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  • Don’t take this the wrong way, but youre 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 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% 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, 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

  • haven’t figured out how to reproduce, synthesize, or imitate.

  • It’s a part of you that is literally irreplaceable.

  • 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 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 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 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 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 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 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 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 likeThis 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

  • 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 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, 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 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’ve 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.

Don’t take this the wrong way, but youre pretty replaceable.

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ブラッド 第1部 トゥルーブラッドクラッシュコースA&P #29 (Blood, Part 1 - True Blood: Crash Course A&P #29)

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    Darya kao に公開 2021 年 01 月 14 日
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