Take a guess as to how much blood is in your body right now? Just like a ballpark guess.
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.