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