pretty replaceable. When it comes to your body, science has figured
out how to hack, synthesize, or replace a surprising amount of its parts and processes. We have implants to keep heart beats steady,
and steel rods to mimic bones. Weve got drugs that can replace hormones,
and antibiotics to cover for your immune system, and pretty soon youll be able to just 3D
print a new ear if you need one.
Really! But one thing we absolutely cannot manufacture
-- despite what True Blood would have you believe -- is blood.
And yet blood is a thing that we all need. And sometimes, because of injury or illness,
we need extra blood. In fact, every two seconds, someone in the
U.S. Needs a blood transfusion.
This could be a victim of a car accident, someone undergoing
surgery, or a cancer patient who needs new blood to maintain their health during chemotherapy. And because we cant grow it on trees, or
make it in a lab, or even it store it for all that long, the blood that people need
-- nearly 16 million pints a year in the U.S. -- Has to come from people who have donated
it. So lets talk blood, shall we? The meal of choice for vampires and female
mosquitoes, blood is red, sticky, salty, and kind of metallic tasting.
It is indeed thicker than water, and super
viscous -- which is why Hitchcock used chocolate syrup as a stand-in in a certain classic shower
scene. For most purposes, blood comes in eight different
types, and it accounts for about 8% of your body weight. You might remember from our episodes on tissues
that blood is a type of connective tissue, which means its made of living cells suspended
in a nonliving matrix, which in this case is the fluid ground substance called plasma. And of course one of bloods main missions
is to transport and distribute oxygen, nutrients, waste products, and hormones around the body.
But it also helps regulate and maintain body
temperature, pH levels, and the volume of fluids in your body. Plus it protects you from
infection and from the loss of blood itself. Perhaps second only to your brain, your blood
is the one component of your body that we havent figured out how to reproduce, synthesize,
or imitate. Its a part of you that is literally irreplaceable.
Its Saturday and you feel like doing a
good deed, so you head over to your local Red Cross for a blood drive. You get your finger pricked and then somebody
directs you toward a lounge chair, swabs your inner elbow with alcohol, and then comes at
you with a hollow needle. Once the bag is full -- they usually take
about a pint -- you get unhooked and grab a cookie and a juice to replace the blood sugar you lost.
And the whole process takes around 20 minutes. But for your blood, the day is just beginning.
Soon it will be taken to a lab, where itll be tested for infectious diseases and separated
into different parts before heading out to hospitals.
So, hold up: What exactly do I mean by different
parts? Well, the blood that flows from your arm into
that bag is whole blood, a mixture of cells and cell fragments called formed elements,
along with water, and lots of dissolved molecules. A patient who needs a transfusion may only
need some of those things and not others, so the parts are separated. Once your blood makes it to a lab, technicians
put it in a centrifuge, which spins it around fast enough to send the heavier components
to the bottom of the tubes, and bring the less dense elements to the top. In the centrifuge, three distinct layers emerge.
Down at the bottom youve got a heavy red
layer of erythrocytes, or red blood cells that carry oxygen and carbon dioxide. They make
up about 45 percent of your total blood volume. Then youve got this thin little whitish
layer in the middle. Those are your warriors, the leukocytes or white blood cells, that
defend your body from toxins and foreign microbes.
And there are also the cell fragments, called
platelets, which help with blood clotting and make up less than one percent of your
blood. Finally, up at the top you see the yellowish plasma,
which accounts for about 55%of your blood volume. Plasma is actually 90 percent water, but the
other 10 percent is chock full of 100 different solutes, including proteins, electrolytes,
gases, hormones, and waste products. The most of abundant of these solutes are
electrolytes -- which you may have heard of as the secret ingredient in sports drinks.
But theyre
really just positively-charged cations -- like calcium, sodium, and potassium -- and negatively-charged
anions, like phosphate, sulfate, and bicarbonate. Together these ions help regulate your bloods
chemistry, maintaining its pH levels and proper osmotic pressure, and allowing other tissues to do their jobs, like
making muscles contract and sending action potentials. But when measured by weight, the bulk of the
solutes in your blood are really the plasma proteins. Most of these proteins -- like albumin, and
alpha and beta globulins -- are made by the liver, and do things like balance the osmotic
pressure between the blood and surrounding tissues, and transport lipids and ions.
Others run defense for you, like the gamma globulin
antibodies that are released by plasma cells during an immune response, or fibrinogen proteins, which
are vital to forming blood clots and stopping bleeding. All right, bleeding. I want to talk about that. Because, for the very reason that I mentioned
at the beginning -- that we cant replace your blood with some synthetic wonder-fluid
-- the LAST THING that your circulatory system wants is for you to fritter away your blood,
in some sidewalk scrape or kitchen accident.
So, it has a whole system in place to prevent
you from losing too much of it, through a process known as hemostasis. So imagine youre slicing a nice garlic-cheese
bagel one morning, and you lacerate the distal phalanx of your pollex -- in other words,
you cut the tip of your thumb. And now youre bleeding all over your breakfast. At the very first sign of a rupture, the blood
vessel actually constricts itself, to slow the flow of blood through it.
Then little cell fragments called platelets
gather at the site of the injury, creating a plug that dams the breech and keeps the
blood from leaking further. Now these free-floating platelets dont clump
together during regular circulation -- that would be terrible -- but when the endothelial
cells lining a blood vessel wall tear, the underlying collagen fibers are suddenly exposed.
And they chemically react with the platelets, turning them all sticky and glue-like at the
scene of the injury. But that platelet plug still isnt as strong
as it could be -- it needs reinforcement to complete the clotting process. This reinforcement comes in the form of fibrin
threads, protein strands that join together to make a sort of mesh that traps the platelets
and blood cells.
Eventually, the threads actually pull the
opposite sides of the wound together, to close the vessel wall, so the endothelial cells
can be replaced. Over a few days, the blood vessel heals, and
the blood clot dissolves. Or at least, thats how it is supposed to
happen. People who suffer from disorders related to
hemostasis may have trouble with unwanted clotting, or the inability to form clots.
In the family of disorders known as hemophilia,
a patient can usually complete the first two steps of hemostasis just fine, but they cant
make an effective fibrin clot. So its not that they bleed more than anyone else, its just that they bleed
longer. Which, I guess means that they bleed more. As a result, they may need frequent blood
transfusions throughout their lifetime.
Which brings me right back around to that
Saturday morning blood drive. Another thing youre going to need to know
before you give blood is what type you have -- do you have A, B, AB, or O? These different types all do the job equally
well, they just sort of have a different flavor related to your immune system. All the cells in your body have a plasma membrane
with specialized glycoprotein markers on them that act like name tags or labels, sort of
like This cell is Property of Hank. These markers are your antigens.
And your bodys immune system is totally
fine with your particular antigens, but if it detects antigens from someone elses
cells -- including viruses or bacteria -- then itll send out antibodies to bind to those markers, often
to tag them for destruction by the immune system. Your red blood cells have specialized antigens on
them, called agglutinogens, that activate antibodies that work by binding invading cells to each other,
which causes coagulation, or the clumping of blood. Which agglutinogens you have on your
erythrocytes defines your blood type. But theyre classified in two different ways.
In the most important blood classification
-- the kind people are most familiar with -- there are only two kind of agglutinogens,
simply A and B. And your blood can either have one, or both, or neither of these molecules. So the name of your blood type refers to what
kind you have or dont have: A-type has A antigens, B-type has B, AB has both, and
O has neither. So, why do you need to know what type you
are before you give or receive blood? Well, like I mentioned: If you have either
of these antigens, your body will be fine with it, because it doesnt produce any
antibodies that label it for attack.
So if you dont have a particular antigen
on your blood cells -- say the type B -- then you do have antibodies that are going to label those
B antigens for attack, should they enter your space. So AB-type folks are called universal recipients,
because they have both antigens, and therefore no antibodies for either. So they can accept
A, or B, or AB, or O blood. Meanwhile, O-type doesnt have A or B antigens, so those folks
have antibodies for both.
That means that they can only accept other O blood. And yet that lack of antigens means that Type
O blood can mix with other types of blood without getting attacked, which is why its
known as the universal donor. But just to complicate things a little bit
more, youve got a whole other set of antigens with totally different protocol. These are
your Rhesus, or Rh antigens, named after the species of monkey they were first identified
in.
Much like A and B, you either have the Rh
antigens, in which case youre Rh positive, or your dont, and are Rh negative. Most of the population is Rh positive, so
they dont have the anti-Rh antibodies, which means they can accept either positive
or negative blood. But negative types should stick to just the Rh negative blood. And since the presence of A-B antigens is
controlled by different genes than the Rh ones, we end up with eight different blood types --
four separate groups, each with two variations.
And now, hopefully, you understand why its
so hard to replace blood, and why True Blood is...Not true. Ive not actually ever seen
that show. Along the way, you also learned the basic
components of blood -- including erythrocytes, leukocytes, platelets, and plasma -- as well
as the basic process of hemostasis that stops bleeding, and how antigens are responsible
for the blood type that you have. Thanks to all of our Patreon patrons who make
Crash Course possible through their monthly contributions.
If you like Crash Course and
want to help us keep making it for free for everyone in the world, you can go to patreon.Com/crashcourse Also, a big thank you to Bryan Drexler
for co-sponsoring this episode. Crash Course Anatomy and Physiology is filmed
in the Doctor Cheryl C. Kinney Crash Course Studio. This episode was written by Kathleen
Yale, the script was edited by Blake de Pastino, and our consultant, is Dr.
Brandon Jackson.
It was directed and edited by Nicole Sweeney, the sound design was by Michael Aranda, and
our graphics team is Thought Cafe..
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