Hello everyone and welcome to the Institute of Human Anatomy.
Today we're going to use the cadavers to help us understand the endocrine system.
Yes, that system that releases those raging hormones.
We're often trained to think of the sex hormones when we hear the word "hormones", like testosterone
and estrogen.
However, the endocrine system releases about 50 different hormones throughout the human
body, so a lot more than just testosterone and estrogen.
So if you're a person who's been interested or maybe even suffered from endocrine dysfunction,
like thyroid disorders, PCOS, adrenal disorders, reproductive disorders - this video will help
set the stage and create a better understanding to those disorders or those dysfunctions.
So, we are going to define the endocrine system, obviously, and talk about its general function,
define hormones and how they have a physiological response and focus really on those two bosses
of the endocrine system - the hypothalamus and the pituitary gland.
So as always, let's get to this anatomical awesomeness.
[Intro] so, how do we define the endocrine system?
Well the endocrine system is a control system within the human body.
Now, we're typically trained to think of this as one of the major control systems of the
human body.
And yes, the nervous system does exhibit a large amount of control.
However, the endocrine system is also extremely powerful, and the two systems together essentially
coordinate all the function of the human body.
Now, the endocrine system does it a little differently than the nervous system.
It secretes things called "hormones".
These are signaling molecules that are secreted into the body fluids that are by these little
tiny blood vessels called "capillaries".
And once those little signaling molecules or hormones get into the bloodstream, they
can go everywhere in the human body.
So far, we've defined the endocrine system as one of the control systems of the human
body - works with the nervous system to coordinate that control, also exhibits that control through
the release of hormones into the bloodstream.
But what's specifically releasing the hormones into the bloodstream?
Simply it's the glands that make up the endocrine system.
You've probably heard of glands like the pituitary gland or the thyroid glands.
Those are examples of glands within the endocrine system that secrete their hormone into the
blood.
So keep in mind that if you're an endocrine gland, you have to have a strong relationship
with the cardiovascular system and blood vessels literally have to penetrate through that gland
so that those hormones can be secreted into the bloodstream.
So one other thing to consider is this idea that the hormones are going throughout the
entire human body because they're circulating throughout the bloodstream - how does a hormone
have an effect on one tissue and not another.
Why doesn't it affect every tissue it passes through if the blood vessels are going through
those tissues?
And the answer is this lock and key analogy.
We all know that - yes I’m going to have to wash these later - but we all know that
keys have different shapes.
If I take this key to my house, it should open the door at my house.
However, if I took it to your house, beyond being extremely creepy, it shouldn't work
on that lock.
If it did, it's a whole other level of creepy.
But coming back to this analogy is, all these keys have different shapes and therefore different
locks or you could say "receptors".
Think of these as individual hormones and they're circulating throughout the body and
certain cells have certain locks or very specific receptors for that key or hormone.
So for example, if the thyroid gland secretes T4, which is one of the hormones of the thyroid,
that hormone's going to circulate throughout the entire body.
However, it can only affect tissues that have the specific receptor for that specific hormone.
Now that we've given a definition of the endocrine system, talked about the details of hormones,
we have to go a little bit further to understand the endocrine system by talking about these
structures that I nicknamed "the bosses of the endocrine system" - the hypothalamus and
the pituitary gland.
Now, a little bit of a side note - if you ever get into endocrine dysfunction, say like
somebody has a thyroid dysfunction, in some cases with people the thyroid gland can be
fine, and so they'll - then trace back...
Could it be a problem with the hypothalamus in the pituitary gland?
Keep that in mind for our future videos when we go into specific endocrine dysfunctions.
You kind of have to check, in some cases, three different locations or three different
structures to get a full understanding if somebody have endocrine dysfunction.
But back to this idea of these two bosses, the hypothalamus and the pituitary gland,
the pituitary gland got nicknamed the master gland of the endocrine system because this
thing releases up to eight different hormones that control other endocrine glands.
For example, it secretes thyroid stimulating hormone that tells the thyroid gland to activate.
It secretes adrenal corticotropic hormone - a little bit of a mouthful, but if you hear
the name "adrenal, adrenal gland" it tells the adrenal gland to secrete certain hormones.
So again, that's why it got nicknamed the "master gland".
However, as we learned more and more about the interactions of the endocrine system in
the nervous system, we learned that the endocrine system or the pituitary specifically had a
boss of its own called the hypothalamus, and this is located in the brain.
So let's take a look at this.
If you look here at the cadaver, you can see - I’m showing you the central area of the
brain here.
And specifically here where I’m laying the probe is the hypothalamus and I’ll trace
about the area there.
Now when you're looking at that, it kind of looks like a glob of nervous tissue, it doesn't
look like much.
But don't look - let the looks of this thing deceive you.
This thing can pack a punch with how small it is.
So we can do multiple videos alone just on the hypothalamus.
It does so many amazing things like regulates the autonomic nervous system, controls emotion
and behavior, gets involved in eating and drinking - so, appetite, circadian rhythms,
sleep/wake cycles and even body temperature.
But I haven't even mentioned the function because I just got so busy function dropping
there...
I haven't mentioned the function that we really are concerned about with this video and that
is its control or regulation of the endocrine system.
You see, the hypothalamus secretes some hormones of its own to tell the pituitary gland what
to do.
So, kind of an interesting relationship.
You could kind of think of the hypothalamus as the president and the pituitary as the
vice president, or you know, maybe the owner and manager, but they work seamlessly together
to control the whole endocrine system.
So now that we've had this discussion about the hypothalamus and pituitary, I still need
to show you the pituitary on the cadaver dissection.
So let's take a look.
We come up to the same dissection...
You can see - let's reference that hypothalamus again in that region, but below the hypothalamus
is the pituitary gland and it sits in this region here which is bony and it's called
the hypophyseal fossa.
And this is just a trench or a ditch that the pituitary sits in in a bone called the
sphenoid bone.
Now, one other note is the pituitary gland is sometimes referred to as the hypophysis,
hence the name hypophyseal fossa that it sits inside of.
Another important distinction or something cool is that the anterior pituitary gland
is here and the posterior is there.
So it's divided into two segments and you can even see a little bit of a color difference
between the anterior pituitary and the posterior pituitary.
One other thing I have to mention is this connection between the hypothalamus and the
pituitary gland.
Yes, I’m slightly depressed that it got broken in one of our cadaver labs - the student
got really excited about it, but we can still illustrate this connection that should be
between these two structures - the hypothalamus and the pituitary.
And again, this is called the infundibulum.
So, back to this idea of the anterior and the posterior pituitary gland, that's a really
important distinction because the hypothalamus interacts differently with the anterior pituitary
gland than it does with the posterior pituitary gland.
Why you might ask...
Well, that's an answer in embryology.
I often tell people, anatomy gives you the "what's", like this is this, this is that,
where embryology tends to give you the "whys".
And the anterior pituitary gland developed from a different embryonic tissue when you're
inside mom than the posterior.
The anterior portion developed from the roof of the mouth called the hypophyseal pouch
and that migrated up and became the anterior pituitary gland and the posterior pituitary
gland might - or I should say, developed from the neurohypophyseal - let me say that again,
neurohypophyseal bud and this came from a structure called the neural tube in the embryo.
And the neural tube just became the brain and spinal cord primarily, but it butted off
of that neural tube and then became the posterior pituitary gland.
And because of those differences in embryological origin, you get this difference in interaction
with the hypothalamus.
And to really show that, I’ve got a little whiteboard session for us.
So, to the whiteboard.
So, welcome to the whiteboard session and our rendition of the relationship between
the pituitary and the hypothalamus, let me orient you to the rest of the drawing here...
This is representing bone or the hypophyseal fossa that the pituitary gland is sitting
down into.
I’ve divided the pituitary into an anterior portion of a posterior portion.
FYI, this isn't perfectly to scale.
The anterior pituitary is actually larger and makes up about 75% of the total size of
the gland.
Gland's not big in and of itself, it's about the size of a pea as you saw on the cadaver
dissection.
Some other structures we need to mention that we've drawn in here are these green doodads.
Doodads is a technical term.
Those are representing neurons and they are interacting with the blood supply.
We need to talk about the difference of the blood supply here and what's going on here.
So, typically in the human body, this is a more typical pattern that you see versus this.
So we'll start here.
Typically you'll see a small artery, called an arteriole, take blood into a tissue.
And then when it goes into the tissue, it branches into what we call a capillary bed
or this capillary network.
And the capillaries are these tiny little blood vessels that we've drawn in this web
and those allow for exchange of nutrients and waste products between the bloodstream
and the tissue.
Then all of those capillary vessels start to converge into one and the blood leaves
typically on a tiny vein that we call a venule and then it goes back to the heart.
So, that's the typical pattern in the human body with blood supply.
You take blood into a tissue, branch into a capillary bed, converge and take it out
through a vein back to the heart.
On the anterior pituitary, you're seeing - yes, we take the blood in through like an arterial
here, branches into a capillary similar, but what we see next is it goes directly to another
capillary before it leaves through, say, a venule or a tiny vein before going back to
the heart.
Flowing from one capillary to another capillary is what we call a portal system.
It allows for quick exchange from whatever is going on in this capillary that it ensures
that it's going to go to this next capillary and allow for some exchange as well before
it goes to systemic circulation or back to the heart.
There's a few examples of this in the human body, this being one of them, another being
the liver - they call it the hepatic portal system.
Here we're dealing with a different portal system specific to the hypothalamus and the
pituitary.
And we'll talk about how that works in just a second.
But I actually want to start with the posterior pituitary because it's a little bit more simple
as you can see.
And we mentioned that the whole gland - anterior plus posterior.
A lot of people say it releases eight and I often refer to it releasing eight hormones
most of the time.
Technically there's nine - one gets left out, isn't talked about as much, I’m not sure
why - just doesn't have as I guess strong functionality as some of the other hormones
but I’ll mention it in a second.
But one of the reasons the posterior pituitary is a little bit more simple to learn is; one,
because it only secretes two hormones; oxytocin and antidiuretic hormone.
Antidiuretic hormone is also known as vasopressin and it just has a simplistic blood supply
compared to the anterior.
So, let me give you the example of how this works...
These neurons are going to manufacture either oxytocin or antidiuretic hormone depending
on which neuron it is.
They manufacture those in this structure called the cell body.
So every neuron has - that we're talking about here has a cell body where it's kind of a
manufacturing portion of the neuron and then it transports that neuron - or I’m sorry,
that hormone down this thing called the axon - this long portion of the neuron is called
the axon and then stores it in a vesicle - these sac-like structures inside the neuron until
it's ready to be released.
The end of a neuron is called the axon terminal FYI.
So let's say we are dealing with oxytocin.
Oxytocin was produced, gets transported down here waiting for the signal for release, hypothalamus
says "release it", gets released near the blood vessels and guess what happens?
It gets sucked in or we could say absorbed into the bloodstream and moves out and then
it's in systemic circulation.
That would be the same idea if it was anti-diuretic hormone, just different neuron manufactures
it, it transports it down.
When it gets released, goes into the bloodstream and then it can circulate to where it needs
to go throughout the human body.
So, as we move on to the anterior pituitary, we know already from previous discussion that
it's going to get the job done a little bit differently than the posterior pituitary.
One thing that's important to know is the seven hormones that are going to be secreted
by the cells surrounding these blood vessels from the anterior pituitary.
They are growth hormone, thyroid stimulating hormone, prolactin, adrenocorticotropic hormone,
luteinizing hormone, follicle stimulating hormone - and one that doesn't get mentioned
very often is called melanocyte stimulating hormone.
Now ,that's a mouthful and a lot to remember.
The nice thing about those hormones is they kind of imply what they're going to do in
the name like prolactins for lactation, thyroid stimulating hormone stimulates the thyroid
hormone, even the tongue twister - adrenocorticotropic has adreno in the name to help you know about
the adrenal gland.
But how do these get released or what tells them?
Because if you compare it to the posterior, no hormones were necessarily getting produced
in the actual tissue of the posterior pituitary, they're getting produced up here.
We have this interesting relationship with this portal system.
There's going to be some hormones released by these hypothalamic neurons here that go
into the bloodstream, come down into this capillary network, release out into the tissue
and then tell the cells here to secrete another hormone.
So, the best way to do this is to give you a specific example...
There's a hormone called thyrotropin-releasing hormone that is produced in some of the neurons
of the hypothalamus.
It's a mouthful I understand, but thyrotropin-releasing hormone gets produced in the cell body, moves
down the axon.
When it's stimulated to be released, it pops out of the axon terminal, goes into the bloodstream.
Because of this portal system, the first place that hormone is going to go is down to this
capillary network and get out of the capillary into the surrounding tissue.
Remember our receptor analogy - it's only going to bind to receptors that will fit that
specific hormone.
So, there's going to be receptors on certain cells that will bind to the thyrotropin-releasing
hormone.
And when it binds to those cells, it tells those cells in their "Hey, I need you to now
make thyroid stimulating hormone", and then the thyroid stimulating hormones produced
by the cells here gets dumped into the bloodstream and then it leaves to go to the systemic - to
systemic circulation which you can probably guess is going to go to the thyroid gland.
That's just one of the seven potential examples that I mentioned.
Because remember, there were seven hormones here.
And so, that's the same idea.
With whatever hormone that you want to be secreted down here, whether it's thyroid stimulating
hormone, prolactin, adrenal corticotropin hormone, any of those seven hormones I mentioned,
there's going to be first a releasing hormone up here that says "Okay, come out here, go
down there, tell the cells and hear what to do".
And you're thinking, "That's kind of a complicated process".
And I agree.
And you're saying "Why would the human body do this.
This posterior pituitary is a lot more simplistic in a way".
But again, the answer was that embryological answer.
This posterior pituitary is essentially an outgrowth of nervous tissue, which was what
I mentioned earlier in the video.
This came from glandular tissue in the roof of the mouth and migrated up and joined together
to form the whole pituitary, so there had to be some way to create an interaction between
the hypothalamus and the anterior pituitary and it did it through a blood supply.
Where this one you kind of just said "All right, we're just going to outgrow so - grow
out to this area here, create the hormones in these neurons and then we secrete it out
in a little bit more of a simplistic approach".
Hopefully that can kind of give you an idea between the two and uhh, simplify it a little
bit as you're trying to learn the details of future endocrine dysfunctions or endocrine
regular good functions that we want to happen.
Thanks for watching everyone.
Hopefully your minds were blown with endocrine awesomeness or maybe your minds were blown
because you noticed I wasn't wearing the same old black t-shirt that I wear in every single
video.
I was wearing a "mind blown" t-shirt.
So, if you're interested we've got some new t-shirts.
We'll put the link below.
We still have our other t-shirts and hoodies that we've had in the past.
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