the different parts of our body communicate?

I think we often consider the body

to be this one complete thing, this self.

But really our body is composed of lots of parts.

There are lots of organ systems.

And each of those has organs.

And all of those organs are made of tissues.

And all of those tissues are made of cells.

And it's crazy, but there are 100 trillion--

or at least roughly 100 trillion cells in our body.

So it's curious then how do those 100 trillion

different parts communicate?

Well, one way is through the nervous system

and through the pre-laid tracks of nerves.

But not every part of the body is connected by nerves.

I mean how, for example, would part of the brain

go about communicating with part of the kidney?

Well, to talk about that we're going

to have to talk about the endocrine system.

And the endocrine system is a system

of organs that are called glands.

And these glands secrete little chemical messages

that are called hormones.

And they release those little chemical messages

called hormones into the bloodstream

so that they can circulate from one part of the body

to another part of the body in order to initiate an effect.

And there are many parts of the body that

use these hormones to communicate.

But certain organs are really defined

by this method of communication and we

call them endocrine glands.

And so one of the major endocrine glands

is the hypothalamus.

And the hypothalamus is located right here.

It's a member of the forebrain.

And as a member of the brain, it receives a lot of those signals

that we talked about from the nervous system.

So those nerve signals are funnelling into the brain.

And the hypothalamus then, as a kind

of dual member of the endocrine system,

funnels those signals into the pituitary gland.

And so because it plays that dual role

between the endocrine system and the nervous system,

it often gets taglined as the control

center of the endocrine system.

In addition to stimulating the pituitary gland,

the hypothalamus actually make some hormones itself also.

And so it makes ADH and oxytocin.

And ADH is antidiuretic hormone.

And it's a main regulator of our fluid volume in our body.

And then oxytocin is a hormone that stimulates the uterus

to contract for females during pregnancy.

And so that's the hypothalamus, member

of the brain and member of the endocrine system

where it all begins, the control center.

And then right below the hypothalamus

is the pituitary gland.

And the pituitary gland is located right here,

dangling right below.

And so the hypothalamus is about the size of a grape.

And the pituitary gland is actually

about the size of a green pea.

But this little green pea is so important

that it's called the master gland.

And it's called the master gland because the pituitary gland

takes that stimulation from the hypothalamus

and it directs it to all of the other endocrine glands,

or at least almost all of the other endocrine glands,

such that their function is ultimately

dependent on the pituitary gland to work well.

And so that little green pea is a really important part

of the endocrine system.

And so one of the endocrine glands

that the pituitary directs is the thyroid gland.

And the thyroid gland is located right here in your neck.

It wraps around your trachea.

And your trachea is your windpipe.

And so you can feel this thyroid gland on your neck

as you swallow.

If you hold your hands right around your Adam's apple

and swallow, that meaty thing moving up and down,

that's your thyroid gland.

And one of its main jobs is regulating

your body's metabolism.

And it does that through the thyroid hormones T3 and T4.

And another name for T3 is triiodothyronine.

And another name for T4 is thyroxine.

But the thyroid uses these hormones, the thyroid hormones,

to stimulate the body's metabolism, which is crucial

because that's how our body gets energy.

And then right behind that thyroid gland

are four spots known collectively

as the parathyroid.

And the main role of the parathyroid

is regulating our body's blood calcium level.

And the level of calcium in our blood

is hugely important because calcium

does a lot of stuff in our bodies.

It's involved in muscle contraction.

It's involved in bone growth.

And all of those functions are really sensitive to the level

of calcium that's floating around in our blood.

And so the parathyroid glands, those four spots

on the back side of our thyroid, regulate calcium

through the parathyroid hormone, or PTH.

And then moving down the torso, we have the adrenal glands.

And the adrenal glands are located

right on top of the kidneys here.

And they're called the adrenal glands

because they're adjacent to or right next to the kidney

system, which is called the renal system in medical speak.

But we really need to further divide the adrenal glands

into two parts, the outer part and the inner part.

So the outer part is the cortex and the inner part

is the medulla.

And the reason for the distinction

is that the inside and the outside of the adrenal glands

have two different functions.

And so we'll start with the outside or the cortex.

And that's where the steroids, the adrenal corticosteroids,

are made.

And two major examples of steroids

made in the adrenal cortex are cortisol and aldosterone.

And cortisol is one of the body's stress hormones.

So it functions to increase blood sugar in times of stress

so we have energy.

And it also has some anti-inflammatory functioning.

And then aldosterone is one of the major regulating

hormones of our body's blood volume

and how much fluid is in our veins and arteries.

And so that's the cortex.

And then the medulla makes a class

of hormones called catecholamines.

And two major examples of catecholamines

are epinephrine and norepinephrine.

And I'm going to shorten those as epi and norepi.

And sometimes epinephrine is called adrenaline.

And that might be a little bit more familiar to you.

But these catecholamines are really

involved in our body's fight or flight response, that

adrenaline response that we have to a stressful or scary

situation.

And so the medulla and the cortex

make up the adrenal glands.

But moving down the list and down the body,

we have the gonads.

And in females, those are the ovaries,

and in males, the testes.

And the gonads release the sex hormones.

And so in males, the testes produce testosterone.

And in females, the ovaries produce

estrogen and progesterone.

But these sex hormones are mainly

involved in the development of our secondary sex

characteristics like pubic hair, and larger frames in males,

and breasts in women.

But they're also involved in progressing us

through those life stages that accompany those sex

characteristics, like puberty and menopause.

And then last, but not least, we have the pancreas.

And it's located right here in the upper part of the abdomen.

And I saved the pancreas for last

because it isn't involved as directly

with the pituitary glands as the other endocrine hormones were.

But it still uses those hormones to stimulate an effect

in a different part of the body.

And the effect that the pancreas stimulates

is control over the blood sugar.

And it does that through the hormones insulin and glucagon.

And the pancreas is vitally important

because without its hormones insulin and glucagon,

we can't regulate how much sugar is

in the body's blood versus the cells.

And that can lead to major diseases like diabetes.

And so with the pancreas, we can conclude

our list of major endocrine glands.

And so as we look at these glands and at these hormones

and we think about all of the different effects that

are being stimulated in our body by them,

it becomes pretty clear that there aren't just a few

of these circulating in our bloodstream.

There are literally loads of hormones circulating

through our vasculature at any given moment.

And so that poses a potential problem.

If, say, that you're in the brain

and you're trying to tell something to the kidney,

you're trying to send him a message,

and you put that in the bloodstream

and you just float it down to him,

how do you know that it's going to get there?

I mean, isn't that what every other endocrine

gland is trying to do?

Well, it turns out that hormones are a lot like radio waves.

In your city or in your town, there are many different radio

stations and there are many different songs

being played at any given time by those radio stations.

And even maybe from the next town

over, there are radio waves filling the air of your town.

But unless you're tuned in specifically to that station,

you're not going to pick up on the song that's

being transmitted.

And in a very similar way, a hormone

is not going to be received unless there's

a very specific receptor on the target cell.

And so the receptor and its location

are very important in determining the hormone

function.

And we have classes that we use to help us identify which

hormones fall into which function.

And so the first class are autocrine hormones.

And the autocrine hormones function

at the cell that makes them.

An example of this is the T-cell in the immune system.

It actually secretes a hormone that it

makes called an interleukin, that

signals the cell itself to increase its effectiveness

and its immune function.

And then another class of hormones

are paracrine hormones.

And paracrine hormones function regionally.

And an example of that might be the hormones released

by the hypothalamus that direct the pituitary gland.

And then last, but not least, kind

of the classic class of hormones are the endocrine hormones.

And these are the hormones that function at a distance.

And an example of this might be the pituitary gland stimulating

the gonads, way far away.

And so we have autocrine, paracrine,

and endocrine classes that help us

determine how a hormone functions.

And so I know I just told you a whole lot about hormones.

But this is your introduction into one of the most important

ways that the 100 trillion little tiny individual parts

of your body communicate.