I've invited you all here today because
I wanted to talk to you about

some ugly stereotypes that 
are going around.

I've been hearing a lot of unfair,
unseemly and unscientific

generalizations being made lately
and they mostly have to do with sex,

and your hormones.

People have a nasty habit of equating
hormones with a particular set of behaviors

and conditions most of which have to
do with reproduction or sexual development

or acts that include what my brother John
has referred to as "scoodely poofing."

For example, people will say that hormones
are why Kevin has zits and is being all moody.

Or why Hannah, who is 3 months pregnant
just cried watching a commercial

for car insurance which, let's be honest,
I do that too.

I'm not saying that hormones aren't at
the root of sexual attraction or zits

or occasional bouts of extreme emotion
because they are.

That's just not all that they do.

Not even close.

When people talk about hormones in
the context that I just mentioned,

what they really mean is sex hormones.

Sex hormones are just one kind of hormone
that you have coursing through you

body right now and in fact there are
at least 50 different types of these

chemical messengers at work in your
body at this very minute.

Only a very few of them have anything
at all to do with sex.

The truth is, from birth to death,
just about every cell and function

in your body is under your hormone's
constant influence.

They're floating through your blood and
regulating your metabolism,

your sleep cycle, your response to stress,
and the general and incredibly important

overall homeostasis that keeps you
not dead.

Some hormones are just there to make
other hormones trigger even more hormones

in a kind of chemical relay race that
biologists refer to rather elegantly

as cascades.

These hormones run through your body 
no matter what your mood is or

whether you have zits.

The reality is, we are all hormonal,
all the time.

♪

To begin to understand our hormones
and the endocrine system that

produces, releases, and reabsorbs
them, we have to step back and

take a broad view.

Not just by emphasizing that sex hormones
are not the only hormones you have,

but also by looking at how your hormones
interact with your other organ systems.

If anything, your body has two vases,
two complimentary systems that are

constantly shouting instructions over
each other to all your bits and pieces.

Both your endocrine system and
your nervous system are

constantly trafficking information
around your corpus,

gathering intel, making demands,
controlling your every move,

they just have totally different
ways of doing it.

Your nervous system uses lightning fast
electro-chemical action potentials

delivered by an expressway made of
neurons to specific cells and organs.

But your endocrine system prefers
a slower, wider stream of data.

It secretes hormones that travel through
your blood, not through neurons

so they move more slowly, but they
also produce widespread effects

that last a whole lot longer than
action potential.

Now, compared to your heart, or brain,
or other arguably more glamorous

organs, your endocrine system's
organs and glands are kind of

small and lumpy.

They are also rogues and instead of being
all nestled together like in your other

organ systems, these guys are scattered
all over the place from your brain

to your throat, to your kidneys,
to your genitals.

A gland is just any structure that makes
and secretes a hormone.

The master gland in your body, 
is the pituitary which produces

many hormones that signal other
glands like the thyroid, parathyroid,

adrenal, and pineal glands to make
their own hormones.

The endocrine system also includes
a few organs like the gonads,

the pancreas, and the placenta
in pregnant women, all of which

have some other non-hormonal
functions and are made up of

multiple tissues types.

Technically the hypothalamus in your brain
is in the endocrine club too

since in addition to all of its busy brain
duties, it does produce and release hormones.

Thanks to all these glands and organs
you've got these hormones diffusing

through your blood doing all sorts of
different things, but the thing

to remember about them is that a hormone
can only trigger a reaction in specific cells.

Their so called target cells that have
the right receptors for them.

Just like some keys can open many locks,
while others only work with one,

so too can the hormone target cell
relationship either by widespread

or localized.

You're probably going to want an example
of that, so your thyroid at the bottom

of your throat, produces the hormone
thyroxine which stimulates metabolism

and binds to receptors in most of the
cells in your body but your pituitary

which is nestled all comfy under your brain,
produces follicle stimulating hormone

which helps regulates the growth and triggers
sexual maturity and it only targets

specific cells in the ovaries
and testes.

How do hormones bind to their
target cells?

Chemically most hormones are either
made of amino acids including their

more complex structures like peptides
or proteins, or they're derived from

lipids like cholesterol.

This is key because a hormone's
chemical structure determines if it's

water soluble like most amino acid
based ones are,

or lipid soluble like steroids are.

Solubility is important because your
cell membranes are made of lipids.

That means that water soluble ones
can't get across them so target cells

for those kinds of hormones have
receptors on the outside of their membranes.

Lipid soluble hormones on the other hand
can just basically glide right through that

cell membrane so their receptor sights
are inside the target cells.

Either way, when a target cell is activated
the hormone alters its activity by

either increasing or decreasing
some of its functions.

Usually with the goal of maintaining your
body's homeostasis in one way or another.

If hormones are keeping your body
in balance, what's putting

your body out of balance?

I don't know ... Could I interest
you in some pie?

Have a couple of nice generous
helpings of strawberry rhubard

pie and just to make things interesting,
let's say that they are ala mode,

your blood glucose level is going to
go through the roof.

The pancreas regulates your blood sugar
by releasing two different hormones:

insulin and glucagon.

Once you have a belly full of that pie,
the beta cells in your pancreas

release insulin which helps lower
your blood sugar by increasing

the rate at which your cells store the
sugar either a glycogen or

as fat for later use.

Now let's say you've done the opposite.

You've eat no pie, you are pie-less.

In fact, you've eaten nothing for hours.

If your blood sugar drops too low then
alpha cells in the pancreas will instead

send out glucagon which helps
raise your blood sugar levels

in part by decreasing the storage
of sugar in your cells and

triggering the release of glucose
back into the blood.

Lots of different endocrine related
illnesses like diabetes or

hyperthyroidism tend to be the
result of either hyper - too much,

or hypo - too little secretion of
certain hormones which throw

your homeostasis off balance.

There are lots of more common
and less obvious ways your

hormones can get out of balance,
not because of some disorder,

but because these signaling chemicals
are just caught up in a chain reaction

which can take awhile to subside.

Some hormones just exist to control
other hormones, which in turn

control still more hormones so as
soon as one starts to trickle out,

you can pretty quickly wind up with
a cascade on your hands.

You get a few different hormone cascades
going on in any given moment but

one of the big ones, one that's really
worth understanding is the

hypothalamic pituitary adrenal
axis, or the HPA axis

because you don't want to have to
say that every time.

This is a complex series of interactions
between three glands that ultimately

regulates lots of your body's daily
processes like digestion, sexuality,

immune response, and how you
handle stress.

It's complex not just because of all
the glands involved,

it's also one of the more crucial
instances of your endocrine system

coordinating with your nervous system.

Presumably it's behind that "fight
or flight" response that everybody

keeps talking about.

The HPA axis is essentially the
endocrine system's companion

to the sympathetic nervous system.

The sympathetic nervous system in
times of high stress does things like

speed up your heart rate and direct
blood away from your digestive organs

into the muscle so many of the other
effects of the stress response are

carried out by your endocrine system.

Getting your nervous and endocrine
systems to work together in times

of crisis, is where they hypothalamus
comes in.

It's the hub of where the two systems meet
and keeps tabs on what's going on

all over your body, analyzing your blood
for signs that something might be off.

Let's revisit our fight or flight scene
from a few lessons ago ...

The old burning house scenario.

So you're sleeping, you're dreaming
about petting pandas or whatever

and your smoke alarm goes off.

The action potentials in your brain
trigger neurons in your hypothalamus

to release the peptide hormone CRH,
or corticotropin-releasing hormone.

The CRH makes the very short trip
through the bloodstream to the

interior pituitary gland where because
it is water soluble, it binds to receptors

on the outside of the target cells,
there it triggers the release of

adrenocorticotropic hormone or ACTH.

The ACTH travels again through the
blood stream into the adrenal cortisis of

the adrenal glands on top of you kidneys.

When the ACTH binds to receptors on
cells in the adrenal cortex,

it triggers the release of a frenzy of 
different freak-out compounds

known as glucocorticoid and the 
mineral ocorticoud hormone.

These hormones help us deal with
day to day stress by keeping our

blood sugar and blood pressure
balanced, but under major stress

like waking up in a burning building stress,
these hormones like cortisol cause the

classic fight or flight response, ramping up
your blood pressure,

dumping glucose into your bloodstream,
shutting down non emergency services

like your immune system and sperm
and egg development, and guess what?

Now that all these stress hormones are
pulsing through your blood,

the hypothalamus back in the brain
senses those and because its job

is to monitor and maintain balance
whenever possible,

it stops secreting CRH which eventually
causes the other glands to stop

secreting their panic hormones.

Because this element of the stress
response is hormonal rather than

electrical, it comes on more slowly
than the nervous system part

and it takes longer to subside too 
as those stress hormones linger

in the blood before being broken
down by enzymes.

We're a long way from teenage crushes and
zits and crying over commercials,

at this point aren't we?

As a lifelong owner of hormones,
I hope you will join me in

dispelling the stereotypes that surround
these powerful and important chemicals

and give them the respect they 
rightly deserve.

Today we looked at the endocrine system
and how it uses glands to produce hormones.

These hormones are either amino acid
based and water soluble,

or steroidal and lipid soluble and may
target many types of cells

or just turn on specific ones.

We also touched on hormone cascades

and how the HPA axis affects
your stress response.

Thank you to our headmaster of learning,
Thomas Frank and to all of our

Patreon patrons who help to make
crash course possible through

their monthly contributions.

If you like Crash Course and you want to
help us keep making free educational

content for the whole world,
you can go to Patreon.com/crashcourse.

Crash Course is filmed in the
Dr. Cheryl C. Kenny Crash Course studio.

This episode was written by
Kathleen Yale, edited by

Blake de Pastino and our consultant
was Dr. Brendan Jackson.

It was directed by 
Nicholas Jenkins,

the editor is Nicole Sweeney,
the script supervisor was

Stefan Chin and the sound
designer was Michael Amanda

and the graphics team
is Thought Cafe.