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  • 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 equatinghormoneswith 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 asskoodilypooping.”

  • For example, people will say thathormonesare why Kevin has zits, and is being all moody,

  • or why Hannah, who's three months pregnant, just cried watching a commercial for car insurance

  • -- which, let's be honest, I do that too.

  • Now, 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 abouthormonesin the contexts that I just mentioned,

  • what really they mean is "sex hormones."

  • But sex hormones are just one kind of hormone that you have coursing through your body right now.

  • In fact, there are at least 50 different types of these chemical messengers at work in your

  • body at this very minute, but 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 hormones' constant influence.

  • They're floating through your blood, 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, ascascades.”

  • These hormones run through you no matter what your mood is, or whether you have zits.

  • So the reality is: We're all hormonal ... all of the time.

  • OK, to begin to understand our hormones -- and the endocrine system that produces, releases,

  • and re-absorbs them -- we have to step back and take a broad view.

  • Not just by emphasizing that sex hormones aren't the only hormones you have -- but also by

  • looking at how your hormones interact with your other organ systems.

  • Because, if anything, your body has two bosses -- two complementary systems that are constantly

  • shouting instructions over each other, to all of 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 electrochemical 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 an action potential.

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

  • system's organs and glands are kinda small and lumpy.

  • They're also rogues -- 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 a just any structure that makes and secretes a hormone. And 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 tissue types.

  • And 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.

  • So, thanks to these glands and organs, you've got all 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 it.

  • So, just like some keys can open many locks, while others only work with one, so too can

  • the hormone-target-cell relationship either be widespread or localized.

  • You're probably gonna 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 regulate growth and triggers sexual maturity, and it only targets specific

  • cells in the ovaries and testes.

  • So how do hormones bind to their target cells?

  • Well, 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.

  • And 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 for them 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 sites are inside their 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.

  • So, 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?

  • If you have a couple of nice, generous helpings of strawberry-rhubarb pie -- and just to make

  • things interesting, let's say they're a la mode -- your blood glucose level is gonna go through the roof.

  • And the pancreas regulates your blood sugar by releasing two different hormones -- insulin and

  • glucagon. Once you have a belly full of that pie, 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 as glycogen or as fat for later use.

  • Now, let's say you've done the opposite: You've eaten no pie -- you're 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 their 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.

  • But 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 a while 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've got a few different hormone cascades going on at 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.

  • And 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.

  • Specifically, 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 system, in times of high stress, does things like speed up your heart rate

  • and direct blood away from the digestive organs and to the muscles.

  • But many of the other effects of the stress response are carried out by your endocrine

  • system. And getting your nervous and endocrine systems to work together in times of crisis

  • is where the hypothalamus comes in.

  • It's the hub of where the two systems meet -- it keeps tabs on what's going on all

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

  • So, let's revisit our fight-or-flight scene from a few lessons ago -- the old Burning House Scenario.

  • So you're sleeping, dreaming about petting pandas with Emma Watson or whatever, when the smoke alarm

  • goes off. Well, 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 anterior pituitary

  • gland, where, because it's water soluble, it binds to receptors on the outside of its target cells.

  • There, it triggers the release of adrenocorticotropic hormone, or ACTH. The ACTH travels -- again

  • through the bloodstream -- to the adrenal cortices of the adrenal glands on top of your kidneys.

  • When the ACTH binds to receptors on cells in an adrenal cortex, it triggers the release

  • of a frenzy of different freak-out compounds known as glucocorticoid and mineralcorticoid hormones.

  • Typically these guys 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 them.

  • And because its job is to monitor and maintain balance whenever possible, it then stops secreting

  • CRH, which -- eventually -- causes the other glands to stop secreting their panic hormones.

  • Now, 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.

  • So. We're a long way from teenage crushes and zits and crying over commercials at this point, aren't we?

  • As a life-long owner of hormones, I hope you'll 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 effects your stress response.

  • Thank you to our Headmaster of Learning, Thomas Frank, and to all of our Patreon patrons who

  • help 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 Doctor Cheryl C. Kinney Crash Course Studio. This episode

  • was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon

  • Jackson. It was directed by Nicholas Jenkins, the editor is Nicole Sweeney, the script supervisor

  • was Stefan Chin, our sound designer is Michael Aranda and the graphics team is Thought Café.

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

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内分泌系、パート1 - 腺とホルモン。クラッシュコース A&P #23 (Endocrine System, part 1 - Glands & Hormones: Crash Course A&P #23)

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    alice に公開 2021 年 01 月 14 日
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