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- [Voiceover] Red blood cell functionality

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really is dependent on the amount

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of oxygen and carbon dioxide.

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One of the main functions
of the red blood cell

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is to transport your
oxygen from your lungs

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to your body's tissues.

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But then on the return
trip, the red blood cells

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can take up some carbon dioxide.

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Allowing the carbon dioxide,
which is a waste product

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to be taken from your
tissues and transported

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to your lungs where it can exhale it.

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So here's a picture of
your red blood cells.

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You can see they're kind of looking

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like this biconcave disc, almost like,

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a donut that the center
wasn't punched out of.

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But these are tiny, tiny, tiny things.

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What's more interesting is what's inside.

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Even though they're
called a red blood cell,

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there's no nucleus.

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The nucleus was removed before

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the red blood cell made it to maturation.

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So, what's inside instead?

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Tons and tons of hemoglobin.

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This molecule used on the
right-hand side of the screen

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is a hemoglobin molecule.

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Don't worry about trying to figure out

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what all the lines mean and everything.

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The important parts is, notice how

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there are four of those heme
groups that contain iron.

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This iron is what allows oxygen to bind,

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or for that matter, carbon dioxide.

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So since each hemoglobin molecule

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has four heme groups with four irons,

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each hemoglobin molecule can bind

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a total of four oxygen molecules.

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We can have four oxygen
molecules per hemoglobin.

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Each red blood cell will have

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hundreds and hundreds of hemoglobin.

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In one cubic millimeter, you can get

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about five million red blood cells.

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If you're wondering what five million

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red blood cells looks like,

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or how big a cubic millimeter is,

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take your thumb and index
finger and put them together.

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Now take your other finger
and put across the top

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so you form a little tiny
triangle at the very tip

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of your thumb and index finger.

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That tiny triangle is
roughly a cubic millimeter.

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It'll have about five
million red blood cells

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in that little space.

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So how do they start off?

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Your red bone marrow, which is found

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in the ends of your big
bones, your long bones,

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or it's found in your bones like

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the skull and the vertebrae.

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This red bone marrow has
stem cells inside of it.

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Stem cells are special because they

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can become pretty much anything.

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Now. this particular stem cell can become

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any type of formed element in the blood.

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As you can see, this stem cell can become

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erythrocyte or a red blood cell,

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it can become leukocytes
or white blood cells,

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or it can become the
platelets at the very bottom.

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That means this one stem cell can end up

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making a total of seven
different particles.

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It's not important to
know all the in-between

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names of everything,
what's important for us

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is knowing that the stem cell is found

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in the red bone marrow
at the end of long bones,

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and that stem cell can make all

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the white blood cells, the red blood cells

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and the platelets from the
same original stem cell.

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So we have the erythrocyte red blood cell.

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We have the five different
white blood cells:

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the neutrophil, the eosinophil,

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basophil, monocyte, lymphocyte.

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All five of those white blood cells

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have their own functionality, but they

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are all made from the same stem cell.

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Platelets, they come
from the megakaryocyte.

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That megakaryocyte just breaks pieces off

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to create these little platelets
that float in your blood.

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Life span comes up about
120 days in humans,

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so give or take, three to four months.

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It's controlled by a hormone.

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The hormone erythropoietin
is produced from the kidneys.

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This erythropoeitin goes
to the red bone marrow

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causing it to produce blood cells.

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So it's all based on O2 availability.

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If you have a low amount of oxygen

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available in your blood,
the kidney senses it

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saying "Hey, we got a problem here."

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The kidneys initiate erythropoietin

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which goes to your red bone marrow

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the red bone marrow
increases red blood cells

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then red blood cells increase in number

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meaning there's more
hemoglobin to carry oxygen,

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oxygen levels go back
up, and just like that,

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you're back to that set point,

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that normal level of O2 availability.

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It's kind of weird to think about,

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but your kidneys are really the ones

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that control your blood production.

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They're the ones that can tell if there is

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a low amount or a high
amount of O2 availability.