- Hey, if you took out all the blood cells in your body and laid them out, end to end, you'd be dead.

The average human adult contains about this much blood, 15 pounds of my body is just liquid meat and not all blood is red.

Across the tree of life we can find blue blood, green blood, purple blood, even clear blood.

Why does it come in so many colors?

Of all the juices in your body, blood is by far the most important.

But how much do you really know about it?

'Cause without this stuff we couldn't feed babies, cool off our bodies, eat, or even cry.

Blood's the reason your pee is yellow and also why your poop is brown.

We're taking an adventure through this wild universe to learn all about blood and what it does.

Why does blood come in so many colors?

Why are some blood so rare?

And maybe weirdest of all, why is your blood different than mine?

(light music) Hey, smart people, Joe here.

Ow!

Everyone knows blood is red.

But ever hear someone say that blood is blue when it isn't carrying oxygen?

That's actually not true, it's just an optical illusion.

Blue light is better at reaching the surface of the skin than red light so it just appears that our veins are blue.

But some animal's blood really is blue, some is green, some is even clear.

Blood in all of its forms is really super weird.

But before we can understand all of those, we need to know what's in our blood.

Blood gets around your body in many ways, through big arteries and veins that branch smaller and smaller all the way to capillaries, the smallest of which are so tiny that only a single blood cell can squeeze through.

Together, these form a closed loop that begins and ends at your heart and lungs, and there's a lot of ingredients to this little crimson cocktail.

But that recipe starts with four main components.

About half your blood is plasma, it's mostly water with the rest being proteins, fats, salts, and other chemicals.

Its main job is to carry stuff where it needs to go, things like nutrients, blood cells, and chemical messages all over the body.

It also carries waste to your body's dump and recycling centers.

And since it's mostly water, plasma also helps keep your fluid and electrolyte levels balanced.

Without blood, you would overheat super fast, 'cause when you're hot, water's absorbed by the blood and your plasma volume increases to give you a reservoir for sweat making.

That's right.

Next time you're sweating, think about the fact that all the water dripping down your face was sucked out of your blood vessels.

And that's true for pretty much every liquid our bodies make.

Milk, made from blood.

The spinal fluid that cushions and cleans your brain, also made from blood.

Saliva, that starts as blood too.

Even tears, you couldn't cry without plasma.

And our next three blood ingredients use plasma as a liquid highway to get where they need to go.

You ever wonder why you don't bleed to death when you get a paper cut or prick your finger?

Platelets, they aren't true blood cells, they're more like cell fragments.

When a blood vessel gets damaged, a chemical alarm bell draws platelets to that injury.

They stick to each other and to the broken vessel, which creates a plug like Flex Seal for your circulatory system, and you've got a lot of 'em.

One drop of blood has millions of platelets in it.

That's bonkers.

But it's nothing compared to blood's biggest army, red blood cells.

A body like mine has almost 30 trillion red blood cells.

That's hundreds of times more blood cells than there are stars in the Milky Way.

You have four times as many red blood cells as all your other cells combined.

If you were to count them, one by one, it would take you almost a million years.

So your body's basically all red blood cells with a sprinkling of other cell types.

A red blood cell's main job, of course, is carrying oxygen from the lungs to the rest of the body and then helping move carbon dioxide back to the lungs so that you can breathe it out.

That oxygen carrying job is done by a literal atom of iron inside a protein called hemoglobin.

Just like rust, iron is what gives our blood its red color and also what makes it taste like metal.

The smallest blood vessels in your body are just 1/10 of a human hair across, that is tiny.

So red blood cells have to be tiny too.

They're about a million times smaller than one of your muscle cells.

There are around a billion red blood cells in just one drop of blood.

Your body makes about two million new red blood cells every second, old blood cells constantly being replaced by new ones because, well, red blood cells are kind of dead.

See, like all your blood cells, your red ones get their start in the bone marrow, and as they mature, the nucleus gets thrown away.

This is a big trade off.

Not having a nucleus makes red blood cells flexible enough so they can squeeze through those tiniest of blood vessels, and it leaves more room to pack the cells full of hemoglobin.

But it also means they have no DNA blueprints to repair damage or make more of themselves.

So one red blood cell only lasts around four months, which really isn't very long compared to all the other cells in your body.

It makes you think, in some ways, they kind of don't fit our definition of living cells at all.

Your biggest cellular army is basically a hoard of oxygen-hungry iron zombies.

And finally, you've got white blood cells, a huge family of cells whose main job is about figuring out self versus non-self, what belongs in your body and what doesn't, and putting up a fight to attack anything that shouldn't be there.

So that's our basic recipe for blood, but each of us has our own special twist on that recipe.

Pop quiz, hotshot!

What's your horoscope sign?

Okay, you probably know that.

Now, what's your blood type?

I'm willing to bet a lot of you did not know the answer to that one.

More Americans know their astrological sign than their blood type, which is pretty wild, because only one of those things could potentially save your life, and it ain't your horoscope.

This test is gonna tell me my blood type.

Even though all of our blood does the same jobs, I might not be able to give you mine.

Why is that?

Scientists are still trying to unravel this blood type mystery, and the story starts about 200 years ago.

Back in the old days, doctors were really into mixing different bloods together, different people, different animals, sometimes people blood with animal blood, and it didn't usually end well.

They noticed sometimes combining different bloods made them clump up.

It's called agglutination, basically Latin for glued together.

One Austrian doctor, Karl Landsteiner, wanted to know if this happened because there was something wrong with the blood or if this sticky situation was something blood was doing on purpose.

So he did what any normal person would do.

He asked some friends for some of their blood and got to mixing it up.

He found that there were rules, patterns to the clumping, and after studying those patterns, it seemed like human blood could be split into three groups.

When he mixed plasma from group A with red blood cells from group A, the mixture stayed nice and liquid.

And the same for group B, but if you mix plasma from group A with red blood cells from B, or vice versa, the mixture clumped up.

He noticed something even weirder about group C, adding plasma from this group to either of the others red blood cells resulted in clumping, but C type red blood cells could be added to plasma from any group, no problem.

Now it's been more than 100 years since those early experiments, but Landsteiner's work is still the basis for the major blood types we use in medicine today, the ABO system.

And today we know why these different bloods don't always play nice together.

Comes down to our immune system.

Your immune system is constantly tasting bits of cells of debris, looking for foreign invaders and other bad stuff that it should attack.

These bits that it tastes are called antigens.

Not all antigens come from outside of our body like viruses.

Some we make ourselves and your red blood cells are covered in them.

Your specific flavor of blood antigens depends on which genes you inherit.

Your immune system learns not to attack your own red blood cell antigens, but it means that it is ready to attack any that don't match.

Today, we know there are hundreds of different red blood cell antigens, but what this test is looking at are called the A and B antigens.

This is the blood type that most people think of.

Blood group A has A antigens on its red blood cells with anti-B antibodies in the plasma.

Blood group B has B antigens with anti-A antibodies.

You can mix A blood with A blood, and B blood with B blood because the antibodies and antigens are never gonna clash.

Blood group C has neither A or B antigens, which is why it was renamed O, after the German word for without.

Landsteiner was able to put O red blood cells into any group's plasma because without A or B antigens, they don't trigger an immune system alarm.

And that's why O is the universal red blood cell donor.

But O plasma does have both anti-A and anti-B antibodies ready to attack, so you can't let it mix with other red blood cells from other groups.

The opposite is true in AB blood.

It has both antigens on the red blood cells, but that plasma won't attack either A or B blood cells.

That's why the universal human plasma donor and the universal blood acceptor is AB.

That's why when Landsteiner mixed plasma from one type with red blood cells from another, bad things happen.

The immune cells swimming in the plasma recognize the antigens on the red blood cells.

That clumping was the immune system going into battle mode.

A person's combination of antigens and antibodies is what determines your blood type in the ABO system.

(sighs) I gotta prick my finger again.

Ow.

All right, now we just need a little bit of blood.

Here we go.

This card contained antibodies against A antigens, and B antigens on red blood cells.

My blood didn't react to either, which means I don't have A or B antigens.

In other words, I've got type O blood.

But there's something else.

40 years after his initial discovery, Landsteiner and his colleagues found another way to categorize blood, by whether or not the red blood cells also carried a different antigen called the Rhesus factor.

If your red blood cells have the RhD protein on them, your blood type is positive, if they don't have it, your blood type is negative.

That's why if you're Rh negative, you can have a similar immune reaction to Rh positive blood.

This test tells me that I do have the RhD protein on my blood, so I'm O positive.

These eight blood types are just the ones we get from the ABO and Rh systems.

But there are more than 30 systems for categorizing human blood, just depending on what doctors are looking for.

And humans aren't the only animals with blood types.

Dogs have more than a dozen of 'em, so do chickens.

Horses have seven main ones, and many primates even share components of the ABO system with us.

Huh?

It's almost as if we share a common evolutionary ancestor.

And there are some humans that have special super rare blood types.

About one in one million people in Europe and one in 10,000 people in India have what's called the Bombay phenotype, a rare condition where they don't have any of the blood types in the ABO system.

They live normal lives as long as they don't receive blood from someone who does have those antigens.

Rh-null lacks all of the antigens in the Rh system.

It's sometimes called golden blood and it's so rare that it's likely more people win the Powerball jackpot each year than are born with this blood type.

Like the Bombay phenotype, people with this kind of blood can only get transfusions from others who share their unique trait.

Scientists still don't fully understand why blood types even exist.

Many evolutionary biologists think that having these different blood types gave certain populations advantages in particular habitats or helped 'em survive certain threats.

For example, type O people are more susceptible to plague, cholera, mumps, and tuberculosis.

And people with type A are more susceptible to smallpox.

That is as various groups of humans faced these and other challenges in the past, certain blood types could have provided a local survival advantage, and people with type A blood are more prone to blood clots.

That's a health risk today.

But imagine in times without hospitals and modern medical care, this could have been an advantage in childbirth or in response to injuries.

Even today, some simpler animals like jellyfish and sponges survive without any blood.

They have slow metabolisms and really high surface area, which means they can absorb and release important molecules and nutrients directly through their skin.

Insects don't have blood either.

They use a similar fluid called hemolymph that transfers gases through openings in their exoskeletons.

Across the tree of life, we think that blood or something like it first showed up more than 500 million years ago.

It's also likely that blood evolved many different times in different types of animals, creating unique, and sometimes strange blood recipes along the way.

The blood of some ice fishes is completely clear.

Their cold ocean habitat has a ton of oxygen so they can survive without hemoglobin or red blood cells.

The green blooded skink has lime-colored life juice thanks to a pigment made when red blood cells break down.

It's the same thing that makes your bruises turn green.

And the blood of some marine worms and brachiopods carries a protein called hemerythrin.

When it grabs onto oxygen, it turns purple.

And you might owe your life to the literal blue bloods of the world, horseshoe crabs.

These ancient animals use a different blood protein, hemocyanin, to bind to oxygen, and it gets its blue hue from using copper instead of iron.

Horseshoe crab blood also has a superpower.

It holds a chemical that can detect bacteria and certain toxins at super low concentrations.

It's used to make sure that drugs and vaccines and medical devices are safe for humans.

Despite how much we've learned about blood in the past few centuries, it still holds many secrets.

That's why humans have never been able to accomplish what many scientists view as the holy grail of biomedical research, creating synthetic blood.

Because, like everything in our bodies, blood is more than just the parts that make it, it's a flowing universe of cells and molecules that interact with each other in these really complicated ways that we're still figuring out.

So the next time you see a drop of blood, don't get the ick.

Remember that there's a universe of complexity inside, one that fuels us, defends us, heals us, and connects every corner of our bodies in ways that we rarely appreciate.

Stay curious.

Not messing around this time.

I don't wanna do this, I'm gonna do it anyway.

All right.

How sure am I?

Positive.

Don't use that.

It's bad.

Okay.