And you might think that's because smaller people need smaller doses. But you'd be wrong, because kids aren't just tiny adults. In fact, children sometimes need larger doses of medicine.

Welcome to the weird science of allometry.

Allometry simply refers to the study of how physiology changes with size, and it plays a big part in understanding how much medicine kids should get.

You might think bigger bodies would need larger doses of medicine, since they have more of whatever tissue or cells the drug if targeting. And there are a lot of medications that are prescribed based on a patient's weight, what's sometimes referred to as a weight-based dose.

For instance, an adult might be prescribed five milligrams of medicine for every kilogram they weigh.

But what's really weird is that this dose isn't constant throughout a person's lifetime. You often can't take the dose per weight an adult would take and simply multiply it by the child's weight to figure out how much they should get.

And that's largely because a person's cells use less energy as their body grows. In other words, bigger people have lower mass-specific metabolic rates than smaller ones.

Even at the cellular and sub-cellular level, oxygen and calorie consumption is just slower in larger bodies. Not only that, but it's a non-linear relationship.

So, as bodies get smaller, metabolism increases fast, and that matters when it comes to medicines, because it impacts how fast your body processes drugs.

Generally speaking, the faster the metabolism, the faster you break down a drug. And that means that, even if you're small, you might need to take more to have the amount you need stick around long enough for it to work.

Let's consider acetaminophen, which you might know as Tylenol. It's an over-the-counter drug that you can take to reduce pain and fevers. The regular adult dose for acetaminophen is 650 milligrams, so an adult weighing roughly 80 kilograms, the average mass for adults in North America, would generally take that amount every 4-6 hours as needed.

And if you'd never heard of allometry, you might think that a child one quarter that weight, at 20kg or so, would get 162.5mg, but that's way too little. The child will metabolize it super quickly, so the drug won't really have a chance to be effective. With junior-strength tablets, the recommended dose is actually 240mg, and for a 44kg child, the recommended amount is 640 milligrams. That's basically the same as what's recommended for adults, even though the child is about half the weight.

And if you calculate the weight-based doses for everyone, the children actually take about two times as much, and other drugs can be even more extreme.

The adult dose of oseltamivir, which is an antiviral used to treat influenza infections, is 150mg a day, so a little under 2mg per kg per day for our 80kg adult. But pediatric doses range from 3-6mg per kg per day.

Now as for why metabolic rate scales with mass in this strange, nonlinear way, well, physiologists don't know for sure. But by looking at people and animals of all sizes, they've come up with some good hypotheses.

The first, and perhaps most obvious, is that little bodies belong to individuals that are still growing, and it takes more energy to build tissues than to maintain them.

So, that's probably part of it, but there has to be more going on, because the allometric pattern holds across different sized adults, too.

That's where the shapes of smaller and larger bodies might be coming into play. Because a smaller body has a larger surface area relative to its tiny volume, it loses core heat faster. And that means it needs to burn more energy, and therefore run a higher metabolic rate, to stay warm.

But even that still doesn't fully explain the allometric scaling of metabolic rates seen in nature, because it only makes sense in so-called "warm-blooded" species like us that strictly regulate their internal temperature with the heat they produce. And this metabolic rate scaling occurs in reptiles and other so-called "cold-blooded" species, as well.

There may be another way that overall shape impacts all of this, though, which helps explain that.

Some mathematical work suggests metabolic rates in large animals are limited by their internal plumbing. See, branching blood vessels have to strike a balance between their width, the pressure of the blood moving against their walls, and the force it takes to move that blood around. In a bigger animal with a larger, longer, and branchier circulatory system, blood flow ends up being slower.

Since it's slower and the body is larger, it takes longer for blood and the oxygen it carries to get places. And the animal can't beat its heart faster or harder to speed things up, because, if it tried, its heart would fail. So as an animal grows in size, its cells simply can't keep using oxygen at the same rate. 

More work is needed to sort out how all these ideas apply to humans specifically. But whatever the reason for children's elevated metabolisms, the effect on drug dosing can be pretty dramatic.

So ultimately, it's not that there are separate formulations because kids always need less medicine. If anything, it's so that a patient or their caregiver can more carefully adjust the dose they need for their body size.

Liquid medications in particular can be carefully tailored to a child's weight, and there are other reasons to have special formulas for kids, too, like that pills can be hard to swallow or even a choking hazard, especially for younger children.

So lots of kid's medicines avoid the issue by using chewables or liquids instead. Plus, not all medications are recommended for children of all ages. They might be ineffective or have more severe side effects in smaller kids, or they just haven't been thoroughly tested yet. 

The bottom line is, if it isn't sold in a separate children's version, it shouldn't be given to a child without consulting a pediatrician. And you should always stick to the children's formulations when treating kids, because that's the best way to ensure they get the right amount, even though it may be more than what you'd take. 

[Present Michael] So remember, if your human body experiences technical issues, it's best to consult a professional.

Thanks for watching this SciShow compilation. If you'd like to keep going, we have a user's guide to the human brain as well.

And, if you'd like to help us make more videos, you can support us at Patreon.com/SciShow.

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