Download CrashCourseBiology30SkeletalSystem-1

Name: _____________________________
Date: ________________ Period: ______
The Skeletal System – It’s Alive!
Crash Course Biology #30
1. List some functions of the skeletal system.
2. Skeletons have multiple forms, including:
a. _______________________ skeletons – jellies and worms
b. _________________________ – insects and molluscs
c. __________________________ – skeleton inside the body
3. Endoskeletons allow animals to grow ____________ by supporting more mass.
4. Adult humans have ______ bones, including:
__ tiny ones in each ear
____ in your hands
__ shaped like a horseshoe in your throat
__ skull bones
____ face bones
Hu
ma
leas ns al
so
t 32
ha
the
y ar teeth, ve at
en’t
but
bon
es!
5. What is the study of bones called?
6. Most new bone tissue starts out as ___________________ and is made of specialized cells called
_____________________________. These cells divide and secrete collagen and other proteins to
form a cartilage model, or framework, for the bones to form on.
ost
e-
7. Blood vessels work their way into the cartilage and bring plump little cells called
or
ost
eo=b
one
________________________. The bone-building that they do is called _________________________:
a. secrete a combination of __________________ and a polysaccharide
b. absorb ___________________ and salts from the blood in all the capillaries around them,
especially calcium and phosphate, and they begin depositing those minerals onto the
matrix
c. With the help of enzymes secreted by the osteoblasts, these chemicals bond to form
calcium phosphate which crystallizes to make your ________ _______________
d. about two-thirds of your bone matrix is proteins, like collagen, and the other third is
_______________ __________________
8. Bones tend to have the same basic structure. The matrix actually forms in two layers
a. outer layer, called the _________________ or cortical bone, is hard and dense and
makes up about 80% of the bone's mass
b. In the middle, the _________________ or trabecular bone, is softer, and more porous, and
contains the marrow and fatty tissues in larger bone
9. The marrow makes almost all blood cells, including new red blood cells, by a process called
_____________________________.
10. On the outside, the larger bones of your body have a similar structure, a main shaft and
rounded ends. As a child grows, the new tissue forms at the border between the two, a place
called the ____________________ _________
11. On the diagram of a long bone, label the diaphysis and the epiphyses.
12. What gland in the body is the primary producer of growth hormone?
13. The thickness and strength of the bone must continually be maintained
by the body. Over the course of each year of your adult life, about ____%
of your skeleton is completely broken down and then rebuilt from scratch,
in a process called ________ _________________________.
14. In bone remodeling, osteoblasts (bone-makers) and
_________________________ (bone-breakers) work closely together.
15. Osteoclasts work at sites of microscopic fractures in the bone matrix:
a. secrete acidic ________________ ions to dissolve the calcium
phosphate
b. secrete ______________ that digest collagen
c. this process is called ______________________
d. regulated by the ________________________ glands in your neck
16. Osteoclasts and osteoblasts also maintain proper blood calcium.
a. When the ______________ 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.
b. When blood calcium levels are too high, the _______________ gland signals osteoblasts
to take calcium out of the blood and lay it down on the bone collagen through more
ossification.
c. The thyroid also regulates how much calcium and vitamin ___ are reabsorbed by the
_______________.
Calcitonin promotes calcium deposit in bone by
stimulating _________________________ activity
PTH activates _________________________
activity to degrade bone and release calcium
into the blood
When blood calcium levels drop,
the _______________________ glands
release parathyroid hormone (PTH)
18. After the video, write osteoblast,
osteoclast, parathyroid, and thyroid
in the appropriate blanks of the
diagram at right.
When blood calcium levels rise,
the ________________ gland
releases calcitonin
17. The relation of active osteoblasts to active osteoclasts can change. The more you ____________
your bones, the more your osteoclasts work to break down the bone matrix so that it can be
reformed.
ANSWERS
The Skeletal System – It’s Alive!
Crash Course Biology #30
1. List some functions of the skeletal system. Provides structural support, protects organs, makes
locomotion/movement possible, manufactures blood, repairs itself
2. Skeletons have multiple forms, including:
a. hydrostatic skeletons – jellies and worms
b. exoskeletons – insects and molluscs
c. endoskeleton – skeleton inside the body
3. Endoskeletons allow animals to grow larger by supporting more mass.
4. Adult humans have 206 bones, including:
3 tiny ones in each ear
27 in your hands
1 shaped like a horseshoe in your throat
8 skull bones
14 face bones
Hu
ma
leas ns al
so
t 32
ha
the
y ar teeth, ve at
en’t
but
bon
es!
5. What is the study of bones called? Osteology
6. Most new bone tissue starts out as cartilage and is made of specialized cells called
chondrocytes. These cells divide and secrete collagen and other proteins to form a cartilage
Os
model, or framework, for the bones to form on.
te
-/
Os
teo
-
7. Blood vessels work their way into the cartilage and bring plump little cells called osteoblasts.
The bone-building that they do is called ossification:
a. secrete a combination of collagen and a polysaccharide
b. absorb minerals and salts from the blood in all the capillaries around them, especially
calcium and phosphate, and they begin depositing those minerals onto the matrix
c. With the help of enzymes secreted by the osteoblasts, these chemicals bond to form
calcium phosphate which crystallizes to make your bone matrix.
d. about two-thirds of your bone matrix is proteins, like collagen, and the other third is
calcium phosphate.
8. Bones tend to have the same basic structure. The matrix actually forms in two layers
a. outer layer, called the compact or cortical bone, is hard and dense and makes up
about 80% of the bone's mass
b. In the middle, the spongy or trabecular bone, is softer, and more porous, and contains
the marrow and fatty tissues in larger bone
9. The marrow makes almost all blood cells, including new red blood cells, by a process called
hematopoiesis.
= B
one
10. On the outside, the larger bones of your body have a similar structure, a main shaft and
rounded ends. As a child grows, the new tissue forms at the border between the two, a place
called the epiphyseal plate.
11. On the diagram of a long bone, label the diaphysis and the epiphyses.
12. What gland in the body is the primary producer of growth hormone? Pituitary gland at the
base of the brain
13. The thickness and strength of the bone must continually be
maintained by the body. 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.
epiphysis
14. In bone remodeling, osteoblasts (bone-makers) and osteoclasts
(bone-breakers) work closely together.
diaphysis
15. Osteoclasts work at sites of microscopic fractures in the bone
matrix:
a. secrete acidic hydrogen ions to dissolve the calcium
phosphate
b. secrete enzymes that digest collagen
c. this process is called resorption
d. regulated by the parathyroid glands in your neck
epiphysis
16. Osteoclasts and osteoblasts also maintain proper blood calcium.
a. 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.
b. When blood calcium levels are too high, the thyroid gland signals osteoblasts to take
calcium out of the blood and lay it down on the bone collagen through more
ossification.
c. The thyroid also regulates how much calcium and vitamin D are reabsorbed by the
kidneys.
17. The relation of active osteoblasts to active osteoclasts can change. The more you stress your
bones, the more your osteoclasts work to break down the bone matrix so that it can be
reformed.
Calcitonin promotes calcium deposit in bone by
stimulating osteoblast activity
PTH activates osteoclast activity to degrade
bone and release calcium into the blood
When blood calcium levels drop,
the _parathyroid glands release
parathyroid hormone (PTH)
When blood calcium levels rise,
the thyroid gland releases calcitonin
18. After the video, write osteoblast, osteoclast, parathyroid, and thyroid in the appropriate blanks
of the diagram below.
Crash Course Biology #30
The Skeletal System – It’s Alive!
Next Gen Science Standards (High School Life Sciences):
DCI LS1.A:
• Systems of specialized cells within organisms help them perform the essential functions of life.
(HS-LS1-1)
• Multicellular organisms have a hierarchical structural organization, in which any one system is
made up of numerous parts and is itself a component of the next level. (HS-LS1-2)
• Feedback mechanisms maintain a living system’s internal conditions within certain limits and
mediate behaviors, allowing it to remain alive and functional even as external conditions
change within some range. Feedback mechanisms can encourage (through positive
feedback) or discourage (negative feedback) what is going on inside the living system. (HSLS1-3)
Image Attribution
Thyroid and Parathyroid Gland. By National Institute of Diabetes and Digestive and Kidney Diseases
(NIDDK)
Long Bone Structure. Cropped from Rice University Image [CC BY-SA 4.0]. Accessed at
https://opentextbc.ca/anatomyandphysiology/chapter/6-4-bone-formation-and-development/.
Video Info: This video can be accessed via YouTube: https://youtu.be/RW46rQKWa-g
Video length: 13:10. Using the table of contents provided by CrashCourse with the YouTube video,
you can click and watch certain sections as desired.
Transcript (retrieved from https://nerdfighteria.info/v/RW46rQKWa-g/ and edited and re-formatted):
This, my friends, is a walrus baculum. It's basically a..a penis bone, found in most placental mammals,
interestingly not in humans. And this 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, uh, good for being giant, or 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 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.
<Intro music>
Endoskeleton
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
shape, and that can be used to produce movement. Insects have exoskeletons of course, made of
the nitrogenous carbohydrate chitin, and 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 in armored plates like some race of
iron men, having our 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 heaver 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,
and 1 weird 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!
Bio-Lography (3:27)
Now, you'd think that we'd have nailed down the basics of the human skeleton a long time ago,
because our teeth and our bones are the biggest and 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?
Most 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, 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's greatest contribution was showing the world that everything we though 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, you
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 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 a 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'd 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.
New Bone Formation (6:13)
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 other of your 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 your 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. First, they secrete this gelatinous goo that's a
combination of collagen and a polysaccharide that act kind of like an organic glue. Then, they start
absorbing a bunch of minerals and salts from the blood in all the capillaries around them. And,
unsurprisingly, they're 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.
Bone Structure (8:00)
Now, even though bone can take all kinds of forms, from big, flat plates protecting the 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 bone. The marrow, of course, makes not only new red blood cells, but almost all of
your different blood cells by a process called hematopoiesis. I'd need like, 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 one 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 the biggest bone in your body. The main shaft is called the diaphysis, and each rounded end is
an epiphysis. The 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, and 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 a pea, nestled at the
base of your brain. As adults, this and other glands produce less growth hormone which slows down
our bone lengthening.
Bone Remodeling (9:47)
But, even though lengthening is a limited-time-only process, the thickness and strength of the 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 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, the 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 to 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 phosphate, and the calcium,
phosphates, and 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 level of calcium in your blood. The glands that call all the plays during the bone-breaking part of
the 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 all...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 your osteoclasts work to break down the bone
matrix so that it can be reformed. 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 kinda hard to talk about bones without also talking about muscles. And that's
what we're gonna 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 http://umzoology.tumblr.com. 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. If you have any
questions for us, of course, we will be in the comments below, as will 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!