Understanding the Endocrine System

Hi everyone, Dr. Mike here. In this video we're taking a look at an introduction to the endocrine system. We're going to talk about what hormones are, what triggers their release, how do they get transported through the bloodstream, all the various different types, the receptors, and so much more. Let's take a look. All right, to begin we need to define what the endocrine system actually is. A lot of us have an idea. but it has a very specific definition. And that definition is, it's a collection of cells, tissues, and glands that release chemical messengers into the bloodstream. So a couple of points here. They can be cells, tissues, and glands, meaning that while we might have explicit, what we call endocrine tissues, meaning their job is to release these chemicals into the bloodstream, we've got other cells and tissues of the body which might perform one function, but also an endocrine. So for example, the stomach, you don't think about it as being endocrine tissue. You go, well, that's part of the digestive system, not the endocrine system. It tears foods apart and chemically digests it so that we can absorb the various nutrients. But the stomach also produces chemical messengers that it drops into the bloodstream, which means that the stomach can be defined as endocrine tissue. Now, these chemical messengers we call hormones if they are dropped into the bloodstream. That's important because there are many chemical messengers of the body. And we know that the nervous system has chemical messengers that it releases from neurons that target tissues directly. And they aren't called hormones. They are called neurotransmitters. So it's important to understand this because the endocrine system is a communication network. And so is the nervous system. So if this is an endocrine cell tissue or gland, it's going to be releasing. A chemical messenger, like I said, which we call a hormone, which will jump into the bloodstream and travel to areas that will have or other tissues that will have receptors specific for it so it can bind to it, right? So that is endocrine. Now let's compare this to the nervous system because like I said, it's a communication system as well. So if for example, this was a neuron, so there's the cell body, there's the axon. and there's the terminal, so a very simplistic neuron, and there is its target cell with its receptor, that neuron is going to be releasing its chemicals, which are neurotransmitters that will bind. Now again, they're chemical messengers. What are the differences between the endocrine and the nervous system as modes of communication? Well, firstly, if we look at the nervous system, it is fast. It travels extremely quickly. Compare that to the endocrine system which is a lot slower. So those hormones are only going to travel at the speed in which the blood is moving. So that's dependent upon heart rate, for example, and blood pressure. So endocrine, a lot slower. The nervous system is direct. It is sending a signal from A to B and there's nothing in between. But have a look at the endocrine system. The endocrine system... Because it's going through the bloodstream, we know that there are all these little stop-offs on the way to the station that this hormone can get released. So it is not direct. It is definitively indirect. And that's really, really important. A couple of other things is that when it comes to thinking about the acting time of the hormone, or in this case, the neurotransmitter. It is short-acting. So you release the neurotransmitter, it has its effect, it then gets recycled or gobbled back up by the cell or the neuron, and it's short-acting. But hormones are longer-acting. I don't want to say long, because then I'll have to give you a time frame, but it's longer-acting. So they have a longer half-life in the system compared to neurotransmitters. So as you can see, there are some important differences. between the endocrine system and the nervous system as communication networks. Now I want to talk to you about what these hormones actually are. So I said they're chemical messengers, but what are the different types of chemical messengers? Well firstly let's have a look at the categories. So first thing is there's three major types of hormone categories. You can have protein slash peptide hormones. You can have steroid hormones. And you can have amino acid derived hormones. Amino acid. And to be more specific here, these are from one amino acid called tyrosine. So these are the three categories. So we've got protein-based, lipid-based, but no carbohydrate-based. There are no solely carbohydrate-based hormones. That's important. Some of the steroid hormones can have... sugars attached to them, sugar moieties, and we can call them, and proteins as well. And so that would be called glycoproteins, for example. But generally speaking, no carbohydrate hormones. So let's have a look at these differences. Firstly, protein peptide. Let's distinguish between the two. Broadly, if you've got more than 100 amino acids, it's recognized as a protein. If it's got less than 100 amino acids, it's recognized as a peptide. So that's an important first point. The protein or peptide-based hormones, they are the most abundant. So if you ever get a question in an exam asking you to classify the type of hormone and you have no clue, just guess protein or peptide-based. So what are some examples? The examples of the protein or peptide-based hormones are all of the pituitary hormones. So all the hormones released by The pituitary gland, they are protein or peptide based. What are these hormones? All right. So for example, if we have a look at the anterior lobe of the pituitary gland, that's going to include the gonadotropins and those gonadotropins are follicle stimulating hormone and luteinizing hormone. We've got the adrenocorticotropic hormone, we've got growth hormone, we've got prolactin, and we've got, if we have a think about the last one, thyroid stimulating hormone. So these hormones from the anterior pituitary are protein or peptide based. They also include those of the posterior pituitary. And these include antidiuretic hormone and oxytocin. So these are protein and peptide based. But that's not all. Importantly, other protein peptide based includes those of the pancreas. So importantly, what are these? Well, these are going to be insulin and glucagon. There we go. There is not an exhaustive list, but a good list of the protein peptide based hormones that you should be aware of. When it comes to steroid based hormones, these are made from cholesterol. So not amino acids, but cholesterol. So they are lipid soluble, not water soluble, lipid soluble. And if we have to think about where they are produced, most of these steroid hormones come from the adrenal cortex, come from the adrenal cortex. Now let's have a look at this, the adrenal cortex. If we have a look at, and I'm going to incorporate this here as well. So I'm going to just draw up, I'll do it in blue. If we have a look at the, I'll draw it over here. So I've got the kidney and on top of the kidneys, the adrenal gland. And the adrenal gland has two major sections. It's got this outer section here, which we call the cortex. So that's the adrenal cortex. And then you've got the inside here, which is what we call the adrenal medulla. So the adrenal cortex, it produces steroid-based hormones made from cholesterol. Now I'm going to draw this up like this. So imagine we take this whole adrenal gland into a cross section and have a look into it. What you're going to find is this. You're going to have the medulla here. So there's the medulla. And then you're going to have the cortex here. So I'll write it like this. There's the cortex. Now the cortex has three major layers associated with it. We've got the glomerulosa, and it's called the zona glomerulosa, the zona fasciculata. and the zona reticularis. And they produce different hormones. So the glomerulosa produces a hormone called aldosterone. The fasciculata produces a hormone called cortisol. And the reticularis produces androgens, which are male sex hormones. Now, all three of these, because they're in the cortex, they are steroid hormones. Now importantly, aldosterone is also known as a mineralocorticoid. So this is just for completion's sake, right? Mineralocorticoid. Cortisol is known as a glucocorticoid. What's the difference? Mineralo, mineral. Because what aldosterone does is it tells sodium, which is a mineral, to be reabsorbed. Corticoid is at the cortex, so that's the corti. And oid... tells you it's a steroid. So cortisol, glucose, it plays around with blood glucose levels, increases blood glucose levels, again, corticoid because it's in the cortex and it's a steroid. All right. So these are the steroid hormones of the adrenal cortex, but that's not all of them. What we also have is those of the gonads, which is obviously going to be the ovaries and the testes. So they also produce steroid hormones. The steroid hormones that they produce include estrogen, progesterone, and testosterone. So we've got our steroid hormones here. Now, when it comes to the amino acid derived from tyrosine, It's important because I've drawn the medulla here. Now, I know it's a bit of an angle because that's how, unfortunately, when I'm looking at this whiteboard, it's a bit of an angle. But just so you know, the amino acid derived actually come from the adrenal medulla. The adrenal medulla. And the adrenal medulla produces the catecholamines. Catecholamines. Now what are the catecholamines? The catecholamines, specifically the medulla produces one important catecholamine which is or you could say two noradrenaline slash adrenaline Similar but different. Mostly it's adrenaline that gets released from the adrenal medulla. But catecholamines also include dopamine. Now dopamine isn't produced here at the adrenal medulla, but it is a catecholamine and it is amino acid based, tyrosine based. But there's another hormone that is amino acid based. And this is from the thyroid gland. And the hormone from the thyroid gland is unsurprisingly... thyroid hormone. Or you can argue thyroid hormones, which is T3 and T4. Just remember, T3 is the active form. Right. So what we've got here are the different types. We've got proteins, peptides, steroids, and amino acids. You can see the most abundant are the protein peptides. Steroid-based in the adrenal cortex and gonads, and the amino acid-based include the catecholamines, noradrenaline, adrenaline, and dopamine, and also the thyroid hormones. What I want to do now is have a look at what stimulates these hormones to get released, right? So we've spoken about all the different types. Let's talk about what stimulates these hormones to get released. So there are three main stimuli that trigger hormones to get released. So let's draw these up first and then we'll write them down. We've got you. Now you might go, hey I remember you drawing this thing before. So let's have a look, let's see what it is. Okay, hopefully you're looking at this going, I think I know what this is. Okay, there's the first one. Let's draw up the second one. What's this one doing? Hmm. This might look a little bit similar to what I drew up before. Okay, there's the second one. Let's draw up the third one. Alright, looks very similar to the second one, doesn't it? Alright, these are the three ways. So all of these down here are endocrine cells, tissues or glands that need to release hormones. How are we triggering them to be released? First one, what we've got is a neuron. is triggering the adrenal medulla to release a hormone. Do you remember what I just said before, right? What does the adrenal medulla release? The adrenal medulla releases adrenaline. The adrenal medulla releases adrenaline. That's a hormone and it releases that adrenaline into the bloodstream. So here because there's a neuron triggering this hormone to be released, this is called a neural stimulus. So you can have neurons stimulating hormones to be released. I'm going to come back to this one in one second but let's first go through all of them. Here we've got an endocrine cell or tissue or gland releasing a hormone that's going to jump into the bloodstream and in the bloodstream it's going to travel to another uh, endocrine cell tissue or gland and tell it to release its hormone into the bloodstream. Okay. Because it's a hormone telling a gland to release another hormone, this is a hormonal stimulus. So that's the second one, a hormonal stimulus. And the third one looks very similar. This here can be a nutrient, right? Or a mineral, for example, some substance. So let's just say it's a nutrient. And let's say this nutrient is glucose. And this glucose nutrient is jumping into the bloodstream. And... binding to what would glucose bind to the pancreas right beta cells within the pancreas and it tells it to release insulin and then that insulin is going to jump into the bloodstream so that's not a hormone that's a nutrient and so we call this a humoral stimulus a humoral stimulus so these are the three different types of stimuli hormones can be released from neurons triggering them Other hormones triggering them. An important point here, when a hormone is getting released to tell another hormone to be released, it generally has in its name the suffix tropic. It's tropic. Like you could say an example of this is going to be corticotropic hormone. It goes, it's released by the hypothalamus and tells the anterior pituitary to release. adrenocorticotropic hormone. So corticotropic hormone tells another hormone to be released, ACTH, adrenocorticotropic hormone. You might say, that's also got tropic in it. That's right, because its job is to travel to the cortex to tell it to release more hormones. So when you see tropic, just know it's not the last hormone to be released. And then finally, you've got the humeral when it's a nutrient or it could be calcium, right? If it's low blood calcium, in this instance, it would be traveling to the parathyroid gland. to release parathyroid hormone to try and increase blood calcium levels. So this is the stimulus. I said I was going to tell you more about this. Yes, I do want to tell you more about this because it's an important thing to understand because none of these systems work in isolation. We've got here a neural stimulus to a hormone being released. This neuron, because it ultimately is releasing, triggering the release of adrenaline, this has got to be part of the sympathetic nervous system. So I'm going to just take a little bit of a detour to talk about the sympathetic nervous system because it is important. So here's the brain from a side view and we're going to have a look. And there's the brainstem and spinal cord. And for good measure, there's the cerebellum. What you probably already know is that the autonomic nervous system has two divisions. You're sympathetic, which is fight or flight, and you're parasympathetic, which is rest and digest. Now you're sympathetic, the fight or flight. Its neurons exit the spinal cord from either the thoracic or the lumbar region. That's why the sympathetic nervous system, I'll write the whole thing for you, that's why the sympathetic nervous system is sometimes also referred to as the thoracolumbar system. Now, I said that there's neurons that come out from either the thoracic or lumbar. Let's draw up this particular neuron coming out of the thoracic. And there's one neuron and it speaks to a second neuron that will go to a target tissue. Let's say it's the heart. Now, what generally happens here is that the first neuron called the preganglionic neuron releases a hormone called, sorry, a neurotransmitter. Again, just another chemical messenger, but in this instance released by neurons called acetylcholine. That binds to the postganglionic, which releases another neurotransmitter. which is called noradrenaline, or if you're from the US, norepinephrine. All right. All of them are two neuron chains, except in one instance. There is one instance where a neuron will exit and this neuron will not synapse with another neuron. It will synapse in the medulla of the adrenal gland, right? So let's draw the kidney underneath here. Right? And so this pre-ganglionic neuron is just like that one. So what does it release? It releases acetylcholine. So it's releasing acetylcholine here. And it's the acetylcholine that triggers this adrenal medulla. And this adrenal medulla is acting just like the post-ganglionic neuron, triggering it. So if it's acting like the post-ganglionic neuron, what does that release? Noradrenaline. So in this instance, it is the hormone version of noradrenaline. which is adrenaline. And because that adrenaline jumps into the bloodstream, it goes to the whole body and you have an entire body effect of fight or flight. I thought that was an important point to highlight. Now, what I want to talk about is how, once these hormones are released, we know that they jump into the bloodstream. How do they travel through the bloodstream? So a couple of points. What were the different classes I said, right? So let's just say we have peptide or protein, we have steroid and we have amino acid, specifically tyrosine based, right? How did these travel through the bloodstream? So firstly, peptides, as you probably know, proteins are mostly negatively charged. And as we know, water. has both a positive and negative charge. So if you don't know this already, water being hydrogen, oxygen, hydrogen, H2O, the hydrogen have a slight positive charge, water has a slight negative charge. Now proteins and peptides, which are these three-dimensional folded structures, are mostly negatively charged. And the negative charge loves the positive charge of water. So we say that proteins love water. They're hydrophilic. So they can jump straight into the bloodstream, which is mostly water. and mix beautifully. So peptides are actually, they travel freely within the bloodstream, freely within the bloodstream. So that means not bound, not bound to something to carry it. Steroids on the other hand, they're lipid soluble and we know fats and lipids don't like water. So they need to be bound and they are bound. to plasma proteins. And there's many different types of plasma proteins. One important one is albumin, but pretty much all of them are produced in the liver. Most of them are produced in the liver. And why is this important? It's important because if your liver doesn't function very well anymore because of hepatitis or cirrhosis or whatever liver disease that somebody might get, it's going to affect the way these steroid proteins are bound and can travel. And why is that important? It's important because if you compare bound to free proteins, the active form is the free protein. So regardless of the two, right? Free hormone, I said free protein. Free hormone is the active hormone. Now you might think, so that means none of the steroid hormones are active, not when they're bound, but they can be released and they become free. So that also means that a good thing for the steroid hormones is they have a longer half-life in the bloodstream. They have a longer half-life, meaning they don't get... destroyed as quickly. And that's because they're carried by these proteins, right? And they don't get targeted for basically metabolism and get to be excreted. Longer half-life. But it also means that it increases their pool, available pool, because they're bound to those proteins. But if the liver doesn't work... very well. These proteins aren't produced and the steroid hormones have a shorter half-life and they don't work. And then the balance of free to bound is off and there can be a problem there. What about the amino acid ones? Well, these, the catecholamines, if we think of the catecholamines, and I'll just tell you what they are again, just in case you forgot, that's adrenaline and noradrenaline and dopamine. They are free, right? They're transported freely. However, I said there's another amino acid-based hormone. What was it? Do you remember? It was thyroid hormone, T3, T4. So again, I should say thyroid hormones. Now, interestingly, T3, T4 tells you about how many iodine molecules are attached to it. right it's important because once iodine is bound to this tyrosine based hormone it no longer is water soluble so it needs a carrier protein it needs a carrier protein because it's no longer water soluble so keep that in mind the peptides free floating not bound to carry proteins steroids they are a good thing is that you gives them a longer half-life and keeps a pool of them available. However, the active form is the free form only. If the liver doesn't work, steroid-based hormones can go off in the bloodstream. And the amino acid-based ones, the catecholamines, they're free-floating, but the thyroid need to carry a protein. So now they're transporting the bloodstream, they get to their target tissues, what happens at the target tissue? This is really important because if you have a think about hormones in the bloodstream, Their concentration is really, really low. So hormone concentration, hormone concentration generally sits at around about 10 to the negative seven to 10 to the negative 12 molar. And you might think, what the hell's that? All right, let's just put this into context. 10 to the negative seven. If you've got 1.0 molar, and I wanna figure out what 10 to the negative seven is, I'm going to move it seven decimal places into the negative. One, two, three, four, five, six, seven. So there's the new decimal point. And I'm going to put zeros under here. So that's the concentration of 10 to the negative seven. It's 0.0000001 molar. That is pretty much the strongest concentration of hormones that you'll get in the blood. 10 to the negative 12, well, you're going to have to add... what, 7, 8, 9, 10, 11, 12, 5 more zeros in that direction, right? So my point is that they're in very low concentration. What does this mean? It means two things, that hormone receptors, hormones and their receptors must have strong affinity and specificity, spell that right, specificity. Okay. So first affinity, affinity is how strongly will it bind to the receptor. So that means the hormone and the receptor must have a perfect key lock mechanism. So it must bind really, really tightly. So that's the first thing because it's such low concentration. Second thing is it's specificity. It needs to be very specific. The hormone must be very specific. to the receptor so that you don't get any cross hormone binding or you don't get other chemicals binding to it. So the specificity must be high as well. Two important things. Now once the hormone is bound to the receptor, well firstly where are these receptors located? That's a really, really important point. So where are these receptors located? Let's draw up a cell. There's a nice big cell and there's the nucleus inside of that cell. And we know that inside the nucleus we've got our DNA. Alright, I said you've got different types of hormones, right? We've got our peptides, we've got our steroids, we've got our... And I'm going to break up the amino acids into the two, right? So catecholamines. and the thyroid hormones. So we're going to go through these. So firstly, peptide. Because peptides are water-soluble, they were transported freely, right? But when they get to a cell, remember the cell is surrounded by a phospholipid bilayer, so a fatty layer. The peptides cannot pass through the fatty layer. So they must bind to what we call... membrane receptors, membrane receptors. So peptides, peptides will bind to membrane receptors. Steroids, steroids are lipid soluble. They are bound to proteins in the bloodstream, then they are released and they can, because they're lipid soluble, they can be transported through the layer, the phospholipid bilayer, and they will bind to receptors in the cytoplasm. And those receptors that they bind to generally are associated with what we call transcription factors. And they then will, once they're bound, once they're bound, will translocate into the nucleus and then have its effect on the DNA, transcribing then translating the DNA. This is going to be steroid. And like I said, this is a cytoplasmic receptor, cytoplasmic receptors, and generally they're transcription factors. All right, so we've done that. What about the catecholamines? The catecholamines are the catecholamines. work like the peptides. So they are membrane receptors because amino acids and peptides are pretty much the same thing. However, thyroid hormone is different. Thyroid hormone needs to get, because it's got to carry a protein, right? It's got to carry a protein because of the iodine that's attached to it. Then when it gets to the cell, it's released, but it's not lipid soluble. Thyroid hormone, even though it acted like it was lipid soluble in the bloodstream, can't just transport through the membrane. So it needs specific membrane transporters. So membrane transporters. And this allows for thyroid hormone to enter the cell. And then... The thyroid hormone can freely diffuse into the nucleus and in the nucleus it binds to receptors that are on the DNA. So these are what we call nuclear receptors. Nuclear receptors. And again they're going to help transcribe DNA. They're going to say, turn things on, turn things off. So we can tick that one off. So again, the peptides and the catecholamines, they both work together through the membrane receptors. The steroids, they work through the cytoplasmic receptors and the thyroid hormone works via the nuclear receptors. Brilliant. Now they can have their particular effect. All right. is my introduction to the endocrine system. I hope that makes sense. I'm Dr. Mike.

Let's take a look. All right, to begin we need to define what the endocrine system actually is. A lot of us have an idea.

but it has a very specific definition. And that definition is, it's a collection of cells, tissues, and glands that release chemical messengers into the bloodstream. So a couple of points here. They can be cells, tissues, and glands, meaning that while we might have explicit, what we call endocrine tissues, meaning their job is to release these chemicals into the bloodstream, we've got other cells and tissues of the body which might perform one function, but also an endocrine. So for example, the stomach, you don't think about it as being endocrine tissue.

You go, well, that's part of the digestive system, not the endocrine system. It tears foods apart and chemically digests it so that we can absorb the various nutrients. But the stomach also produces chemical messengers that it drops into the bloodstream, which means that the stomach can be defined as endocrine tissue. Now, these chemical messengers we call hormones if they are dropped into the bloodstream. That's important because there are many chemical messengers of the body.

And we know that the nervous system has chemical messengers that it releases from neurons that target tissues directly. And they aren't called hormones. They are called neurotransmitters.

So it's important to understand this because the endocrine system is a communication network. And so is the nervous system. So if this is an endocrine cell tissue or gland, it's going to be releasing.

A chemical messenger, like I said, which we call a hormone, which will jump into the bloodstream and travel to areas that will have or other tissues that will have receptors specific for it so it can bind to it, right? So that is endocrine. Now let's compare this to the nervous system because like I said, it's a communication system as well. So if for example, this was a neuron, so there's the cell body, there's the axon. and there's the terminal, so a very simplistic neuron, and there is its target cell with its receptor, that neuron is going to be releasing its chemicals, which are neurotransmitters that will bind.

Now again, they're chemical messengers. What are the differences between the endocrine and the nervous system as modes of communication? Well, firstly, if we look at the nervous system, it is fast. It travels extremely quickly.

Compare that to the endocrine system which is a lot slower. So those hormones are only going to travel at the speed in which the blood is moving. So that's dependent upon heart rate, for example, and blood pressure.

So endocrine, a lot slower. The nervous system is direct. It is sending a signal from A to B and there's nothing in between.

But have a look at the endocrine system. The endocrine system... Because it's going through the bloodstream, we know that there are all these little stop-offs on the way to the station that this hormone can get released. So it is not direct.

It is definitively indirect. And that's really, really important. A couple of other things is that when it comes to thinking about the acting time of the hormone, or in this case, the neurotransmitter. It is short-acting.

So you release the neurotransmitter, it has its effect, it then gets recycled or gobbled back up by the cell or the neuron, and it's short-acting. But hormones are longer-acting. I don't want to say long, because then I'll have to give you a time frame, but it's longer-acting.

So they have a longer half-life in the system compared to neurotransmitters. So as you can see, there are some important differences. between the endocrine system and the nervous system as communication networks. Now I want to talk to you about what these hormones actually are.

So I said they're chemical messengers, but what are the different types of chemical messengers? Well firstly let's have a look at the categories. So first thing is there's three major types of hormone categories.

You can have protein slash peptide hormones. You can have steroid hormones. And you can have amino acid derived hormones. Amino acid. And to be more specific here, these are from one amino acid called tyrosine.

So these are the three categories. So we've got protein-based, lipid-based, but no carbohydrate-based. There are no solely carbohydrate-based hormones.

That's important. Some of the steroid hormones can have... sugars attached to them, sugar moieties, and we can call them, and proteins as well. And so that would be called glycoproteins, for example.

But generally speaking, no carbohydrate hormones. So let's have a look at these differences. Firstly, protein peptide. Let's distinguish between the two.

Broadly, if you've got more than 100 amino acids, it's recognized as a protein. If it's got less than 100 amino acids, it's recognized as a peptide. So that's an important first point.

The protein or peptide-based hormones, they are the most abundant. So if you ever get a question in an exam asking you to classify the type of hormone and you have no clue, just guess protein or peptide-based. So what are some examples?

The examples of the protein or peptide-based hormones are all of the pituitary hormones. So all the hormones released by The pituitary gland, they are protein or peptide based. What are these hormones?

All right. So for example, if we have a look at the anterior lobe of the pituitary gland, that's going to include the gonadotropins and those gonadotropins are follicle stimulating hormone and luteinizing hormone. We've got the adrenocorticotropic hormone, we've got growth hormone, we've got prolactin, and we've got, if we have a think about the last one, thyroid stimulating hormone.

So these hormones from the anterior pituitary are protein or peptide based. They also include those of the posterior pituitary. And these include antidiuretic hormone and oxytocin. So these are protein and peptide based. But that's not all.

Importantly, other protein peptide based includes those of the pancreas. So importantly, what are these? Well, these are going to be insulin and glucagon.

There we go. There is not an exhaustive list, but a good list of the protein peptide based hormones that you should be aware of. When it comes to steroid based hormones, these are made from cholesterol.

So not amino acids, but cholesterol. So they are lipid soluble, not water soluble, lipid soluble. And if we have to think about where they are produced, most of these steroid hormones come from the adrenal cortex, come from the adrenal cortex.

Now let's have a look at this, the adrenal cortex. If we have a look at, and I'm going to incorporate this here as well. So I'm going to just draw up, I'll do it in blue.

If we have a look at the, I'll draw it over here. So I've got the kidney and on top of the kidneys, the adrenal gland. And the adrenal gland has two major sections.

It's got this outer section here, which we call the cortex. So that's the adrenal cortex. And then you've got the inside here, which is what we call the adrenal medulla.

So the adrenal cortex, it produces steroid-based hormones made from cholesterol. Now I'm going to draw this up like this. So imagine we take this whole adrenal gland into a cross section and have a look into it.

What you're going to find is this. You're going to have the medulla here. So there's the medulla.

And then you're going to have the cortex here. So I'll write it like this. There's the cortex. Now the cortex has three major layers associated with it.

We've got the glomerulosa, and it's called the zona glomerulosa, the zona fasciculata. and the zona reticularis. And they produce different hormones. So the glomerulosa produces a hormone called aldosterone.

The fasciculata produces a hormone called cortisol. And the reticularis produces androgens, which are male sex hormones. Now, all three of these, because they're in the cortex, they are steroid hormones.

Now importantly, aldosterone is also known as a mineralocorticoid. So this is just for completion's sake, right? Mineralocorticoid. Cortisol is known as a glucocorticoid. What's the difference?

Mineralo, mineral. Because what aldosterone does is it tells sodium, which is a mineral, to be reabsorbed. Corticoid is at the cortex, so that's the corti.

And oid... tells you it's a steroid. So cortisol, glucose, it plays around with blood glucose levels, increases blood glucose levels, again, corticoid because it's in the cortex and it's a steroid.

All right. So these are the steroid hormones of the adrenal cortex, but that's not all of them. What we also have is those of the gonads, which is obviously going to be the ovaries and the testes. So they also produce steroid hormones. The steroid hormones that they produce include estrogen, progesterone, and testosterone.

So we've got our steroid hormones here. Now, when it comes to the amino acid derived from tyrosine, It's important because I've drawn the medulla here. Now, I know it's a bit of an angle because that's how, unfortunately, when I'm looking at this whiteboard, it's a bit of an angle. But just so you know, the amino acid derived actually come from the adrenal medulla.

The adrenal medulla. And the adrenal medulla produces the catecholamines. Catecholamines.

Now what are the catecholamines? The catecholamines, specifically the medulla produces one important catecholamine which is or you could say two noradrenaline slash adrenaline Similar but different. Mostly it's adrenaline that gets released from the adrenal medulla. But catecholamines also include dopamine.

Now dopamine isn't produced here at the adrenal medulla, but it is a catecholamine and it is amino acid based, tyrosine based. But there's another hormone that is amino acid based. And this is from the thyroid gland. And the hormone from the thyroid gland is unsurprisingly... thyroid hormone.

Or you can argue thyroid hormones, which is T3 and T4. Just remember, T3 is the active form. Right. So what we've got here are the different types.

We've got proteins, peptides, steroids, and amino acids. You can see the most abundant are the protein peptides. Steroid-based in the adrenal cortex and gonads, and the amino acid-based include the catecholamines, noradrenaline, adrenaline, and dopamine, and also the thyroid hormones.

What I want to do now is have a look at what stimulates these hormones to get released, right? So we've spoken about all the different types. Let's talk about what stimulates these hormones to get released.

So there are three main stimuli that trigger hormones to get released. So let's draw these up first and then we'll write them down. We've got you. Now you might go, hey I remember you drawing this thing before. So let's have a look, let's see what it is.

Okay, hopefully you're looking at this going, I think I know what this is. Okay, there's the first one. Let's draw up the second one. What's this one doing? Hmm.

This might look a little bit similar to what I drew up before. Okay, there's the second one. Let's draw up the third one.

Alright, looks very similar to the second one, doesn't it? Alright, these are the three ways. So all of these down here are endocrine cells, tissues or glands that need to release hormones.

How are we triggering them to be released? First one, what we've got is a neuron. is triggering the adrenal medulla to release a hormone. Do you remember what I just said before, right? What does the adrenal medulla release?

The adrenal medulla releases adrenaline. The adrenal medulla releases adrenaline. That's a hormone and it releases that adrenaline into the bloodstream.

So here because there's a neuron triggering this hormone to be released, this is called a neural stimulus. So you can have neurons stimulating hormones to be released. I'm going to come back to this one in one second but let's first go through all of them.

Here we've got an endocrine cell or tissue or gland releasing a hormone that's going to jump into the bloodstream and in the bloodstream it's going to travel to another uh, endocrine cell tissue or gland and tell it to release its hormone into the bloodstream. Okay. Because it's a hormone telling a gland to release another hormone, this is a hormonal stimulus. So that's the second one, a hormonal stimulus.

And the third one looks very similar. This here can be a nutrient, right? Or a mineral, for example, some substance. So let's just say it's a nutrient.

And let's say this nutrient is glucose. And this glucose nutrient is jumping into the bloodstream. And... binding to what would glucose bind to the pancreas right beta cells within the pancreas and it tells it to release insulin and then that insulin is going to jump into the bloodstream so that's not a hormone that's a nutrient and so we call this a humoral stimulus a humoral stimulus so these are the three different types of stimuli hormones can be released from neurons triggering them Other hormones triggering them.

An important point here, when a hormone is getting released to tell another hormone to be released, it generally has in its name the suffix tropic. It's tropic. Like you could say an example of this is going to be corticotropic hormone.

It goes, it's released by the hypothalamus and tells the anterior pituitary to release. adrenocorticotropic hormone. So corticotropic hormone tells another hormone to be released, ACTH, adrenocorticotropic hormone.

You might say, that's also got tropic in it. That's right, because its job is to travel to the cortex to tell it to release more hormones. So when you see tropic, just know it's not the last hormone to be released. And then finally, you've got the humeral when it's a nutrient or it could be calcium, right?

If it's low blood calcium, in this instance, it would be traveling to the parathyroid gland. to release parathyroid hormone to try and increase blood calcium levels. So this is the stimulus.

I said I was going to tell you more about this. Yes, I do want to tell you more about this because it's an important thing to understand because none of these systems work in isolation. We've got here a neural stimulus to a hormone being released. This neuron, because it ultimately is releasing, triggering the release of adrenaline, this has got to be part of the sympathetic nervous system. So I'm going to just take a little bit of a detour to talk about the sympathetic nervous system because it is important.

So here's the brain from a side view and we're going to have a look. And there's the brainstem and spinal cord. And for good measure, there's the cerebellum. What you probably already know is that the autonomic nervous system has two divisions.

You're sympathetic, which is fight or flight, and you're parasympathetic, which is rest and digest. Now you're sympathetic, the fight or flight. Its neurons exit the spinal cord from either the thoracic or the lumbar region.

That's why the sympathetic nervous system, I'll write the whole thing for you, that's why the sympathetic nervous system is sometimes also referred to as the thoracolumbar system. Now, I said that there's neurons that come out from either the thoracic or lumbar. Let's draw up this particular neuron coming out of the thoracic.

And there's one neuron and it speaks to a second neuron that will go to a target tissue. Let's say it's the heart. Now, what generally happens here is that the first neuron called the preganglionic neuron releases a hormone called, sorry, a neurotransmitter.

Again, just another chemical messenger, but in this instance released by neurons called acetylcholine. That binds to the postganglionic, which releases another neurotransmitter. which is called noradrenaline, or if you're from the US, norepinephrine. All right.

All of them are two neuron chains, except in one instance. There is one instance where a neuron will exit and this neuron will not synapse with another neuron. It will synapse in the medulla of the adrenal gland, right? So let's draw the kidney underneath here. Right?

And so this pre-ganglionic neuron is just like that one. So what does it release? It releases acetylcholine. So it's releasing acetylcholine here.

And it's the acetylcholine that triggers this adrenal medulla. And this adrenal medulla is acting just like the post-ganglionic neuron, triggering it. So if it's acting like the post-ganglionic neuron, what does that release? Noradrenaline. So in this instance, it is the hormone version of noradrenaline.

which is adrenaline. And because that adrenaline jumps into the bloodstream, it goes to the whole body and you have an entire body effect of fight or flight. I thought that was an important point to highlight.

Now, what I want to talk about is how, once these hormones are released, we know that they jump into the bloodstream. How do they travel through the bloodstream? So a couple of points. What were the different classes I said, right?

So let's just say we have peptide or protein, we have steroid and we have amino acid, specifically tyrosine based, right? How did these travel through the bloodstream? So firstly, peptides, as you probably know, proteins are mostly negatively charged. And as we know, water. has both a positive and negative charge.

So if you don't know this already, water being hydrogen, oxygen, hydrogen, H2O, the hydrogen have a slight positive charge, water has a slight negative charge. Now proteins and peptides, which are these three-dimensional folded structures, are mostly negatively charged. And the negative charge loves the positive charge of water. So we say that proteins love water. They're hydrophilic.

So they can jump straight into the bloodstream, which is mostly water. and mix beautifully. So peptides are actually, they travel freely within the bloodstream, freely within the bloodstream.

So that means not bound, not bound to something to carry it. Steroids on the other hand, they're lipid soluble and we know fats and lipids don't like water. So they need to be bound and they are bound.

to plasma proteins. And there's many different types of plasma proteins. One important one is albumin, but pretty much all of them are produced in the liver. Most of them are produced in the liver. And why is this important?

It's important because if your liver doesn't function very well anymore because of hepatitis or cirrhosis or whatever liver disease that somebody might get, it's going to affect the way these steroid proteins are bound and can travel. And why is that important? It's important because if you compare bound to free proteins, the active form is the free protein.

So regardless of the two, right? Free hormone, I said free protein. Free hormone is the active hormone. Now you might think, so that means none of the steroid hormones are active, not when they're bound, but they can be released and they become free. So that also means that a good thing for the steroid hormones is they have a longer half-life in the bloodstream.

They have a longer half-life, meaning they don't get... destroyed as quickly. And that's because they're carried by these proteins, right?

And they don't get targeted for basically metabolism and get to be excreted. Longer half-life. But it also means that it increases their pool, available pool, because they're bound to those proteins.

But if the liver doesn't work... very well. These proteins aren't produced and the steroid hormones have a shorter half-life and they don't work.

And then the balance of free to bound is off and there can be a problem there. What about the amino acid ones? Well, these, the catecholamines, if we think of the catecholamines, and I'll just tell you what they are again, just in case you forgot, that's adrenaline and noradrenaline and dopamine.

They are free, right? They're transported freely. However, I said there's another amino acid-based hormone. What was it? Do you remember?

It was thyroid hormone, T3, T4. So again, I should say thyroid hormones. Now, interestingly, T3, T4 tells you about how many iodine molecules are attached to it.

right it's important because once iodine is bound to this tyrosine based hormone it no longer is water soluble so it needs a carrier protein it needs a carrier protein because it's no longer water soluble so keep that in mind the peptides free floating not bound to carry proteins steroids they are a good thing is that you gives them a longer half-life and keeps a pool of them available. However, the active form is the free form only. If the liver doesn't work, steroid-based hormones can go off in the bloodstream. And the amino acid-based ones, the catecholamines, they're free-floating, but the thyroid need to carry a protein.

So now they're transporting the bloodstream, they get to their target tissues, what happens at the target tissue? This is really important because if you have a think about hormones in the bloodstream, Their concentration is really, really low. So hormone concentration, hormone concentration generally sits at around about 10 to the negative seven to 10 to the negative 12 molar. And you might think, what the hell's that?

All right, let's just put this into context. 10 to the negative seven. If you've got 1.0 molar, and I wanna figure out what 10 to the negative seven is, I'm going to move it seven decimal places into the negative. One, two, three, four, five, six, seven.

So there's the new decimal point. And I'm going to put zeros under here. So that's the concentration of 10 to the negative seven. It's 0.0000001 molar.

That is pretty much the strongest concentration of hormones that you'll get in the blood. 10 to the negative 12, well, you're going to have to add... what, 7, 8, 9, 10, 11, 12, 5 more zeros in that direction, right?

So my point is that they're in very low concentration. What does this mean? It means two things, that hormone receptors, hormones and their receptors must have strong affinity and specificity, spell that right, specificity. Okay. So first affinity, affinity is how strongly will it bind to the receptor.

So that means the hormone and the receptor must have a perfect key lock mechanism. So it must bind really, really tightly. So that's the first thing because it's such low concentration. Second thing is it's specificity. It needs to be very specific.

The hormone must be very specific. to the receptor so that you don't get any cross hormone binding or you don't get other chemicals binding to it. So the specificity must be high as well. Two important things. Now once the hormone is bound to the receptor, well firstly where are these receptors located?

That's a really, really important point. So where are these receptors located? Let's draw up a cell. There's a nice big cell and there's the nucleus inside of that cell. And we know that inside the nucleus we've got our DNA.

Alright, I said you've got different types of hormones, right? We've got our peptides, we've got our steroids, we've got our... And I'm going to break up the amino acids into the two, right? So catecholamines. and the thyroid hormones.

So we're going to go through these. So firstly, peptide. Because peptides are water-soluble, they were transported freely, right?

But when they get to a cell, remember the cell is surrounded by a phospholipid bilayer, so a fatty layer. The peptides cannot pass through the fatty layer. So they must bind to what we call...

membrane receptors, membrane receptors. So peptides, peptides will bind to membrane receptors. Steroids, steroids are lipid soluble.

They are bound to proteins in the bloodstream, then they are released and they can, because they're lipid soluble, they can be transported through the layer, the phospholipid bilayer, and they will bind to receptors in the cytoplasm. And those receptors that they bind to generally are associated with what we call transcription factors. And they then will, once they're bound, once they're bound, will translocate into the nucleus and then have its effect on the DNA, transcribing then translating the DNA. This is going to be steroid.

And like I said, this is a cytoplasmic receptor, cytoplasmic receptors, and generally they're transcription factors. All right, so we've done that. What about the catecholamines? The catecholamines are the catecholamines.

work like the peptides. So they are membrane receptors because amino acids and peptides are pretty much the same thing. However, thyroid hormone is different. Thyroid hormone needs to get, because it's got to carry a protein, right?

It's got to carry a protein because of the iodine that's attached to it. Then when it gets to the cell, it's released, but it's not lipid soluble. Thyroid hormone, even though it acted like it was lipid soluble in the bloodstream, can't just transport through the membrane.

So it needs specific membrane transporters. So membrane transporters. And this allows for thyroid hormone to enter the cell. And then...

The thyroid hormone can freely diffuse into the nucleus and in the nucleus it binds to receptors that are on the DNA. So these are what we call nuclear receptors. Nuclear receptors. And again they're going to help transcribe DNA.

They're going to say, turn things on, turn things off. So we can tick that one off. So again, the peptides and the catecholamines, they both work together through the membrane receptors. The steroids, they work through the cytoplasmic receptors and the thyroid hormone works via the nuclear receptors.

Brilliant. Now they can have their particular effect. All right.

is my introduction to the endocrine system. I hope that makes sense. I'm Dr. Mike.

Transcript for:Understanding the Endocrine System

Transcript for:
Understanding the Endocrine System