Endocrinology - Overview

ARMANDO HASUDUNGAN:
Armando Hasudungan,

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In this video, we're going to
talk about the endocrine system.

Now the endocrine
system, the main purpose

is to maintain a
homeostatic environment

through the use of hormones.

So when we think of
the endocrine system,

we should think of hormones.

Hormones are essentially
signaling molecules.

Now the endocrine
system works in close

proximity with the
nervous system,

in that both the endocrine
system and the nervous system

tries to maintain a homeostatic
environment by sending out

signals.

The main difference
between the two

is that the nervous system
is a quick response.

So when a stimulus
arrives at a neuron,

the neuron can pass on these
signals, these commands,

as neurotransmitters that will
then target a particular cell.

The neurotransmitter will bind
onto a specific receptor that

will cause the target
cell to initiate

a short-term quick response.

Now, the endocrine
system, on the other hand,

will send signals not
via neurotransmitters,

but through hormones.

And these hormones will
travel via the bloodstream

where it will then target a
cell and its specific receptors.

So what happens is,
with an endocrine cell,

is that when a stimulus
or command comes,

this will stimulate
the endocrine cell

to secrete hormones into
the bloodstream like so.

These hormones will
then target and bind

onto a specific receptor
on a target cell.

This will cause a
target cell to initiate

a long-term slow response.

So that is a major difference,
in that the nervous system

is a short-term quick response,
whereas the endocrine system is

a long-term, slow response.

The endocrine cell
typically secrete hormones

into the bloodstream.

This is normal.

This type of signaling is called
endocrine signaling, hence

the name, endocrine system.

However, hormones
does not always

have to be secreted into the
bloodstream to target a cell.

The endocrine cell
can also secrete

hormones that target a
cell directly close to it,

like this cell, for example.

And so when it does this, this
type of secretion is known

as paracrine signaling--
"para" as in "across."

And this will initiate a
long-term slow response.

When the endocrine
cell is secreted--

when the endocrine cell,
sorry, secretes hormones

into the bloodstream,
which is the basics

for the endocrine
system, we have

hormones in the
bloodstream, the hormones

can travel in the bloodstream
as a free form, which can

be cleared quickly by the body.

Free form as in it's just a
hormone traveling in the blood.

Or the hormone can actually
travel bound to a protein.

These hormones that are
bound to a protein and travel

through the blood are
typically lipid hormones

because lipids hate water.

They need to travel
bound to proteins.

We call these
protein-bound hormones.

So now let's talk a little
bit more about hormones.

Hormones, as I mentioned,
are signaling molecules.

Hormones can be grouped
into three types--

amino acid derivatives, peptide
hormones, or lipid derivatives.

Lipid derivatives, for example,
are steroid hormones or thyroid

hormones, if you know a little
bit about your hormones.

And so these hormones, they
will bind onto a target cell,

onto the specific receptor,
that will initiate a desired

response, a long-term response.

Here, I'm drawing these hormones
binding onto target receptors

on this plasma membrane
of the target cell

to initiate a response.

Well, peptide hormones,
and most hormones derived

from amino acid, they
bind to receptors

on the plasma membrane.

Whereas the
lipid-derived hormones,

they cross the cell
membrane and bind

to receptors in the
cytoplasm as shown.

Thyroid hormones and
steroid hormones,

they cross the plasma
membrane and bind

to receptors in the cytoplasm
to initiate a response.

And this is because the
thyroid and steroid hormones

are lipid-derived.

OK, now that we know a
bit more about hormones,

these signaling
molecules, let's look

at an example of an
endocrine response.

A good example is to
look at blood glucose.

So here in the bloodstream,
we have low glucose levels.

And this is not very good
because we need glucose.

Glucose is a source of energy
for tissues in our body.

So low blood glucose
is a stimulus.

And the body will
have to try to fix

this to maintain homeostasis.

So it will try to increase
blood glucose levels.

And this is when the
endocrine system kicks in.

The stimulus, which is
low blood glucose levels,

will stimulate an
endocrine cell,

known as the pancreas cell.

Actually, it's called
the alpha cell,

but let's call it pancreas
cells for simplicity.

The pancreas cell will then
secrete a hormone called

glucagon into the bloodstream.

Glucagon will travel
through the bloodstream

to the liver, which
is the target cell.

Glucagon is not a lipid
hormone because, one, it

is not bound to a protein when
it travels through the blood;

and two, it binds to receptors
on the cell membrane.

When glucagon binds to the
receptors on the liver cell,

glucagon will
stimulate the liver

to break down glycogen to
secrete glucose in the blood.

And so the response
by this liver cell

is that it will
secrete more glucose

in the blood, which will
increase blood glucose

levels like so.

All these glucose is
being secreted out.

When blood glucose level is
increasing, this will send a--

this will send a feedback back--

it will send a feedback.

It will send a negative
feedback signal

to stop stimulating
the pancreas cell.

Because when you have normal
to high blood glucose levels,

you don't need any--

you don't have low blood
glucose stimulation,

and you don't need any more
glucose to be secreted.

I hope you understand this
concept of negative feedback.

So in the example
we just saw, we

looked at the hormone
called glucagon,

which is secreted by the
alpha cells of the pancreas.

Now let's look at some
other major hormones,

such as glucagon, and
where they come from.

We will not look
at what they do--

or I'll try to mention them,
but just look at what they are.

Before we continue,
we should know

that we have many
hormones in the body that

perform different functions
or have different responses.

Another terminology
to learn is also

what's called the what's
called endocrine glands.

Endocrine glands are essentially
groups of endocrine cells

that are dedicated to
perform a specific function.

So let's look at the
first two or three most

important endocrine
glands, I think.

The first is situated
in the brain here.

This is known as--
this first one is known

as the hypothalamus,
and this is an--

endocrine tissue.

And the hypothalamus is
responsible for the production

of antidiuretic
hormone and oxytocin.

It is also responsible
for the production

of the regulatory hormones.

And we will look
at what these do.

Probably the most
important endocrine glands

are the pituitary glands,
which are located right

below the hypothalamus here.

And there are two lobes
of the pituitary glands.

There is the anterior lobe
and the posterior lobe.

The posterior lobe
secretes oxytocin

and antidiuretic
hormones that were

produced by the hypothalamus.

So the hypothalamus
produces these hormones

and sends them to the posterior
lobe, which then the posterior

lobe will secrete
it into the blood.

The anterior lobe of
the pituitary glands

actually secrete many
hormones, including

ACTH, which stands for
Adrenocorticotropic Hormone;

TSH, Thyroid Stimulating
Hormone-- obviously stimulates

the thyroid; GH, Growth
Hormone, for growth;

PRL, Prolactin, for milk
production in the breast;

and then FSH, which is
Follicle-Stimulating Hormone,

and LH, Luteinizing
Hormone, which

are responsible for the
reproductive system.

So these hormones that are
secreted by the anterior lobe,

they are regulated by the
hormones from the hypothalamus.

So if you remember,
the hypothalamus

secretes regulatory
hormones that

regulates the secretion
of the anterior hormones,

if that makes any sense.

There's another endocrine
gland in the brain known

as a pineal gland, which
is around this area,

and it secretes melatonin.

Melatonin is actually
responsible for the body clock.

Now let's make our way down.

In the trachea,
our throat area, we

have-- wrapping
around the trachea

we have the thyroid gland.

Now the thyroid gland
secretes a few hormones.

Thyroxine, which
is abbreviated T4,

and triiodothyronine,
which is T3.

It also is responsible for
metabolism, essentially.

Then you have calcitonin.

Now the thyroid gland, it also
has other endocrine glands

on it.

So if we zoom in
to this area here,

we're looking at this person
from a posterior point of view,

from the back.

So we have the thyroid gland.

And then we have these four
lobes on the thyroid gland.

These are known as
the parathyroid gland.

"Para" as in a cross,
but it's just on it.

So there are four
parathyroid glands.

And these are behind
the thyroid gland.

And they secrete the
hormone parathyroid hormone.

Simple enough.

Parathyroid hormone is
important in the regulation

of calcium and phosphate in
the in our blood, in our body.

Now right below
the thyroid gland

we have another
endocrine gland, you

can say, known as the thymus.

Now the thymus is not really a
big deal in the endocrine system

world, but it is a big deal
in the immune system world.

But in endocrine world,
the thymus actually

undergoes atrophy
during adulthood

and it begins
secreting thymosin.

Of course, this person,
we have-- in this person,

we have the lungs,
connected to the trachea,

and the heart between the lungs.

Now if you didn't
know, the heart

is also classified
as an endocrine gland

because it secretes hormones.

The heart secretes the
hormone naturetic peptide,

which is responsible in
blood pressure regulation.

It's actually decreases
blood pressure

when there is an increase
in blood pressure.

Then we have the
digestive tract.

Now the digestive
tract, they secrete

a lot of variety of hormones.

And what I mean by
the digestive tract,

I mean the stomach, the
duodenum, the small intestines,

et cetera.

And they produce many hormones,
such as gastrin, somatostatin,

cholecystokinin, et cetera.

Another important-- well,
another very important endocrine

gland or tissue is the pancreas.

And it secretes two main
hormones, insulin and glucagon.

Now we talked about glucagon in
that it increases blood glucose

levels.

Well, insulin works opposite.

It decreases blood
glucose levels.

And you might know
diabetes people,

they have very low insulin
levels-- or type 2 diabetes,

they have low insulin
levels, or type 1,

they have depleted
insulin levels.

And so you have a lot--

you have very high
blood glucose levels,

and you can't decrease this
because you have no insulin.

And if you wait until
the end of the video,

I will provide links to
some of these hormones

so you can watch them in more
detail and see what they do.

The kidneys also
secrete hormones.

They secrete erythropoietin,
which stimulates red blood cell

production in the bone marrow.

And also secretes calcitriol,
which I don't know what it does.

Above the kidneys, we also
have the very important adrenal

glands.

If we take a cross-section
of the adrenal glands--

we have two adrenal
glands, of course,

because we have two kidneys.

If we cut a cross-section
of the adrenal glands,

we have the adrenal
cortex, the outer part

of the adrenal glands.

And then we have the
adrenal medulla, the middle

of the adrenal glands.

And this line I'm drawing
for the adrenal medulla,

it's meant to be in the in the
middle of the adrenal glands,

not on the outside.

It's a mistake.

Anyway, the adrenal
cortex secretes

cortisol and aldosterone.

Cortisol is
essentially for stress,

and aldosterone is to promote
sodium reabsorption, as well

as potassium, secretion
in the kidneys.

And then we have
the adrenal medulla,

which secretes adrenaline
and noradrenaline,

also known as epinephrine or
norepinephrine in America.

These hormones are important in
the fight-or-flight response,

as well as the rest
and digest response.

Adipose tissue, which
is essentially fat,

also secrete hormones.

It secrete leptin, which is
important in fat metabolism.

The final endocrine tissue,
or endocrine glands,

I want to talk about
are the gonads.

Now because we have a male
and female version of humans,

we have two types of gonads.

We have the testes for the male
and the ovaries for females.

So the testes--
in the testes, we

have cells that secrete
androgens such as testosterone,

which is important for
promoting male characteristics,

as well as sperm production.

And then we have, for
the gonads, the ovaries,

for the female, we
have important hormones

being secreted, such as
estrogen and progesterone,

which is important in
female characteristics,

and egg production.

OK, so those were
the main hormones

that were secreted
by the main endocrine

tissue or endocrine glands.

But I'd like to concentrate
and look into more detail

on the pituitary glands, which
is a very important endocrine

gland because it, for one,
secretes a lot of hormones;

and two, it secretes
hormones that are regulate--

or stimulate the secretion
of other hormones

from other endocrine tissue,
if that make any sense.

So let's have a closer look
at the pituitary glands.

The pituitary gland is
located within the brain,

and an easy way to
remember this is it's

located sort of below
the hypothalamus.

The pituitary glands
consist of two lobes.

One is called the posterior
pituitary, which is at the back.

It's also known as
the a neurohypophysis.

And then we have the
anterior pituitary, also

known as the adenohypophysis.

I hope I'm pronouncing
the "-physis" right.

Anyway, let's first look
at the posterior pituitary

and what it secretes.

Well actually, the
hypothalamus, it

produces-- it synthesizes
antidiuretic hormone

and oxytocin.

These hormones that are
synthesized in the hypothalamus,

they are synthesized
in these neurons.

And these neurons will
then pass on these hormones

to the posterior pituitary.

And so from here, when a
stimulus arrives stimulating

the secretion of these
hormones, the posterior

pituitary can then secrete it.

So the posterior
pituitary can secrete

the antidiuretic hormone.

The antidiabetic hormone's main
function is for water retention,

so it targets the kidneys,
particularly the kidney tubules.

The posterior pituitary can also
secrete the hormone oxytocin.

Oxytocin essentially
targets the breast.

And it's important-- so it's
important for lactation,

and also, it's important for
the contraction of the uterus

during childbirth.

So remember, for the
posterior pituitary,

the hormones itself are
actually synthesized or made

within the hypothalamus,
the posterior pituitary

only secretes them.

Now let's look at the
anterior pituitary.

Now the anterior pituitary is
different than the posterior

pituitary in that the
anterior pituitary

makes its own hormones.

However, these hormones
they can only--

so most of them can
only be secreted

when there is some
form of confirmation

from the hypothalamus.

So the hypothalamus
actually will

secrete hormones
that will regulate

the secretion of the
anterior pituitary hormones.

The hormone secreted
by the hypothalamus

are known as
regulatory hormones,

and these regulatory hormones,
once they are secreted,

they will cause either
stimulation or inhibition

of the anterior pituitary
hormones being secreted.

Anyway, whatever the
case, if it's all OK,

the anterior pituitary can
secrete prolactin, abbreviated

PRL, which will target
the breasts, essentially,

for milk production.

Anterior pituitary can also
secrete growth hormone,

which targets many tissues,
such as the bone, for growth.

The anterior pituitary can
secrete gonadotropic hormones,

such as follicle-stimulating
hormone and luteinizing

hormone, which will target the
testes or ovaries depending

if it's male or female.

Anterior pituitary
can also secrete

thyroid-stimulating
hormone, which

will stimulate the thyroid gland
to secrete its own hormones.

And finally, the
anterior pituitary

can also secrete
adrenocorticotropic hormone,

which will target
the adrenal glands,

and it will stimulate
the release of cortisol,

et cetera, for example.

So as you can see,
the pituitary glands

secrete a variety of hormones,
and they're controlled

in a very confusing manner.

And I'll provide
actually a video

that will look more into detail
on the on the pituitary glands.

Actually, I'll provide
links hopefully

to many of these
hormones so you can

click on the link on
the screen and they'll

take you to this video
so you can watch it

in a bit more detail.

Thank you.