The Skeletal System: It’s ALIVE! – CrashCourse Biology #30

The Skeletal System: It’s ALIVE! – CrashCourse Biology #30

This, my friends,
is a walrus baculum it’s basically a penis bone
found in most placental mammals. Interestingly, not in humans. And this is a polar bear
skull, which as you can see is more streamlined for swimming in the water than a grizzly bear skull. And over here we have my
giant friend, the rhino head, which is good for being giant,
for fighting off predators and fighting for- I don’t know, why do
rhinos have big heads? And this is the skull
of a pronghorn antelope. It has these horns that come
off, that are covered in these keratin sheaths that
fall off once a year. These are all bones. Parts of skeletons. And they’re all pretty
freaking awesome. And I am surrounded by them here at the Philip L. Wright
Zoological Museum, at the University of Montana. And all of these bones have
adapted to help animals survive, the horns on the pronghorn for
mating displays and self-defense, the streamlined skull of a polar
bear for swimming in the water, and the walrus baculum,
for, longevity, I guess. We’re used to thinking of our
skeletons as being the dead parts of us because that’s what’s
left over after all of our, like, stuff that looks
like us has rotted away. But the fact is, our bones
make up a vital organ system. And I don’t just mean vital
in that, without them you would be a sort of disgusting
dead pile of lumpy mush, but also in the traditional
meaning of vital: meaning it’s alive. It protects your vital organs. It makes locomotion possible. It manufactures your blood. And on top of it all it takes care
of its own repair and maintenance. Your skeleton is alive people. And walrus penises are
just the beginning. So you know what bones are,
but maybe you didn’t know that you don’t have to be a
vertebrate or even a chordate to have a skeleton. Jellies and worms, for instance,
have hydrostatic skeletons, made up of fluid-filled
body cavities. By squeezing muscles around the
cavities, they change their shapes, and that can be used
to produce movement. Insects have exoskeletons,of course made of the nitrogenous
carbohydrate chitin, and most mollusks have
exoskeletons, too, in the form of calcium
carbonate shells. But when it comes to skeletons,
the winningest formula has been the endoskeleton. Even though we’d probably feel
a lot safer if we were covered with armored plates like
some race of Iron Men, having skeletons inside of
our bodies has allowed us to grow larger and have much
more freedom of movement. It’s good stuff. One of the many reasons
you don’t see ants the size of horses
walking around is well, one, it wouldn’t
be able to breathe, but also, a body with
such a huge volume would require an exoskeleton
that was exponentially thicker and therefore heavier and
clumsier, to support it. So, endoskeletons allow
animals to grow larger by supporting more mass,
plus, you don’t have to worry about the embarrassment that
comes with unsightly molting! As adults, humans have 206 bones
of all kinds of shapes and sizes, including 3 tiny ones in each ear, 1 weird one shaped one like
a horseshoe in your throat, 27 in your hands,
and 26 in each foot. You also have at least 32 teeth,
unless you play too much hockey, and even though they’re
included in the skeletal system, they don’t count as bones,
because they’re made up of different material,
namely, dentin and enamel, the hardest material in your body. And you probably think of
the skull as one big bone, but it actually consists
of many separate bones, including 8 plates that cover your
brain and 14 others in your face. Face bones! So, simple, right? Well,
you might want to sit down- You probably already are,
but I’m going to, because it’s time
for Biolo-graphy! Now, you’d think that we’d
have nailed down the basics of the human skeleton
a long time ago, because our teeth and bones
are the biggest, hardest parts of our bodies, and after
we leave this mortal coil, they’re what stick
around the longest. It’s not like they’re super
hard to find and study. Surely all of those
ancient physicians who basically invented medical
science would have inventoried all of our bones pretty
soon after they figured out that we had bones. Right? If the answer was yes, do
you think I’d be sitting here? Must of what we know about
the human skeletal system is thanks to Andries Van Wesel, who was born in what’s
now Belgium in 1514. And in those days, if you were
like, a kung fu master of science, you pretty much got
your own Latin name, so today he’s known
as Andreas Vesalius. Vesalius came from a long line of
physicians to kings and emperors, and while studying in Paris,
he began dorking around in cemeteries and became
interested in what’s now known as osteology, the study of bones. Perhaps Vesalius’ greatest
contribution was showing the world that everything we thought we
knew about osteology was wrong. See, back in those days, if you
wanted to become a doctor, you didn’t study bodies or
see patients, you read stuff written by ancient Romans, whose
work was considered indisputable. Because, y’know, those guys had
long beards and they wore robes! But in his research, Vesalius
discovered that Roman texts about the skeleton,
especially the teachings of the philosopher-doctor
Galen, were way, way off. See, Roman law prohibited the
dissection of human bodies, so none of those guys ever
actually studied human innards. Instead, they dissected
apes and pigs and donkeys, and used that to make
assumptions about the human body, and so for 15 centuries,
young doctors were taught those assumptions. But Vesalius revolutionized
osteology, and all of medicine, by introducing a new practice,
every pre-med student’s favorite! Human dissection! He instructed students
by dismembering corpses in front of them and
cataloging their parts, giving students the
first opportunity ever to directly observe
the inside of the human body. These new methods drew
a lot of attention, particularly from a local judge,
who began donating bodies of the criminals he
executed to Vesalius. Suddenly, the dude was up to
his codpiece in pig thieves and murderers, and by the
time he was 28, he had done enough research that he published
De humani corporis fabrica. On the fabric of the human body, a seven-volume text
on human anatomy, including the first
comprehensive description ever made of the human skeleton. Its beautifully detailed
illustrations are thought to have been created in the studio
of the Renaissance artist Titian, featuring pictures of
flayed corpses positioned in symbolic poses,
and many of the volumes, some of which still exist today,
are bound in human skin. So, the takeaway here is that even
though bones are big and hard, the science behind them
is far from obvious. Even though we tend to think of
our bones as rigid and fixed, your skeleton is as dynamic as
any of your other organ systems. It’s built from scratch with
ingredients in your blood, it’s grown according to
glands in your head, and probably coolest of all,
it’s constantly breaking itself down and rebuilding
itself, over and over again, for as long as you live. Most new bone tissue
starts out as cartilage, which you may know from
your nose and ears. It’s made of specialized
cells called chondrocytes, and in newly forming bones,
these cells start dividing like crazy and secrete
collagen and other proteins to form a cartilage model, or
framework, for the bones to form on Soon, blood vessels work
their way into the cartilage and bring plump little
cells called osteoblasts “oste,” which you’ll be hearing a lot
of today, just means bone and
“blast” means germ or bud. The bone-building that they do is
called, fittingly, ossification. They first secrete a gelatinous goo
that’s a combination of collagen and a polysaccharide that acts
like a kind of organic glue. Then they start absorbing minerals
and salts from the blood in the capillaries all around
them, and unsurprisingly, they are especially absorbing
calcium and phosphate, and they begin depositing those
minerals onto the matrix. With the help of enzymes secreted
by the osteoblasts, these chemicals bond to
form calcium phosphate, which crystallizes to
make your bone matrix. In the end, about two-thirds
of your bone matrix is proteins like collagen, and the
other third is calcium phosphate. Kinda surprising, right? Most of your bone
isn’t even mineral, and even the part that is,
is living tissue. Because it’s all honeycombed
with blood vessels that allow osteoblasts and other
cells to do their jobs. Unlike an insect’s exoskeleton, even the hardest parts of
your bones are alive. Now, even though a bone can
take all kinds of forms, from big, flat plates
protecting your brain to the tiny stirrup in your ear, inside they all tend to have
the same basic structure. If you cut one in half,
you’d see that the matrix actually forms in two layers. The outer layer, called the
compact or cortical bone is hard and dense and makes up
about 80% of the bone’s mass. In the middle the spongy
or trabecular bone, is softer and more
porous and contains the marrow and fatty
tissues in larger bones. The marrow, of course, makes
not only new red blood cells but almost all of your
different blood cells by a process called hematopoeisis. I’d need like about a
week of your time and a Greek dictionary to
explain how it does this, but suffice it to say that
evolution has wisely chosen the innards of our largest bones
to house the blood stem cells that together can produce
1 trillion blood cells in you every day. That’s 10 to the freakin’ 12th. On the outside, the larger
bones of your body have a similar structure. Have a look here at this femur, that’s your biggest
bone in your body. The main shaft is
called the diaphysis, and each rounded
end is an epiphysis. When bones grow, as a child grows, the new tissue forms at the
border between the two, a place called the
epiphyseal plate. As they did when they formed
the original bone tissue, chondrocytes start to
produce new cartilage here, and the osteoblasts
come in and lay down more collagen and
calcium phosphate. So as you grow, the ends of
your bones are actually growing away from each other
until, by the time you’re about 25, the last of these plates
in your bones hardens. By the way, this whole
process is stimulated by growth hormones secreted
from glands all over your body. But the head honcho right
here is the pituitary gland, about the size of pea nestled
at the base of your brain. As adults, this and other glands
produce less growth hormone, which slows down
our bone lengthening. But even though lengthening is
a limited-time-only process, the thickness and strength
of bone must continually be maintained by the body. Because, of course,
like all of your cells, bone cells go through a
lot of wear and tear and need to be be able to
adjust to changing conditions. So over the course of each
year of your adult life, about 10% of your skeleton
is completely broken down, and then rebuilt from scratch in
a process called bone remodeling. Here, the main players
are the osteoblasts, again, and another kind of cell that’s
kind of their complete opposite: osteoclasts, or bone breakers. You’d think maybe that the
cells that form bone tissue and the ones that destroy it
would be in some kind of constant battle in your
body, but during remodeling, they work closely together and
actually communicate nicely. It’s like, they’re
basically frenemies. Remodeling begins when
osteoclasts are sent, by way of hormone signals,
through the capillaries to the sites of microscopic
fractures in the bone matrix. Once they’re in place, they
secrete an acidic cocktail of hydrogen ions to dissolve the
calcium carbonate into calcium ions phosphate, water, and other
material that they carry back to nearby capillaries. Then they secrete enzymes that
specialize in digesting collagen. This whole process
is called resorption, and when the old bone
tissue has been cleaned up, the osteoclasts send out a hormone
shout-out to the osteoblasts, who come in and do
their ossification thing. Bone remodeling is
really pretty amazing, and it’s all ultimately
regulated by hormones that maintain the levels
of calcium in your blood. The glands that call the
plays during the bone-breaking part of remodeling are the
parathyroids in your neck. When the calcium
in your blood plasma falls below the
level of homeostasis, the parathyroid triggers
osteoclasts to take calcium out of your bones
and release it back into the blood. Likewise, when blood calcium
levels are too high, the parathyroid’s cousin,
the thyroid gland, signals osteoblasts to take
calcium out of the blood and lay it down on the bone
collagen through more ossification. And remember last week
when we talked about how the kidneys reabsorb
salts and minerals? Well, the thyroid also
regulates how much calcium is reabsorbed in that process,
as well as the amount of vitamin D, because vitamin D helps
your body absorb calcium through the small intestine. And that is why vitamin D is
good for your bones and stuff. Now, the relation of active
osteoblasts to active osteoclasts can change dramatically
under different conditions. The more you stress your bones,
the more osteoclasts work to break down the bone matrix,
so that it can be re-formed. Bone stress can include stuff
like fractures, of course, but it can also be less
traumatic and more sustained: Exercise causes stress on the
skeleton that helps stimulate bone remodeling, so when
you’re working out, you’re not only building muscle,
you’re also building bone. So, as you can tell, it’s
kind of hard to talk about bones without also
talking about muscles, and that’s what
we’re going to do on the next episode of
Crash Course Biology. Thank you so much to the Philip
L. Wright Zoological Museum at the University of Montana. Sorry, I just hit you. Check out their tumblr at It’s awesome! If you want to review
anything: table of contents! Just click on it, or just
re-watch the whole episode, because you know you liked it. And if you have any
questions for us, of course, we will be in
the comments below, as are all of the super
helpful people who are always answering questions who are not us. Thank you to those
people by the way. And we will see you next
time on Crash Course Biology.

100 Replies to “The Skeletal System: It’s ALIVE! – CrashCourse Biology #30”

  1. I freaking LOOOOOVE bones and physiology! Thank you so so so much for this video I think there is something wrong with me because… I watched it 4 times in a row.

  2. when I study I use hanks voice in my head for reading the lecture section.. for some reasom it helps me retain more.

  3. What bothers me about people who taut Evolution as the development of everything living is that they neglect the fact that everything living could not have been created by happenstance.  Everything that functions within a system to make it as efficient as the human body could not have occurred without a designer.  The complexity of a the human cell become the complexity of tissue which becomes the complexity of an organ, ect. ect.   To say otherwise is to discount the intellect of the person who created the motherboard of the very computer that we work on.  To say that his involvement was superfluous and that given enough time the motherboard would have occurred by chance is an insult to him/them and us.   Evolution as a meaningful explanation for the result of the end product is the equivalent to "yada, yada, yada, which is why we have it now."

  4. You are awesome Hank! I aced my exam because of you and Crash Course! and I blew my teachers away with my knowledge! Nice hair style by the way.

  5. thanks you sir for your help when ever i dont understand a lecture or two in my class of biology and chemistry i try crash course most of the time soo thanks sir for your support and hardwork

  6. I have a question…. is there anywhere were we can buy these books you show in the video, like the Henle one?

  7. I really enjoy this video. Like it's great information, but I'm hesitant to use it in my 7th grade biology classroom due to the penis jokes.

  8. I have an idea of why the middle finger has become the longest fingers of our hands. where do i need to state my idea so scientists can argue about it?

  9. Love your videos. <3 I teach 7th science and would love for you to make a "JV" version for middle schoolers!

  10. So, potentially, could some sort of stimulation or change in pituitary gland expression of growth hormones and such make short people taller? Or because that's partially genetic, would the body not properly respond (either the bones grow but the rest of the body doesn't work with it, or it just doesn't grow because the body says no)?

  11. it is calcium phosphate or calcium carbonate at 10:46 ? i am kinda confused, haha. anyway, thanks so much! Hank, and crash course.

  12. I use this video in conjunction with my anatomy and physiology class to build the bones of the subject matter to my students…. 😉

  13. So, how much time does it take on average to remodel a bone? Also, during that time of remodeling, parts of our bones are basically being broken down right? So why don't we feel that? Or at least grow weak when that is happening? Like, say if my arm bone is being remodeled, how can I still lift heavey objects?

  14. C'Mon! You didn't take advantage of the fact that you were talking about the skeleton and not use Spooky Scary Skeletons in the background at any point? It would have been awesome.

  15. I love messing with the speed, if you slow it down to 0.5 Hank sounds wasted, but if you speed it up to 2 it just sounds kinda silly.

  16. I can’t wait till I finally get my degree(with crash courses help!) and I’m not flat broke just so I can support their Pateron. Thanks for all the great channels and keeping them free!

  17. Every biology teacher I saw were boring, gloomy but you're not. Thanks . I'll be more helpful if you could tell how I'll make biology easy for myself? Because I found this subject really hard. I don't know, why?

  18. ok so …can you make one in more detail that bridges the gap between the video you made on rna and dna transcription and replication? i.e. i want to see on the specific molecular level how the bone is being assembled. what are the actual segments of dna? what are the actual proteins? how does the body gather the material for it? how does it make the shape? there are many details and smaller levels that you didn't get into here. how could we reprogram the system to make the bones out of a different material?

  19. when he said ants the size of horses my head just went " if insects WERE huge would people jam rocks and stuff in their spiracles?" (like they do with exhaust pipes. ther'd have to be animal cruelty giant ant laws)because..people are dicks.. but tbh I think we'd have much bigger problems at approx 30% oxygen atmo. frankly the horse size ants alone could do it. And are we talking one ant species or likely (nearly) ALL ants or ALL insects?

  20. If it were not for Hank, I would have sunk a long time ago. He has kept me afloat!! Seriously, all of his videos are excellent! Thank you so much!

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