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When you think about the body’s infrastructure, you probably think of bones. But what about the heart, the blood vessels, and the lymphatic system? In this episode of Crash Course Biology, we’ll tour the cardiovascular, lymphatic, and musculoskeletal systems, learning how all of them keep a vertebrate’s inner workings connected, powered up, and ready to move.

Introduction: The Body's Infrastructure 00:00
Blood Vessels & Capillaries 01:23
The Heart 04:05
Vivien Thomas 05:29
The Lymphatic System 07:03
The Musculoskeletal System 08:36
Review & Credits 11:04

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CC Kids:
Bones are great.

Lots of animals have them. And the ones that don’t are either blobs,   like jellyfish, or rockin’ a hard  outside coating, like most insects.

But animals need a lot more than a  skeleton to keep their goo sacks gooin’. Inside us, there are incredible systems that act  as the body’s roads, bridges, and structures. Like, take blood vessels.

Humans have 60 thousand miles of them inside our  bodies for oxygen and nutrients to travel along. That windy internal passage is long enough  to wrap all the way around the earth twice! I think my brain is really just now coming  to grips with just how astounding that is!

This is really what parents should tell  kids when they won’t eat their veggies. “Eat your broccoli now Thurston,   so your blood vessels can grow long  enough to wrap around the earth twice.” So, today we’re going to sing the praises  of the cardiovascular and lymphatic systems,   give our muscles the chance to flex, and  sure, we’ll talk some more about bones, too. All systems that serve as infrastructure for these  soft bundles of tissues and organs we call bodies. Hi, I’m Dr.

Sammy, your friendly neighborhood  entomologist, and this is Crash Course Biology. Speaking of flexing, y’all  mind if I flex my theme music? [THEME MUSIC] After our respiratory and digestive systems have  done the job of bringing energy into our bodies,   we multicellular organisms  need a system to transport   life-giving oxygen and nutrients to all the cells. A meal delivery service, if you will.

For that, we can thank the cardiovascular  system, home to our heart and blood vessels. See, inside of all us animals with  backbones and skeletal systems,   or vertebrates, are those miles and miles  of vessels that make up our bloodstream. The bloodstream flows like a river, and  there are a lot of boats on this river,   including hormones, gases, specialized cells, and  more, all traveling to wherever they’re needed.

But perhaps the most important boat in  the bloodstream is the SS Red Blood Cell,   which carries oxygen from the lungs  to the rest of the body—and picks up   carbon dioxide on the return trip to be exhaled. Blood also helps deliver nutrients  from the digestive system to cells,   and removes waste products from those cells  to be released through the urinary system. So, the next time you have  to pee halfway into a movie,   you can thank the efficient rivers  of the cardiovascular system.

Now, red blood cells usually  don’t leave the system. If they do, it probably means something’s  gone wrong and you’re bleeding. Thankfully, animals don’t have to bleed to get  nutrients and waste in and out of the bloodstream.

Instead, we have special vessels called  capillaries that are found all over the body. You can think of it like this: if the bloodstream  is a river; capillaries are the ports. They’re the points of departure or arrival along   the bloodstream for vital  nutrients, gases, or waste.

And they’re littering the  shores of your bloodstream,   popping up anywhere that’s in need of resources. Like, after a good brunch,  capillaries in your intestines   pick up nutrients that then travel through  your bloodstream to where they’re needed. And when you breathe, capillaries in your lungs  pick up and drop off oxygen and carbon dioxide.

Capillaries are so good at this  because they’re typically super thin,   so blood is forced to slow  down as it passes through them. This allows time for nutrients and other important   stuff to pass out of the capillary’s  thin wall and into the body’s cells. Often, molecules flow automatically  from areas of high concentration to   areas of low concentration in  a process called diffusion.

Diffusion also explains why you can  sometimes smell your neighbor’s cooking. The molecules carrying the smell were  highly concentrated in their kitchen,   and they spread out from there. It’s the same principle in many of your body’s  internal systems — including your capillaries.

If there’s a high concentration of carbon dioxide  in the blood and a low concentration in your   lungs, that CO2 will pass out of the capillary  and spread into your lungs to be exhaled. The same goes for oxygen and most nutrients. That’s pretty cool, but the beating heart of your  cardiovascular system is — well, it’s your heart.

And all that blood is kept flowing by  a muscle about the size of your fist. The human heart pumps blood along two circuits:  one from the heart to the lungs and back,   which provides oxygen to the  blood and removes CO2 from it. And a second circuit goes from the heart  to everywhere in the body and back again,   to transport oxygen, nutrients, and waste.

Hearts look different in different animals. For instance, bird and mammal hearts have  four chambers for sending and receiving blood,   but reptiles have three, and fish have two. Then there are invertebrates, which  don’t have our fancy backbones,   and their hearts can vary  even more wildly from ours.

For example, octopuses have three /entire hearts/! Just one reason why I’m low-key  obsessed with these cephalopods! I’m an official card-carrying  member of the OctoNation fandom.

Regardless of spine status, all  those different hearts are doing   the same thing: pumping blood throughout the body. And this is an all-important task. Without the heart beating, the rest of the  system is kind of like an abandoned waterpark.

The tubes are all in place, but  nothing’s flowing through ‘em. So, unsurprisingly, a lot of research focuses  on how to fix the heart when things go wrong. And on that note, would you  join me for an interlude as   I’ve got two tickets to the Theater of Life… [CHAPTER 4 - VIVIEN THOMAS] When Vivien Thomas graduated high school  in 1929, he knew he wanted to be a doctor.

But that was easier said than done. Because, in the 1920s, Black Americans were  banned from many medical schools and careers. But Thomas still managed to get a foot  in the door, when a surgeon named Dr.   Alfred Blalock hired him as a research  technician just a year out of high school.

This move would change the  world of heart surgery forever. Thirteen years later, Blalock was approached by  a cardiologist interested in blue baby syndrome—a   heart complication that prevents a baby from  getting enough oxygen, so their skin turns blue. At the time, 25 percent of babies born with this   condition died before their first  birthday, and 70 percent by age ten.

Thomas came up with a procedure to fix this heart   issue and spent two years  demonstrating its safety. Then, in 1944, he coached Blalock through  the surgery, which worked perfectly. Except, when this discovery was published in   a major scientific journal,  Thomas’s name was left out.

It was just called the “Blalock-Taussig”  procedure after Blalock and the cardiologist. But many people are now starting to call  it the “Blalock-Taussig-Thomas” procedure,   to correct the erasure of Thomas’s  major role in saving lives. Thomas went on to become a surgery  instructor and supervise labs,   and in 1976, he received an  honorary doctorate degree.

The cardiovascular system — and the  heart that powers it — might be the   body’s biggest transportation network, but  animals have another transportation system. And to understand it, we have to circle  back around to those capillaries. You see, fluids from the bloodstream are always leaving through the capillaries and  traveling into the space between cells.

Here, they’re considered interstitial fluid. Cells take up what they need from the  interstitial fluid, but they don’t use it all. Kind of like how your house plant sucks up  some of the water that you pour into its soil.

But it often doesn’t use all the water;  some ends up in the tray beneath the plant. If too much water ended up in that tray,   the roots could become waterlogged  and the plant would be in trouble. And there’s a similar risk with  too much interstitial fluid.

You see, about three liters of the stuff trickles  from your capillaries into your tissues every day. And if that extra fluid doesn’t  get back into your blood,   your tissues are going to swell up, and  your blood is going to be short on fluid. So, something has to regulate this system,   making sure that extra interstitial fluid  high-tails it back into the bloodstream.

That “something” is the lymphatic  system: the cardiovascular system’s   quieter—but equally important—coworker. Through a series of vessels and  tubes all throughout your body,   the lymphatic system takes the extra interstitial  fluid and dumps it back into the bloodstream. But it’s not just the off-ramp for extra fluids.

The lymphatic system also absorbs  fat from the digestive system,   which is too big for nearby  capillaries to pick up. And it plays an important role in the immune  system, but we’ll get into that next episode. So, overall, the lymphatic system is a key  player that keeps multiple other systems running.

Sure, it’s not as marketable as the skeletal  system or as flashy as the cardiovascular,   but without it, those others  would be in a world of trouble. Now, when it comes to us vertebrates,  there’s one system that helps   everything from blowfish to buffalo  make their way through the world. That’s right, we’ve finally made our way back   to bones—the bones and muscles  of the musculoskeletal system.

This is a mash-up of two other systems  that work super closely together:   the muscular system, aka muscles, and  the skeletal system, aka the skeleton. And this combo is what allows vertebrates  to move and gives them structure. The structure part is easy enough  to see…or at least, conceptualize.

If your bones are actually visible right  now, pause the video and go see a doctor! In any case, bones provide a firm support system  for the big sack of water and salts that is you,   so that you don’t collapse into a pile of mush. But there’s more to bones than meets the eye.

Like, they can help store excess  minerals in the body and protect our   squishy internal organs – but, they’re  also the birthplace of blood cells! Yeah, blood cells! When you visit a skeleton in a museum, it can  be easy to think of bones as biology’s rocks.

But they’re made of living cells  surrounded by a super hard exterior. Just like other organs, bones  have lots of stuff inside them,   like blood vessels, tissues, and more. They’re filled with a specific  kind of tissue, called bone marrow,   that’s soft and spongy — and it’s  where most red blood cells are made!

But unlike the animatronic skeletons  in your neighbor’s Halloween decor,   bones can’t move on their own. They need muscles to get anywhere. Many muscles attach to bones,   and they’re basically playing tug-of-war  with your skeleton any time you try to move.

You see, muscles can contract and relax,   but they’re generating force only when  they’re contracting, or bunching up. In other words, they only pull  to move your body, not push. So, when you move your arm up, like  this, your muscles are pulling it up.

But when you put it back down, those  same muscles aren’t pushing it down. Different muscles are pulling it down. That’s because our muscles work in pairs.

When one contracts, the other relaxes,  allowing for some pretty coordinated movements. And most of our biggest movements happen  at joints, where two bones come together. There, groups of muscles contract and relax,  allowing you to do, well, all kinds of stuff.

I think I smell a montage. [Rocky-style training montage] Bones and muscles get a lot of the glory  for keeping our complex bodies together. But we’re so much more than that! Animals work because all of these  systems — cardiovascular, lymphatic,   musculoskeletal, and more — work together.

They’re the foundation that our bodies are built   on — not just stopping us from  being sacks of motionless goo,   but also providing our bodies with oxygen and  vital nutrients while helping us get rid of waste. On top of that, our bodies have systems  for healing and warding off infections. But more on that next time.

I’ll see you then. Peace! This series was produced in  collaboration with HHMI BioInteractive.

If you’re an educator, visit for   classroom resources and professional development  related to the topics covered in this course. Thanks for watching this episode of Crash  Course Biology which was filmed at our studio   in Indianapolis, Indiana, and was made  with the help of all these nice people. If you want to help keep Crash  Course free for everyone,   forever, you can join our community on Patreon.