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

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some ugly stereotypes that 
are going around.

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

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

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and your hormones.

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

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

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

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

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

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

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

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or occasional bouts of extreme emotion
because they are.

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That's just not all that they do.

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Not even close.

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

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what they really mean is sex hormones.

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

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

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

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

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

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

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

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

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overall homeostasis that keeps you
not dead.

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

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

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as cascades.

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

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whether you have zits.

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

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♪

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To begin to understand our hormones
and the endocrine system that

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

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take a broad view.

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

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

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

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

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Both your endocrine system and
your nervous system are

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constantly trafficking information
around your corpus,

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gathering intel, making demands,
controlling your every move,

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they just have totally different
ways of doing it.

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Your nervous system uses lightning fast
electro-chemical action potentials

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

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

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

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

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that last a whole lot longer than
action potential.

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

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

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small and lumpy.

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

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

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

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

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

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

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adrenal, and pineal glands to make
their own hormones.

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

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

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

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multiple tissues types.

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

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

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

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

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

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

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

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so too can the hormone target cell
relationship either by widespread

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or localized.

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

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of your throat, produces the hormone
thyroxine which stimulates metabolism

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

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

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

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specific cells in the ovaries
and testes.

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How do hormones bind to their
target cells?

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Chemically most hormones are either
made of amino acids including their

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

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lipids like cholesterol.

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

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water soluble like most amino acid
based ones are,

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or lipid soluble like steroids are.

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

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

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

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

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

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

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either increasing or decreasing
some of its functions.

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

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

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your body out of balance?

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

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Have a couple of nice generous
helpings of strawberry rhubard

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

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

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

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insulin and glucagon.

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

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release insulin which helps lower
your blood sugar by increasing

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

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as fat for later use.

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

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

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In fact, you've eaten nothing for hours.

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

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send out glucagon which helps
raise your blood sugar levels

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

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triggering the release of glucose
back into the blood.

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Lots of different endocrine related
illnesses like diabetes or

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

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or hypo - too little secretion of
certain hormones which throw

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your homeostasis off balance.

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There are lots of more common
and less obvious ways your

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

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

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which can take awhile to subside.

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

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

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

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

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

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hypothalamic pituitary adrenal
axis, or the HPA axis

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

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

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

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immune response, and how you
handle stress.

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

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

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coordinating with your nervous system.

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

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keeps talking about.

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The HPA axis is essentially the
endocrine system's companion

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to the sympathetic nervous system.

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

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

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

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carried out by your endocrine system.

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Getting your nervous and endocrine
systems to work together in times

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of crisis, is where they hypothalamus
comes in.

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

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

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

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The old burning house scenario.

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

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and your smoke alarm goes off.

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The action potentials in your brain
trigger neurons in your hypothalamus

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

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

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

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

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adrenocorticotropic hormone or ACTH.

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

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the adrenal glands on top of you kidneys.

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

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

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known as glucocorticoid and the 
mineral ocorticoud hormone.


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

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blood sugar and blood pressure
balanced, but under major stress

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

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

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dumping glucose into your bloodstream,
shutting down non emergency services


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

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

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

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is to monitor and maintain balance
whenever possible,

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

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secreting their panic hormones.

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Because this element of the stress
response is hormonal rather than

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

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

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in the blood before being broken
down by enzymes.

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

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at this point aren't we?

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

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dispelling the stereotypes that surround
these powerful and important chemicals

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and give them the respect they 
rightly deserve.

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

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These hormones are either amino acid
based and water soluble,

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

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or just turn on specific ones.

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We also touched on hormone cascades


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and how the HPA axis affects
your stress response.

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

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Patreon patrons who help to make
crash course possible through

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their monthly contributions.

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

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

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

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This episode was written by
Kathleen Yale, edited by

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

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It was directed by 
Nicholas Jenkins,

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the editor is Nicole Sweeney,
the script supervisor was

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Stefan Chin and the sound
designer was Michael Amanda

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and the graphics team
is Thought Cafe.