00:01
Now let's look at the anatomy
of the endocrine system.
00:05
We'll start by looking at
how the endocrine system
is similar and different
to the nervous system,
both of which can carry out
some very similar functions.
00:16
In the endocrine system,
we generally have a cell
of the endocrine organ,
producing something
called a hormone that flows
through the bloodstream in
order to reach its target cell
that will have some sort of
receptor for that given hormone.
00:32
Conversely,
with the nervous system,
we generally have
almost a direct connection
between a nerve via its
axon and the target cell.
00:43
And the end of the
nerve is going to sit
right on top of the cell and
secrete neurotransmitters
a very,
very tiny distance directly
onto the surface
of that target cell.
00:56
In general,
endocrine organs are arranged as glands.
01:02
Glands are generally
made up of cells that
produce and secrete
some sort of substance.
01:07
For example,
the thyroid gland is going to
secrete thyroid hormone
into the bloodstream.
01:16
Difference with the
exocrine system,
something like sweat
glands or maybe
the exocrine portion
of the pancreas
is that those cells are still
going to produce a substance,
but they're going
to enter a duct
and then be released
onto a surface.
01:34
So in the case of sweat glands,
that surface may be,
you know, the skin,
in the case of pancreatic
digestive enzymes
that would be into the duodenum.
01:44
So the endocrine
system is really relying
on its connection to
the bloodstream in order
to submit its hormones
very far distances.
01:55
So again,
by using the bloodstream,
endocrine organs
don't have to be very
close to the cells
that they're targeting.
02:06
We'll start by looking
at some very important
endocrine structures all
the way up in the brain.
02:13
We'll start by looking at a
little portion of the inferior
aspect of the brain
called the hypothalamus.
02:21
This hypothalamus is
going to sit very close
to a very tiny organ
called the pituitary gland.
02:29
Even though it's a very tiny organ,
it's going to carry out
a wide array of very
important functions.
02:36
We're also going to
look at the thyroid gland,
which is gonna be
located in the neck
around the area of the trachea.
02:43
The adrenal glands,
which sit adjacent to the kidneys,
hence the name adrenal,
they're next to the renals
and a different portion of the
pancreas that we saw in the GI system.
02:56
Whereas the majority of the
pancreas has an exocrine function,
there is a small
percentage about
5% that has an
endocrine function.
03:04
And then the gonads
the ovaries and testes
also have important
endocrine functions as well.
03:14
The hypothalamus
is this area of neurons
at the very inferior portion
of the brain that regulate
a lot of functions and
is related in maintaining
balance in the body
called homeostasis.
03:30
And it has a nice mixture
of typical nervous system,
structure and function
and more typical
endocrine structure
and function.
03:42
So we're gonna see that
there are some neurons
that directly regulate things
and there are going to be some
hormones that are secreted
only to carry out their function.
03:50
And this is both going
to have a lot to do
with the underlying
pituitary gland.
03:57
So here we see
the pituitary gland
and it's connected
via this small little area
called the infundibulum
to the rest of the brain.
04:06
The pituitary
gland has two lobes
and it's important to keep
these two lobes separate
because they have
different control
and they secrete
different hormones.
04:16
The anterior lobe is also
called the adenohypophysis.
04:21
And I'm going to say
that name even though it's
a bit of a mouthful,
because adeno means gland.
04:27
Whenever you see that prefix,
like adenocarcinoma,
for example is a
carcinoma of glands.
04:33
Adeno means glands.
04:34
So always think gland
when you see that word,
and that's going to be
important for remembering
the difference between
lobes of the pituitary.
04:42
Posterior lobe on the other hand
is called the neurohypophysis.
04:47
And again,
that name is going to be descriptive
when we talk about
what controls the anterior
versus posterior
lobes of the pituitary.
04:57
The pituitary as we can
see here in this sagittal
crown cross section
sits in a little depression
of a school bone called
the sphenoid bone.
05:05
That little depression is
called the sella turcica.
05:09
It means Turkish saddle.
05:10
It's basically I
guess shaped-like
what a Turkish
saddle looks like.
05:14
It provides this nice
little protected area
for the pituitary
gland to sit in.
05:21
And it's sitting just
below a sinus in the
sphenoid bone called
the sphenoid sinus.
05:29
And because of this,
there's actually
a shortcut to reach
the pituitary gland.
05:35
That makes it a lot easier
than other sorts of brain surgery.
05:39
So for example, if there's a
problem with the pituitary gland,
and you needed to access it,
you wouldn't have to do
what we call a
craniotomy where you
take off a portion
of the cranial bones.
05:51
You could actually go
through the sphenoid,
and something called
Transsphenoidal surgery
by going through a little bit of
bone than the space within it,
the sinus and then
reaching the sella turcica
to access the pituitary gland.
06:08
So the interactions
between the hypothalamus
and the pituitary are
different depending
on which lobe
we're talking about.
06:16
And the hypothalamus
really is what's
regulating the
pituitary in a lot of ways.
06:21
And then in turn,
the pituitary is secreting the
hormones that have an
effect all throughout the body.
06:27
So here are a couple
of what we call nuclei
or collection of cell
bodies in the hypothalamus.
06:34
One's called the
supraoptic nuclei,
the other collection are
the periventricular nuclei.
06:41
And these are going to
have nerves that come off
of them just like any
other sort of gray matter.
06:47
And they're going to travel
down the infundibulum,
down towards the posterior lobe.
06:53
We also have a
lot of blood in here.
06:56
So we have the inferior
hypophyseal artery,
supplying the posterior lobe,
and then the hypophyseal vein.
07:05
And hypophyseal is just
the other word for pituitary.
07:10
It can be a little
confusing when
the terminology
jumps back and forth.
07:14
But just know that that's
a synonym for pituitary,
that's all this is saying is this is the
artery and vein of the pituitary here.
07:21
In the anterior lobe,
it's stimulated a little differently.
07:25
Whereas in the posterior lobe,
those nerves coming from
the hypothalamus directly
reach the posterior lobe
and tell it what to secrete.
07:33
It's a little more indirect
with the anterior lobe.
07:37
So here, these neurosecretory
neurons of the hypothalamus
will actually end
in the infundibulum.
07:45
And they're going to
secrete into the bloodstream
and let these blood
vessels take it from there.
07:52
So there's a superior hypophyseal
artery, again, just means pituitary,
that's going to go
through a capillary network
and form veins here
in the infundibulum.
08:03
And it's those veins that
are going to take these
neurosecretory products
down to the anterior lobe.
08:12
Again, there will be
another capillary network,
and then eventually,
after the hormones are added,
will pass out through
the hypophyseal veins,
and eventually
out into circulation.
08:24
So instead of a direct
neural stimulation,
there's actually a brief
blood vessel pathway
to transport that
tiny little distance
from the infundibulum
to the anterior lobe.
08:36
And because it doesn't
have that direct neural control,
it's called adenohypophysis.
08:43
And so this leads
us to the second
of two portal
systems in the body.
08:50
The first and much
larger one is in the liver
where we had the hepatic
portal venous system.
08:56
But in short,
a portal venous system
is where we have arterial
circulation branching
into smaller and
smaller branches like
arterioles until it
reaches a capillary bed,
which gets into larger and larger
vessels until it forms a venule.
09:11
But instead of going all the
way back to systemic circulation,
that venule branches again
into another capillary bed
before forming a
second set of annuals that
eventually go back
into venous circulation.
09:25
So we've seen that,
again, in the liver where
venous blood coming
from the intestines
was part of the
portal venous system,
and here we see in the
anterior lobe of the pituitary,
another very,
very short portal system,
where those portal
veins if you will,
are meant to transport
substances a very short distance
to stimulate the anterior lobe to
intern secrete more hormones.