- So here's a guy, and like any guy, if we look at
a little bit of his blood, let's say we look at
one microliter of his blood, one microliter, and there are,
of course, a million microliters in one liter.
So if we look at just one microliter, what will we find?
We'll find about 5,000,000 red blood cells,
5,000,000 for every microliter.
And actually, by percent of volume, the blood cells
make up about, you know, maybe 40 percent of his blood.
So there's a lot of them, there's 5,000,000 in one
microliter, red blood cells, I'm just going to write RBCs.
And in that same microliter of blood there will also be
platelets, now do you think there will be more or less
platelets than red blood cells?
So platelets are a lot smaller, so you might think
that there could be more, but actually there is less.
It can range quite a bit, but a number you might see is
200,000 platelets in one microliter.
And that's platelets, those are the little guys that help
make clots when you have problems with your blood vessels.
So let's say this guy is 20 years old.
Do you think that his red blood cells and his platelets
are the same ones that he had five years ago?
Actually, they are not because both of these things
get kind of destroyed, they break down at a certain rate.
And so this guy needs to be constantly making
new red blood cells and platelets.
And actually, it turns out that red blood cells last
about 120 days on average, that's about four months.
And platelets, meanwhile, only last a couple of days,
so you know, a lot less than the red blood cells.
So you need to be constantly making more of these things.
And do you know where you do that?
It turns out you do it in your bone marrow.
So here I'm drawing a bone, and in the marrow
inside that bone, that's where you produce
these red blood cells and these platelets.
Now, the way that red blood cells are made in the
bone marrow, is that they start from some precursor cell.
So there's some kind of cell that's not a red blood cell.
And one way it's different is that it has a nucleus.
So in case you didn't know, red blood cells actually
don't have nuclei when they're in circulation.
So some precursor cell that has a nucleus.
And I'm going to kind of simplify this a little bit.
But basically, this precursor with a nucleus can divide.
So it undergoes the usual process of mitosis to divide.
And so now we have two, and actually it will do that
many, many, many, many times so you'll have a lot of them.
But then some of them can actually become what we know as
red blood cells, and part of that process involves,
as I said, losing the nucleus.
So this whole process happens in years, so I'll draw some
red dots to symbolize that, and then these blood cells
will go into blood vessels, and I'll just draw some
blood vessels here in this bone.
Bones do have blood vessels in them.
So they'll go into those blood vessels and go back out
to circulation and eventually up into the heart
from where they'll get pumped out to the body.
So this is red blood cells, and actually there's
a fancy name for red blood cells which is erythrocyte.
So let me write that here, erythrocyte.
Might be worth knowing, might not be worth knowing.
But anyway, that's another name for a red blood cell.
And actually this whole process of creating red blood cells
can be called erythropoiesis, erythropoiesis, and that just
means creation of red blood cells.
Now, platelets are made differently.
Platelets are actually fragments of cells.
And they come from a big cell called a mega, again
more long words, megakaryocyte.
The important thing about a megakaryocyte is that
it's a big cell, it has lots of cytoplasm,
that's the stuff that's not the nucleus.
And the way that platelets are made is that they
actually just kind of bud off of the megakaryocyte.
So they're bits of cytoplasm that bud off and so they are
obviously surrounded by membrane.
Every megakaryocyte can do this many, many, many, many times
and so they will release many little platelets
in this manner, and then just like the red blood cells,
these platelets are going to go into circulation
and make their way throughout the body.
So we said that red blood cells only last 120 days.
And platelets only last a couple of days.
So a question you might have is what happens to them
after that, do they just disappear?
Of course they don't just disappear.
They are in your bloodstream and your body has to
take them out, and so the organ that's most responsible
for doing this is the spleen, and I'm drawing it here.
As you can tell, it's in the left side of this guy's body.
And it's sort of on the upper part of his abdomen.
And the spleen is just one of the places that blood
goes from the heart, so the heart pumps and it sends
blood to the arms, to your muscles, to your brain,
to your legs, and it also sends some to the spleen.
And when some of that blood passes through the spleen,
and we'll draw a cartoon of that here.
When it passes through the spleen, the spleen is going
to recognize the old red blood cells, the ones that
are damaged, that are worn out.
It's going to take those out of circulation and basically
chew them up and break them down and get rid of them.
And the cells that do this are called monocytes.
So let's just draw a quick cartoon of that.
So here we have a red blood cell which is old,
so I'm going to draw it a little bit deformed.
It's a little bit old and decrepit, and it's lived a good, long
happy life, and it's time for it to go.
So in the spleen it's going to get engulfed by a cell
called a monocyte, I'll write that name here,
a monocyte, we're putting a lot of new names out here.
A monocyte is basically like a macrophage, if you've heard
of that, it's quite similar, so this monocyte in the spleen
is going to phagocytose or basically chew up this
old red blood cell and break it down
into its parts so that they can be reused.
And I will mention that one of the things that needs
to be reused and I'll actually draw it here because
I think this is important, is iron.
So red blood cells have iron in them because that's
part of hemoglobin, and your body doesn't want to lose
that iron, so the monocyte is going to recycle that iron
for further use along with many of the amino acids
that make up the proteins in the red blood cell.
So let me just write recycle because it's
important to recycle and not to waste.
Now, I will mention that most of this happens in the spleen.
Most of these red blood cells and platelets get
broken down in the spleen but it also happens a little bit
in the liver, which is an organ next to the spleen here
and the process is quite similar, so this is how you
get rid of old red blood cells and platelets.
Now, how does your body know how
many new red blood cells to make?
Well, to answer that question let's go back to
what is the primary purpose of red blood cells?
Their primary purpose is to bring oxygen out to the
different parts of the body, so if you don't have
enough oxygen, so we'll say low O two, if you don't have
enough oxygen, that means that you probably don't have
enough red blood cells, I mean, it could also mean
that your lungs aren't working or something like that.
But your body most of the time will interpret it to say,
okay, we don't have enough oxygen, that means we need
more red blood cells, so when there's low oxygen,
your body is going to produce something that's going to tell
your bone marrow to produce more red blood cells.
And that's something I'm going to write here.
And it has a similar name to these other words up here.
It's called erythropoietin, it's not erythropoiesis.
It's erythropoietin, which stimulates this whole process
and causes you to make a lot more
erythrocytes or red blood cells.
And like I said, what causes the release of erythropoietin
is low oxygen, and I should mention that that happens
in the kidney, so let me just write kidney here.
So the kidney is going to release erythropoietin.
And you might have heard of erythropoietin.
It has another name which is epo, that's abbreviated name.
And you might have heard of it because some athletes
who feel that they need more red blood cells in order to get
more oxygen to their muscles and perform better, some of
those athletes will actually take epo, they'll take epo so
that their bone marrow makes more red blood cells.
And do you know what epo might be called?
Do you know what kind of substance it
might be in our vocabulary of biology?
Well, epo is a hormone because it's a substance that
communicates between I guess the kidney, basically
really the whole body and the bone marrow.
And likewise, the platelet production system also has
a hormone that controls it, and that hormone is called
something else, it's called thrombopoietin.
And that's a much less important name than erythropoietin.
You're much less likely to ever hear thrombopoietin,
but all I want to do is make it clear that production
of platelets also is controlled by a hormone,
just like production of red blood cells.