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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 mean just
look at your body.
Hold up a hand and 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 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 of how a slice of pizza gives you energy.
Because these disciplines are really about why you’re alive right now, 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 things 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.
They’re 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, to produce everything
from a handshake to a heart attacks, but I think you’ll also start to see that you
really are more than just the sum of your parts.
We have 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, slow, and often creepy road.
The 2nd century Greek physician Galen gleaned what he could about the human form by performing
vivisections on pigs.
Da Vinci poked around 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 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
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 and etcetera 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.
For example, 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,
and 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 is 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 “achey 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
described where one body part is in relation to another.
Imagine a person standing in front of you — this is what’s called the classic anatomical
position — where the body is erect and facing straight ahead, with arms at the sides and
Now imagine slicing that person into different sections, or planes. Don’t imagine it too
The sagittal plane comes down vertically and divides a body or organ in left and right
If you imagine a plane parallel to the sagittal plane, but off to one side, that plane is
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 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 that the heart is posterior to, or behind
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 that 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 to the stomach! I warned you at the beginning: Lots of terms!
But all those terms might have just saved my life. And 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 on directional
terms, all held together with a toothpick.
Thank you 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.
This post was previously published on YouTube.
Photo credit: Screenshot from video.