Say you’re feeling particularly kind one day, so you decide to make a donation to the
local blood bank. You come in, you fill out your paperwork, and sit down for ten minutes while they take
about a pint of blood out of your body. It’s all good though, the blood is going to someone
who really needs it. Plus, they usually have free cookies at these things. And while you’re
noming on that little packet of treats, you can’t help but notice that there’s a lot
of blood around you. Not in a creepy way, but in a way that makes you wonder “If that
just came out of me, how is my body replenishing my own blood supply?” The fact that we can
donate blood is a really cool feature of our physiology, in part because of how blood cells
are made. As I mentioned a few episodes ago, red blood cells, or erythrocytes, don’t
have a nucleus. And without the genetic information inside of a nucleus, they have to come up
with a different way of making copies of themselves. And then we have the task of cleaning and
excreting all the old ones. The creation, maintenance, and filtration of blood is a
unique process that’s different from other cells. Luckily, we’ve got some organs
and physiology up for the task.
So what do I mean when I say that red blood cells don’t
reproduce like normal cells? Well, when our bodies want to create new skin or stomach
cells, old cells will duplicate their inner contents, including their genetic information,
and split into two new identical daughter cells. This is good old fashioned mitosis. But
because red blood cells ditch their nuclei as mature cells, they can’t make direct
copies of themselves. This means the body has to constantly manufacture and eject blood
in order to keep the right number of cells in circulation and function as a human.
It gets complicated, but too many red blood cells can increase the thickness of blood to a dangerous
amount, and too few red blood cells and your tissues can’t get the oxygen they need.
To keep that balance, we eliminate and restock our blood with literally millions of new blood
cells every second of our lives. So yeah, the process is kind of important. Our journey
starts in the bone marrow, the soft tissue inside of certain bones, although similar
processes are happening in the lymph nodes, spleen, and thymus as well. That’s where
we’ll find hematopoietic stem cells, or HSCs. These things are a cool type of cell
that can become any of our blood cells — red blood cell, leukocyte, or platelet-producing
cell, it doesn’t matter, they all start as an HSC. It can also make copies of itself,
which is awesome because you never want to run out of a cell that can become anything. It’s
just like getting a magic genie. The first thing you do is wish for more wishes. It’s
the exact same thing with stem cells, you make more stem cells. From there, the HSCs
start differentiating into more specialized types of cells. At this point, the cell has
a few different options: it can either become a common myeloid progenitor, and have the
option of turning into a platelet-producing cell, a red blood cell, or certain types of
white blood cell. Or it can become a common lymphoid progenitor where it can become a
Natural Killer cell or lymphocyte like a B or T cell. These progenitor cells are kind
of an in between step in cell differentiation. They’re not full blown stem cells where
they can become anything, but they still have options. It’s kind of like in the wizarding
world when you turn eleven. Say you get your letter to Hogwarts, now you know you’re
not going to Ilvermorny or Beauxbatons, but you still need to get sorted into your house.
That’s what it’s like to be a progenitor cell. The future identity of the cell is starting
to take shape. And depending on what kind of chemical environment the cell is exposed
to will determine what type of cell these multipotent cells become. Take the red blood
cell for example. A common myeloid progenitor has a few options in front of it. Not infinite
options, but it still has options. It can become a myeloblast and turn into a number
of white blood cells, or turn into megakaryoblast and produce platelets. Also megakaryoblast
is the coolest cell name of all time. I stand behind this claim a thousand percent — If
you have any challengers for that title, tell me in the comments. But I’m letting you
know ahead of time, you are wrong. Now, the progenitor could turn into a proerythroblast.
The Erthryo- part tells us that it’s a red blood cell, and -blast tells us it's a cell
that creates a certain cell type. You can see more examples of that in the hematopoietic
family tree. A lymphoblast turns into a lymphocyte and a monoblast turns into a monocyte. At
this point, the erythrocyte-in-the-making still has a nucleus, albeit, one that’s
condensing over time. From there, it keeps condensing down and loses its nucleus to become
a reticulocyte, and after ditching even more of its contents and slipping into the bloodstream
it becomes a mature red blood cell. This process of going from proerythroblast to mature
erythrocyte takes up to 5 days although your body can speed it up if it needs more of them.
A few different factors can bump up the production rate, like a lack of oxygen for example. And
if your tissues detect that your blood is running low on oxygen, your kidneys will crank
out a hormone called erythropoietin, or EPO, which tells the bone marrow to get busy making
red blood cells. Then as the kidneys detect that oxygen levels are back to normal, they’ll
back off the EPO production which brings our bodies back to homeostasis. If you’ve ever
traveled somewhere at a high altitude, your body needs to get used to the new oxygen situation
and can adjust EPO accordingly. Likewise, other events like infection will trigger your
body to make more white blood cells, and bleeding will trigger the creation of more
platelets to plug up that new cut. So at this point, you’ve got a certain amount
of red blood cells floating through circulation. They’ll live their lives for about a hundred
and twenty days before being cleaned out and replaced. Now, this process is different from
when you’re making your very first blood cells when you’re still in the womb. After
all, you have to make your very first stem cells before they can start differentiating.
Unfortunately, we know much less about the embryonic version than we know about the adult version.
And as an adult, since you’re constantly making new blood cells, you’ve also got
to get rid of old cells to maintain a consistent blood volume and thickness. That’s where
a few important organs come in, namely the liver and the spleen.
For such a useful,
and you know what, I’ll say it, underrated organ, the spleen itself is pretty tiny.
It’s about the size of an old Gameboy Color and only weighs on average up to 200 grams. Although
scientists would never shame a spleen for its small stature. In fact, an enlarged spleen,
a condition called splenomegaly, is a sign that something bad is happening. It can happen
when someone has liver disease, or other chronic illnesses, like the classic virus mononucleosis,
or mono. But when it’s healthy, the spleen works as a lymphatic and immune center while
also picking off old red blood cells and preparing them for excretion. And if you were to slice
it open, you’d find two main units, red pulp and white pulp. Yep! The same colors
as our blood cells. And lucky for you, the colors roughly match up with the cell type.
The white pulp is built around a central blood vessel and stacks a bunch of B cells and T
cells so it can be ready to pump those cells into the bloodstream during an immune response.
All in all, about a quarter of all the body’s lymphocytes live in the spleen and at most,
about a cup of blood. And that blood is found in the other segments of the spleen known
as red pulp, which is where the heavy duty blood filtration happens. When blood first
comes into the spleen, it fills into empty space that surrounds the white pulp, eventually
collecting into little empty pockets within the red pulp. In particular, all red blood
cells have to pass through the narrow splenic cords, also known as the Cords of Billroth. Sounds
like a DnD character. And remember how red blood cells are ridiculously tiny? Only six
thousandths of a millimeter across? Well, in order to pass through those splenic cords
and stay in circulation, they have to contort themselves into an even smaller shape.
When they’re young and flexible, red blood cells slip through without any trouble. But as they
get older, they also get bigger and stiffer, so they get trapped at this step, which is
when they start the process of getting recycled. The hemoglobin in those old red blood cells
is still a valuable resource for future cell material. So immune cells in the red pulp
recycle the hemoglobin’s iron molecules and either store them in the liver or spleen,
or send them back to the bone marrow to make more hemoglobin for new red blood cells. And
all those globin proteins get broken down into smaller amino acids and recycled for
new red blood cells as well. Meanwhile, all the non useful material from those dead cells
gets transformed into bile and then ends up in your poop. Now clearly, there’s more
in your blood that your body might want to clean out than just blood cells. When people
are looking to “detox”, they’re probably thinking about this part. That’s what we
have a liver and kidney for. Well, hopefully two kidneys. But, you don’t need to buy
a charcoal drink or a special juice or eat anything special — your body’s built to
clean its own blood. Thanks for watching this episode of Seeker Human. I’m Patrick Kelly.