Don't take this the wrong way, but you're 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 heartbeats steady and steel rods to mimic bones; we've got drugs that can replace hormones, and antibiotics to cover for your immune system; and pretty soon you'll 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 can't 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 let's 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 percent of your body weight.

You might remember from our episodes on tissues that blood is a type of connective tissue, which means it's 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 blood's main missions is to transport and distribute oxygen and nutrients and 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 haven't figured out how to reproduce, synthesize, or imitate. It's a part of you that is literally irreplaceable.

(Intro)

It's 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 it'll 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 these 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 you've 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 you've got this thin little whitish layer in the middle. Those are your warriors, the leukocytes or white blood cells, that defend the 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'll see the yellowish plasma, which accounts for about 55 percent 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 they're 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 blood's 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 can't 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 you're slicing up 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 you're 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 breach and keeps the blood from leaking further. Now these free-floating platelets don't 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 isn't 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 that's how it's 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 can't make an effective fibrin clot, so it's not that they bleed more than anyone else, it's 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 you're 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 body's immune system is totally fine with your particular antigens, but if it detects antigens from someone else's cells - including viruses or bacteria - then it'll 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 they're 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 don't 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 doesn't produce any antibodies that label it for attack. So if you don't 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 doesn't 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 it's known as the universal donor.

But just to complicate things a little bit more, you've 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 you're Rh positive, or your don't, and are Rh-negative. Most of the population is Rh positive, so they don't 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 it's so hard to replace blood, and why True Blood... is not true. I have 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 Café.