Think about all the blood in your heart, all the blood in your arteries, veins, and all

the smaller blood vessels. Add up all those sources of blood and you get 5 whole liters. That’s

about this much! This glorious red stuff is so valuable to our physiology as humans, that

we’re going to spend an entire episode introducing you to the cast of characters that make blood

what it is.

For instance, you’ve probably heard about the big players like red blood

cells and white blood cells, but blood is a diverse and multi-faceted substance that

also holds clues to diseases that affect more than just your blood. I want you to think

back to the last time you got a cut. The blood that oozed out of that cut was probably bright

red, shiny and dripped quite a bit more slowly than water. That may give you the impression

that blood is a uniform fluid, but it’s actually multiple substances, all with different

purposes. You can tell because if you look at a vial of blood and let it sit around for

a few hours or if you spun it in a centrifuge, you’d notice three distinct layers emerge

as the denser components of blood sank to the bottom. The creamy colored and lightest

layer on top is the plasma, which is mostly water. But just like a mug of hot cocoa, it’s

what you dissolve in the water that makes it interesting. Yes, I made my cocoa with

water growing up. All you kids who made your hot cocoa with milk, it must be nice.

Plasma is where you’ll find the dissolved solids in your blood. Substances like proteins,

sugars, hormones, and a ton of other things. The red, dense layer at the bottom is made

of erythrocytes, also known as red blood cells, the most common type of cell found in your

blood. We’re talking around 250 million cells per drop of blood. That takes up about

40% of your blood by volume. Which leaves this little layer in the middle here making

up less than 1% of your blood volume. This is where you’ll find your leukocytes, or

white blood cells alongside platelet cells. And even though they only make up a tiny volume

of blood, these cells still have an important function that all comes back to our immune

system. Now, you’ll notice that a lot of cell names we’ve been talking about so far

end in -cyte, which is no coincidence. Whether it’s an erythrocyte or a leukocyte, -cyte

denotes a mature cell. And the prefixes here literally mean red and white. So basically

we have these fancy words that translate back to red cell and white cell. Now, we do still

have a handful of different leukocytes — there isn’t just one single type of white blood

cell. Like B and T cells, what are collectively called lymphocytes are just two types of white

blood cell. If you want to learn more about those cells by the way, we’ll dive into

them and the immune response later in the playlist. Other white blood cells include

a diverse group of immune cells called granulocytes, as well as monocytes which become big chunky-boy

cells called macrophages. Collectively, white blood cells float around in circulation waiting

for some kind of signal that recruits them to the scene of injury or infection. That

signal might be a chemical like interleukin-1 that plays a part in fever, or histamine which

you’re familiar with if you’ve ever sought allergy relief. Now, it might seem surprising

that there are so many immune cells in your blood. After all, there’s a whole separate

immune system for this exact purpose, right? Well, if you want to constantly patrol the

body to find something, traveling through the blood is actually a good strategy. Take

the other component in that little sliver of blood for example, platelets. These little

cell fragments travel through the blood until they bump into an injured blood vessel. And

at that time they start the process of making a blood clot. If this injury took place on

your skin, this blood clot might be better known as a scab. Just like that one Blink

182 song, platelets wander around with no purpose or direction, and they don’t owe

us a single explanation.

So white blood cells help protect us, platelets

clot our blood after an injury, but what do red blood cells do? These donut shaped cells

are squishy, bendable cells with one main purpose — ferry oxygen through the blood

to the rest of the body. And these things are tiny, only 6 microns wide. That’s a

good thing too, because they need to fit through the tiniest blood vessels in the body without

clogging them shut. Red blood cells can carry oxygen because they contain a protein called

hemoglobin, made up of iron, which is the heme- part, and four globin proteins, hence

globin. Oxygen molecules bind onto those hemoglobin proteins so they can hitch a ride

through the bloodstream. All those iron molecules give red blood cells their color too. Just

like how Mars has a bunch of iron on its surface and looks red, it’s the same iron atoms

coloring our blood. And these cells are totally optimized to carry oxygen, or to be oxygenated.

In mammals, red blood cells don’t have a nucleus, which gives them more space for hemoglobin.

And their donut shape gives them more surface area for oxygen to seep through. Plus, oxygen

binds to this protein in a way that when one oxygen molecule binds to hemoglobin, it becomes

easier for more oxygen molecules to hop on that same hemoglobin afterwards. And as those

oxygenated red blood cells are pumping through your arteries, they’ll drop off some oxygen

at active tissues or anywhere in the body that oxygen is needed. Now, red and white

blood cells are the major cells in our blood, and can tell us a lot about our health. But

that other stuff, all the substances dissolved in the plasma can also give us valuable health

data. Like if you go in for a check up and your doctor orders a blood test, they’ll

probably measure things like cholesterol, triglycerides, midichlorians. The usual. There

are definitely times they’d want to measure red and white blood cell counts, but a lot

of the substances you’re used to hearing about are dissolved in plasma. And one of

the ways to use that information is to predict your risk of disease. For example, a fasted

blood sugar test is one of the things doctors might use to diagnose diabetes. This test

is built on the idea that people with diabetes may have elevated levels of glucose in their

blood compared to non diabetics. But all kinds of blood tests can tell us different things

about your health. Like measuring blood electrolytes can give us information about kidney disease,

and counting the number of white blood cells can tell us the status of the immune system.

But doctors’ tests are getting sensitive and specific enough to find all kinds of really

small compounds in the blood that can give them hints about diseases elsewhere in the

body. And not just for body parts directly in the circulatory system, but all over. These

compounds are called biomarkers, literally biological markers. They can be used as clues

for doctors to make better diagnoses, or to predict outcomes more accurately. Now, technically,

metrics like pulse and blood pressure count as biomarkers but scientists are finding all

kinds of new biomarkers in the lab that can help them make decisions about health care.

For example, scientists can measure extremely tiny fragments of genetic material in someone’s

blood called micro RNA and make predictions about that person’s risk of specific cancers. Here’s

what’s cool and exciting about that though — for just about every form of cancer, the

outcomes are much better if the disease is recognized and treated early, or prevented

altogether. And microRNA are very specific to cell type and disease states, so they can

give us very precise information about someone’s cancer diagnosis and where to look. So this

biomarker in your blood might give doctors the ability to detect certain cancers earlier

and target therapies more precisely. Take one of the deadlier forms of cancer, colorectal

cancer. Scientists noticed that if they found a type of microRNA called microRNA-21 in someone’s

blood, there was a very good chance that person had colorectal cancer. Or if they had more

of microRNA-141, participants tended to have a worse chance of survival, which means this

biomarker could give doctors a more accurate prognosis. So not only does blood carry out

so many important functions for our survival, it can give us clues about diseases that may

help us survive longer. But we left out a pretty important detail in this video. You

know that whole ABO blood typing thing? Well, our next video will get you up to speed on

how that system works, and how we’re getting close to turning everyone’s blood into the

universal donor type. Speaking of biomarkers, I’ve had a few concussions throughout my

youth, and every time, doctors ordered a CT scan for me. These days, a few biomarkers

exist for diagnosing concussions, which not only improves diagnosis accuracy but lets

doctors objectively measure how severe the concussion is.  I’m Patrick Kelly, thanks

for watching this episode of Seeker.