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.