I’d like you to take a second and really look at yourself. I don’t mean take stock of your life, which really isn’t any of my business, but I just mean just look at your body. Hold up your hand. Wiggle it around. Take a sip of water. Hold your breath. Sniff the air.

These things are so simple for most of us that we don’t give them a moment’s thought. But each one of those things is oh so much more complex than it feels. Every movement you make, every new day that you live to see, is the result of a collection of systems working together to function properly. In short, you, my friend, are a magnificent beast. You are more convoluted and prolific and polymorphously awesome than you probably even dare to think.

For instance, did you know that, if they were all stretched out, your intestines would be about as long as a three story building is tall? Or that by the time you reach old age, you’ll have produced enough saliva to fill more than one swimming pool? Or that you lose about two-thirds of a kilogram every year in dead skin cells? And you will lose more than 50 kilograms of them in your lifetime. Just tiny, dried-up pieces of you, drifting around your house and settling on your bookshelves, feeding entire colonies of dust mites. You’re your own little world.

And I’m here to help you get to know that world, the body that you call a home, through the twin disciplines of anatomy, the study of the structure and relationships between body parts, and physiology, the science of how those parts come together to function and keep that body alive. 

Anatomy is all about what your body is, physiology is about what it does, and together, they comprise the science of us. It’s a complicated science, I’m not gonna lie to you, and it draws on a lot of other disciplines, like chemistry and even physics. And you’ll have to absorb a lot of new terms - lots of Latin, gobs of Greek.

But this course isn't just gonna be an inventory of your individual parts, or a diagram about how a slice of pizza gives you energy, because these disciplines are really about why you’re alive right now and how you came to be alive, how disease harms you, and how your body recovers from illness and injury.

It's about the big-picture stuff that we either spend most of our time thinking about, or trying not to think about: death, and sex, and eating, and sleeping, and even the act of thinking itself, all processes that we can understand through anatomy and physiology.

If you pay attention, and if I do my job well enough, you’ll come out of this course with a richer, more complete understanding not only of how your body works, everything from handshakes to heart attacks, but I think you’ll also start to see that you really are more than just the sum of your parts.

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We've come to understand the living body by studying a lot of dead ones. And for a long time, we did this mostly in secret. For centuries, the dissection of human bodies was very taboo in many societies, and as a result, the study of anatomy has followed a long and slow and often creepy road.

The second-century Greek physician Galen gleaned what he could about the human form by performing vivisections on pigs. Da Vinci poked around in dead bodies while sketching his beautifully detailed anatomical drawings until the Pope made him stop.  

It wasn’t until the 17th and 18th centuries that certified anatomists were allowed to perform tightly regulated human dissections, and they were so popular that they were often public events, with admission fees, attended by the likes of Michelangelo and Rembrandt. 

The study of human anatomy became such a craze in Europe that grave-robbing became a lucrative, if not legal, occupation until 1832, when Britain passed the Anatomy Act, which provided students with plentiful corpses in the form of executed murderers.

Today, students of anatomy and physiology still use educational cadavers to learn, in person and hands-on, what’s inside a human body by dissecting them. And it’s totally legal. The cadavers are volunteers, which is what people mean when they say they’re “donating their body to science.” 

So what have all of these dead bodies shown us?

Well, one big idea we see over and over is that the function of a cell or an organ or a whole organism always reflects its form. Blood flows in one direction through your heart simply because its valves prevent it from flowing backward. In the same way, your bones are strong and hard and this allows them to protect and support all your soft parts.

The basic idea - that what a structure can do depends on its specific form - is called the complementarity of structure and function. And it holds true through every level of your body’s organization, from cell to tissue to system.

And it begins with the smallest of the small: atoms. Just like the chair you’re sitting on, you are just a conglomeration of atoms -- about 7 octillion of them, to be precise. Fortunately for both of us here, we've covered the basics of chemistry that every incoming physiology student needs to know in Crash Course Chemistry. So I’ll be referring you there throughout the course when it comes to how things work at the atomic level.

But the next level up from the chemistry of atoms and molecules includes the smallest units of living things - cells. All cells have some basic functions in common, but they also vary widely in size and shape, depending on their purpose. For example! One of the smallest cells in your body is the red blood cell, which measures about 5 micrometers across. Now contrast that with the single motor neuron that runs the length of your entire leg, from your big toe to the bottom of your spine, about a meter from end to end. 

Typically, cells group with similar cells to form the next level of organization: tissues, like muscles, membranes and cavity linings, nervous, and connective tissues. When two or more tissue types combine, they form organs -- the heart, liver, lungs, skin, etc. that perform specific functions to keep the body running.

Organs work together and combine to get things done, forming organ systems. It’s how, like, the liver, stomach, and intestines of your digestive system all unite to take that burrito from plate to pooper.

And finally, all those previous levels combine to form the highest level of organization - the body itself. Me and you and your dog - we’re all glorious complete organisms, made from the precise organization of trillions of cells in nearly constant activity.

This ability of all living systems to maintain stable, internal conditions no matter what changes are occurring outside the body is called homeostasis, and it’s another major unifying theme in anatomy and physiology. Your survival is all about maintaining balance of both materials and energy. You need the right amount of blood, water, nutrients, and oxygen to create and disperse energy, as well as the perfect body temperature, the right blood pressure, efficient movement of waste through your body, all that needs to stay balanced.

And by your survival depending on it? I mean that everyone’s ultimate cause of death is the extreme and irreversible loss of homeostasis. Organ failure, hypothermia, suffocation, starvation, dehydration - they all lead to the same end, by throwing off your internal balances that allow your body to keep processing energy.
 
Take an extreme and sudden case - your arm pops off. If nothing’s done quickly to treat such a severe wound, you would bleed to death, right? But what does that really mean? What's gonna happen? How do I die?

Well, that arterial wound, if left untreated, will cause a drastic drop in blood pressure that, in turn, will prevent the delivery of oxygen throughout the body. So the real result of such an injury - the actual cause of death - is the loss of homeostasis.
 
I mean, you can live a full and healthy life without an arm. But you can’t live without blood pressure, because without blood, your cells don’t get oxygen, and without oxygen, they can’t process energy, and you die.

With so many connected parts needed to make your life possible, you can see how we need a hyper-precise language to identify the parts of your body and communicate what’s happening to them. A doctor isn't gonna recommend a patient for surgery by telling the surgeon that the patient has an “achy belly.”

They’re going to need to give a detailed description - essentially, it's like a verbal map. So, over time, anatomy has developed its own standardized set of directional terms that describe where one body part is in relation to another.

Imagine a person standing in front of you. This is what is called the classic anatomical position, where the body is erect and facing straight ahead, with arms at the sides and palms forward. 

Now imagine slicing that person into different sections, or planes. Don't imagine it too graphically, though. The sagittal plane comes down vertically and divides a body or organ in left and right parts. If you imagine a plane parallel to the sagittal plane, but off to one side, that plane is the parasagittal. The coronal or frontal plane splits everything vertically into front and back, and the transverse or horizontal plane divides the body top and bottom. 

Look at that body again and you’ll notice even more divisions, like the difference between the axial and appendicular parts. Everything in line with the center of the body, the head, neck, and trunk, are considered axial parts, while the arms and legs, or appendages, are the appendicular parts that attach to the body’s axis.
 
Everything at the front of your body is considered anterior, or ventral, and everything in the back is posterior, or dorsal. So your eyes are anterior, and your butt is posterior, but you’d also say that your breastbone is anterior to, or in front of, the spine, and the heart is posterior to, or behind, the breastbone.

Features toward the top of your body, like your head, are considered superior, or cranial, while structures that are lower down are inferior, or caudal. So the jaw is superior to the lungs because it’s above them, while the pelvis is inferior to the stomach because it’s below it.

And, there's more: if you imagine the center line running down the axis of a body, structures toward that midline are called medial, while those farther away from the midline are lateral. So the arms are lateral to the heart, and the heart is medial to the arms.
 
Looking at the limbs - your appendicular parts of your body - you’d call the areas closer to the center of the trunk proximal and those farther away distal. In anatomy-talk, your knee is proximal to your ankle because it’s closer to the axial line, while a wrist is distal to the elbow because it’s farther from the center. 

Okay, so pop quiz! 

I’m eating a club sandwich -- I'm not, I wish I was, but imagine I am. I’m so ravenous and distracted that I forget to take out that little frilly toothpick at the top, and I end up swallowing it with a raft of turkey, bacon, and toast. A fragment of the toothpick gets lodged somewhere in here, and my doctor takes an x-ray and says I need surgery. Using anatomical language, how would she direct the surgeon to that tiny wooden stake inside of me? 

She might describe it as being “along the medial line, posterior to the heart, but anterior to the vertebrae, inferior to the collarbone, but superior to the stomach.” That would give the surgeon a pretty good idea of where to look -- in the esophagus, just above the stomach.

I warned you at the beginning, lots of terms! But all those terms might have just saved my life. And hey, it’s the end of your first lesson, and you’ve already started to talk the talk.

Today you learned that anatomy studies the structure of body parts, while physiology describes how those parts come together to function. We also talked about some of these disciplines’ central principles, including the complementarity of structure and function, the hierarchy of organization, and how the balance of materials and energy known as homeostasis is really what keeps you alive. And then we wrapped it all up with a primer in directional terms, all held together with a toothpick.

Thanks for watching, especially to our Subbable subscribers, who make Crash Course available not just to themselves, but also everyone else in the world. To find out how you can become a supporter, just go to Subbable.com.

This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. Our director and editor is Nicholas Jenkins, the script supervisor is Valerie Barr, the sound designer is Michael Aranda, and the graphics team is Thought Café.

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