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Intro to the endocrine system | Health & Medicine | Khan Academy

February 9, 2020

SALMAN KHAN: I’m here at
Stanford Medical School with Neil Gesundheit,
who’s a faculty member here at the med school. NEIL GESUNDHEIT: Hi, Sal. SALMAN KHAN: So, what are
we going to talk about? NEIL GESUNDHEIT: Well,
the topic for today is endocrinology, which
is the study of hormones. And the word hormone is
derived from the Greek word which means arouse the activity. And what hormones do is they’re
chemical messengers that are made at one part of
the body and typically go to another part of the
body to, as suggested, arouse the activity and give
function to another organ. SALMAN KHAN: So
they’re essentially kind of signaling,
a way to communicate between one part of
the body and the other. NEIL GESUNDHEIT: Exactly. They’re very sophisticated
communicators. I think that’s a perfect term. And I think the other
way to think of it is our body communicates
in some ways directly. For instance, nerves
innervate muscle. And when you want to
contract your muscle, you give a signal from your
brain, it goes down the nerve, and it directly
attaches to the muscle and causes it to contract. Whereas, hormones are more like
the Wi-Fi of the human body. They’re wireless. They are made at one place. They go into the bloodstream,
which is like the airwaves, if you will. And then they work on another
part of the body at a distance, without directly connecting
to that part of the body mechanically. SALMAN KHAN: And hormones, are
they a specific type of protein or a specific type of chemical? Or are they really anything that
does what you just described? NEIL GESUNDHEIT: It’s
pretty much anything, but they fall into
two major categories. There are small
molecules that typically derive from amino acids. And those molecules
are just 300 to 500, at most, daltons, which are
molecular mass units, up to large proteins
that can be hundreds and hundreds of
amino acids in size. SALMAN KHAN: I see. So anything, anything that
really has this signaling function. NEIL GESUNDHEIT: That’s right,
would be considered a hormone. And the other thing is
we talk about hormones in three sort of subcategories. We call some of them
endocrine hormones, where they really get
into the bloodstream and work at a far distance. And we’ll give some
examples with your diagram right there in just a minute. But there are others that are
called paracrine hormones. And paracrine hormones are
more regionally active. So they might be made, let’s
say, in one part of the body and work within a small distance
of that site of synthesis. And then the third category,
which is less common, would be autocrine hormones. And the autocrine
hormones are actually made directly at one cell
and work on that same cell or in this cell right next door,
at a very, very small distance. SALMAN KHAN: I see. So, the endocrine
hormones, I think I have a mental model for it. They’re kind of released far
away in the body someplace. If they’re picked up
by the right receptor, they’ll have the right function. The paracrine hormones,
is their effect small because they only are able
to travel a small distance? Or is it something else? NEIL GESUNDHEIT: Typically
the paracrine hormones do get into the bloodstream,
but the concentration of the receptor, the receiving
end, as you suggested, is right close by. So what tends to make
a paracrine hormone work regionally is that the high
concentration of the receptors are very close to the
site of synthesis. And the same with autocrine,
is often they’re made, and there’s a very high
concentration of the receiving end right at that cell,
right next to that cell. SALMAN KHAN: And this
might be a silly question, but it’s called endocrinology. Are there paracrinologists? NEIL GESUNDHEIT: Well,
it’s a good point. I don’t think so. I think we just, perhaps
because the paracrine function of hormones was
discovered later, we still carry this all under
the umbrella of endocrinology. SALMAN KHAN: Right. So all of hormones
is endocrinology, even though endocrine
hormones are the ones that act
at far distances. NEIL GESUNDHEIT: That’s right. I think that’s a good
way to summarize it. Now I like the diagram
that you created here because it illustrates some
of the major endocrine organs, the ones we’ll be focusing
on in later lectures. So the first one that you
showed very nicely in the head, at the base of the brain,
is that orange structure. And that would be
the pituitary gland. That’s right. And the pituitary gland
is called the master gland because from the
pituitary, we make hormones that work on yet other organs. So I’ll give you an example. One of the hormones that’s
made by the pituitary is called thyroid
stimulating hormone, or TSH. And after it leaves
the pituitary, it goes into the
circulation and it acts on the thyroid
gland, where there are high receptors for TSH
on the surface of the thyroid cells. And it stimulates the thyroid
gland to make thyroid hormone, typically thyroxine T4
or triodothyronine, T3. Those would be the two main
circulating thyroid hormones. SALMAN KHAN: And
what do those do? NEIL GESUNDHEIT: Those
regulate metabolism, they regulate appetite,
they regulate thermogenesis, they regulate muscle function. They have widespread activities
on other parts of the body. SALMAN KHAN: But it kind of
upregulates or downregulates the entire body
and the metabolism. NEIL GESUNDHEIT: That’s right. So someone with
hyperthyroidism would have very high metabolism. You may know the
classic picture someone with a high heart rate, rapid
metabolism, weight loss. That would be someone
with excess amounts of thyroid hormone. And then you see pretty
much the inverse picture when someone has a deficiency
of thyroid hormone and someone with hypothyroidism. So it’s critical to maintain
just the right amount of almost all of these hormones,
and the thyroid hormones are good examples of this. But the ultimate regulation
is from that pituitary gland. SALMAN KHAN: This is
kind of the master one. It sends a signal there, and
then that kind of does the– NEIL GESUNDHEIT: That’s right. And we’ll talk later
about feedback loops, because how does the pituitary
know when to stop making TSH? And basically,
like a thermostat, it can sense the levels
of thyroid hormone. And when those levels are
just at the right level, and not too high, it’ll decrease
the amount of TSH it makes. If the levels are too
low, it’ll increase TSH to try to stimulate the
thyroid gland to make yet more thyroid hormone. SALMAN KHAN: Very cool. And what else do we have here? NEIL GESUNDHEIT: OK. So the other hormones,
some of the major ones. The pituitary, in addition to
making the thyroid stimulating hormones, it makes
a hormone called ACTH, adrenal
corticotrophic hormone, which acts on the
adrenal cortex. And the adrenal is
that gland exactly that sits on top of the kidney. And the outer layers
of the adrenal gland are the adrenal cortex, and
those are stimulated by ACTH. SALMAN KHAN: And they’re
not related to the kidney. They just sit on top there. They’re structurally there. NEIL GESUNDHEIT: Right They’re
related only in that sense that the blood supply is rich
like the kidneys’ blood supply, and they happen to
sit above the kidney. And they’re called
adrenal because they’re adjacent to the kidney,
which is the renal part. SALMAN KHAN: That should
have been obvious. I never realized that. NEIL GESUNDHEIT: But
they don’t per se filter blood or do any
of the key functions that the kidney serves. SALMAN KHAN: I see. And what’s their role? NEIL GESUNDHEIT: So
the adrenal glands make the adrenal
hormones like cortisol, which regulates
glucose metabolism and is important to maintaining
blood pressure and well-being. And then it makes
mineralocorticoids like aldosterone, which is
important for regulating salt and water balance. You also have adrenal androgens,
which are somewhat important. And those three hormones
are the main hormones made by the adrenal cortex. The ACTH primarily
regulates the cortisol and the adrenal androgens. And there’s another
system that regulates the mineralocorticoids that
we’ll talk about later. SALMAN KHAN: OK And we have
a few more organs here. NEIL GESUNDHEIT: Yeah. So Also add out
of the pituitary, we make luteinizing hormone and
follicle stimulating hormone. Those would be
abbreviated LH and FSH. And those act on the gonads. So in the male, it’ll act on
the testes, and in the female, it’ll act on the
ovaries to stimulate the development of sperm
in the male and oocytes, or eggs in the female,
and also the production of gonadal steroids, primarily
testosterone in the male and estradiol in the female. SALMAN KHAN: Right And
are we missing anything? NEIL GESUNDHEIT: Well,
there are two other hormones that also derive from
the anterior pituitary. And those would
be growth hormone that’s critical for optimal
growth of long bones. SALMAN KHAN: The pituitary
really does do a lot. NEIL GESUNDHEIT: It does. Yeah. SALMAN KHAN: So it’s HGH,
human growth hormone. NEIL GESUNDHEIT: Yeah. Human growth hormone, and
that would act on long bones, for instance. And then we would
have prolactin, which is important in
women for lactation, being able to breastfeed
after delivering a child. SALMAN KHAN: And insulin is? NEIL GESUNDHEIT:
Insulin is key, but it doesn’t come from the pituitary. So now we’re going to work
our way down a little bit. We talked about the thyroid
gland making thyroid hormone. And then when you get
to the pancreas, which is that yellow structure
right in the middle, inside the pancreas,
there are small islands called the islets of Langerhans. And the islets
within the pancreas make endocrine hormones
like insulin and glucagon. But insulin is vital. Without insulin,
you have diabetes. And without insulin, you don’t
transport glucose into muscle and remove glucose from
the bloodstream normally. The absence of
insulin can produce all of the symptoms of diabetes
that we’ll talk about later. SALMAN KHAN: It seems
just structurally, you have the pancreas right here. You have the adrenal
glands right there, that they’re all near kind
of that interchange on the– because they’re all
so important to get to where they need to get to. NEIL GESUNDHEIT: That’s
a good observation. They all have a lot of
venous drainage from them so that when they
make their hormone, it gets into the bloodstream
rather quickly because they are vital structures. SALMAN KHAN: Very cool. So I think we can
leave it there. And in the next video,
you have some pictures that I think will be
pretty interesting. NEIL GESUNDHEIT: OK. Yeah. In the next video,
we’ll talk about how you have to the right amount
of the hormone or else things go awry. SALMAN KHAN: Very cool. Thanks a bunch. NEIL GESUNDHEIT: OK. Thank you.


  • Reply ym January 29, 2018 at 1:21 pm

    I think him asking questions helped me to be clearer on the topic, thanks!!

  • Reply Gadsden Runningwater March 29, 2018 at 9:33 pm

    He says "aldosterone" wrong.

  • Reply - m7k0z7 - January 25, 2019 at 3:30 pm

    All hail crash course; and our books…

  • Reply muzammil kt February 20, 2019 at 9:19 am


  • Reply Valentin Gonzales March 29, 2019 at 12:13 am

    Who else from ms.norris class

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