Now let's look at the anatomy of the endocrine system.
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.
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.
Conversely, with the nervous system,
we generally have almost a direct connection
between a nerve via its axon and the target cell.
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.
In general, endocrine organs are arranged as glands.
Glands are generally made up of cells that
produce and secrete some sort of substance.
For example, the thyroid gland is going to
secrete thyroid hormone into the bloodstream.
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.
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.
So the endocrine system is really relying
on its connection to the bloodstream in order
to submit its hormones very far distances.
So again, by using the bloodstream,
endocrine organs don't have to be very
close to the cells that they're targeting.
We'll start by looking at some very important
endocrine structures all the way up in the brain.
We'll start by looking at a little portion of the inferior
aspect of the brain called the hypothalamus.
This hypothalamus is going to sit very close
to a very tiny organ called the pituitary gland.
Even though it's a very tiny organ, it's going to carry out
a wide array of very important functions.
We're also going to look at the thyroid gland,
which is gonna be located in the neck
around the area of the trachea.
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.
Whereas the majority of the pancreas has an exocrine function,
there is a small percentage about
5% that has an endocrine function.
And then the gonads the ovaries and testes
also have important endocrine functions as well.
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.
And it has a nice mixture of typical nervous system,
structure and function and more typical
endocrine structure and function.
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.
And this is both going to have a lot to do
with the underlying pituitary gland.
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.
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.
The anterior lobe is also called the adenohypophysis.
And I'm going to say that name even though it's
a bit of a mouthful, because adeno means gland.
Whenever you see that prefix, like adenocarcinoma,
for example is a carcinoma of glands.
Adeno means glands.
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.
Posterior lobe on the other hand is called the neurohypophysis.
And again, that name is going to be descriptive
when we talk about what controls the anterior
versus posterior lobes of the pituitary.
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.
That little depression is called the sella turcica.
It means Turkish saddle.
It's basically I guess shaped-like
what a Turkish saddle looks like.
It provides this nice little protected area
for the pituitary gland to sit in.
And it's sitting just below a sinus in the
sphenoid bone called the sphenoid sinus.
And because of this, there's actually
a shortcut to reach the pituitary gland.
That makes it a lot easier than other sorts of brain surgery.
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.
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.
So the interactions between the hypothalamus
and the pituitary are different depending
on which lobe we're talking about.
And the hypothalamus really is what's
regulating the pituitary in a lot of ways.
And then in turn, the pituitary is secreting the
hormones that have an effect all throughout the body.
So here are a couple of what we call nuclei
or collection of cell bodies in the hypothalamus.
One's called the supraoptic nuclei,
the other collection are the periventricular nuclei.
And these are going to have nerves that come off
of them just like any other sort of gray matter.
And they're going to travel down the infundibulum,
down towards the posterior lobe.
We also have a lot of blood in here.
So we have the inferior hypophyseal artery,
supplying the posterior lobe, and then the hypophyseal vein.
And hypophyseal is just the other word for pituitary.
It can be a little confusing when
the terminology jumps back and forth.
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.
In the anterior lobe, it's stimulated a little differently.
Whereas in the posterior lobe,
those nerves coming from the hypothalamus directly
reach the posterior lobe and tell it what to secrete.
It's a little more indirect with the anterior lobe.
So here, these neurosecretory neurons of the hypothalamus
will actually end in the infundibulum.
And they're going to secrete into the bloodstream
and let these blood vessels take it from there.
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.
And it's those veins that are going to take these
neurosecretory products down to the anterior lobe.
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.
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.
And because it doesn't have that direct neural control,
it's called adenohypophysis.
And so this leads us to the second
of two portal systems in the body.
The first and much larger one is in the liver
where we had the hepatic portal venous system.
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.
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.
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.