Endocrine System: Study with Video Lessons, Practice Problems & Examples

The endocrine system utilizes hormones for long-distance communication, impacting growth, metabolism, and homeostasis. Key glands include the hypothalamus, which regulates the pituitary gland, and the thyroid, responsible for metabolic rate and calcium homeostasis through hormones like T3 and T4. The adrenal glands manage stress responses via cortisol and epinephrine. Hormones can be classified as steroid or water-soluble, influencing their mechanism of action. For instance, steroid hormones alter gene expression, while water-soluble hormones activate signal transduction pathways, demonstrating the complexity of hormonal regulation in physiological processes.

Chemical Signaling

Hi. In this lesson, we'll be talking about the endocrine system, which is a chemical signaling system used throughout the body. Now before we get into the specifics of the endocrine system, I want to review some other types of chemical signaling. Pheromones are chemical signals that are released to the environment and allow organisms to communicate with each other. They're commonly used by insects and mammals, but they are used by other organisms as well. Autocrine signaling is a type of self-signaling where a cell will secrete a chemical signal that stimulates receptors on its own membrane. You can see autocrine signaling happening right here, where this cell is releasing these signaling molecules and they're going to bind to the receptors on that cell's surface. An example of this are the cytokines that are released by T cells. These will actually act as an autocrine signal for those T cells. Juxtacrine signaling is also a close-range type of signaling, but this is where cells signal their neighbors that they have physical contact with. Frequently this will be signaling through something like gap junctions, or in the case of plants, plasmodesmata. You can see an example of these neighboring cells communicating with each other through this physical contact, signaling through those physical connections. Paracrine signaling is when cells release chemicals that will communicate with nearby and neighboring cells. So broader range than juxtacrine signaling. You don't necessarily have to be in physical contact with one another. And you can see paracrine signaling happening right here, where this cell that's close by to this cell is going to release these chemicals, and those are going to stimulate a receptor on the other cell's membrane. These signaling molecules, known as local regulators, can actually act as both autocrine and paracrine signals. Both types of cell signaling will result in cells secreting signaling molecules to their environments. Nitric oxide is an example of a local regulator, and it can act as a hormone causing vasodilation, or dilation of blood vessels, and it also acts in the brain as a neurotransmitter, though its effects there are a little too complicated for us to get into right now. But the point is that these molecules can have many effects in different parts of the body. Another example of paracrine signaling is prostaglandins, which are going to promote inflammatory responses. Additionally, the famous hormones insulin, glucagon, and somatostatin, which are secreted by the pancreas, and we'll be talking about in this lesson, will also have paracrine effects on the pancreas. So, really, I'm just trying to give you these examples to show that these signaling molecules have a really broad array of effects. We're really only going to be talking about some of the effects like the main important ones for our purposes and understanding physiology of certain systems, but they have many different effects and spread all throughout the body. As you can see with these paracrine signals that will act locally as well. And finally, we have endocrine signaling. This is going to be what we focus on, and this is when cells secrete hormones that actually will travel through the bloodstream to reach distant targets. These cells are trying to communicate with cells that are far away from them. One of the defining features of endocrine signaling is it allows for long-distance chemical communication in the body, and it uses the bloodstream as a highway for those hormones to get around. Let's flip the page.

Neuroendocrine Signaling

The endocrine system is tightly linked with the other major signaling system of the body, the nervous system. Synaptic signaling, the signaling of the nervous system, involves these cells called neurons, which you can see an example of here. And these neurons will actually transmit electrical signals through their body. When they reach the target, essentially what happens is that electric signal gets translated into a chemical signal and released to the other cell as a chemical signal. We call this connection between the two cells the synapse, and the cell that has the electric signal going through it, that's my little lightning bolt for an electric signal, is going to translate that signal into neurotransmitters. These chemicals it will release that will cross this very small gap known as the synapse. And they'll bind to receptors on the cell it's trying to communicate in nature.

Now, the parts of the endocrine and nervous system that are linked together are known as neuroendocrine elements. And so, neuroendocrine signaling is going to be when neuronal signals cause hormone secretion. Right? So instead of just simply releasing some neurotransmitters to be picked up by another cell, this is going to involve hormone secretion as part of the endocrine system. This is going to be the type of signaling that links those two communication systems in the body. And the signaling molecules it's going to release, we call neurohormones. These are hormones that are produced by neuroendocrine cells or cells that are involved in nervous signaling and endocrine signaling or synaptic signaling and endocrine signaling, however you want to think of it.

Now, the nervous system and the endocrine system are really going to be linked together in these two organs or brain regions and a gland, really, I should say, the hypothalamus and the pituitary gland. These guys are going to be the major players of the endocrine system, and the hypothalamus is going to be a major player in the nervous system as well. So over here you can see the major endocrine glands. And here you can see, behind me, you can see the nervous system. And here is kind of a zoomed-in image of a person's brain. And so, the hypothalamus sits right above the pituitary gland. And, again, the hypothalamus is going to be that major connection between the nervous system and the endocrine system. It's a very special brain region. It'll actually have a hand in a lot of other functions too. But, one of its main jobs is connecting the nervous system and the endocrine system. And that's because, in part, it is super involved in homeostasis.

So with that, let's flip the page and get into the nitty-gritty details.

Endocrine System

The endocrine system is made up of glands, and these glands secrete hormones into the bloodstream. We call them endocrine glands because they're also glands that will secrete substances into ducts that will be delivered to various other sites in the body, and these are called exocrine glands. Now, in terms of the digestive system, the liver, for example, has a duct that connects to the small intestine, as well as this structure here, the gallbladder, and it's going to secrete bile into that duct, and the bile will eventually be delivered into the small intestine. So in that sense, the liver functions as an exocrine gland in that way. And it's not, mutually the two types of glands are not mutually exclusive. For example, the pancreas here, this particular gland, acts as both an endocrine and exocrine gland. It's going to act as an exocrine gland in digestion, secreting digestive hormones into the small intestine, but it also is a super-important endocrine gland and helps maintain blood sugar homeostasis. So what is a hormone? Technically, it's just a signaling molecule produced in a gland that gets secreted into the bloodstream. And in the bloodstream, it's going to be able to travel great distances in the body to communicate with cells very far away from the source that secreted it. Now the endocrine system has a hand in a ton of different functions in the body. It's going to play a super-important role in development, growth, and reproduction. I mean, for example, think about puberty. That's when your hormones start going crazy. Right? Well, that's going to be a lot of endocrine, signaling happening. Now the endocrine system will also help you respond to the environment in certain ways. A great example of this is the fight or flight response, which will trigger, you know, a surge of hormones in your body, from the endocrine system that help, mediate your, essentially, your defenses in a tense moment. We'll talk more about that later. In addition, the endocrine system is going to be very important for homeostatic mechanisms. For example, we mentioned the pancreas and its role in blood sugar homeostasis, but pretty soon we're going to talk about, for example, how the thyroid and parathyroid help with calcium homeostasis. Now, there are actually three types of hormone structures that hormones kind of get, grouped into, and, you know, these, these labels are can be a little flexible, maybe, I should say. So, for example, we have polypeptide hormones. Sometimes people separate this into, you know, peptide hormones and protein hormones because of the very different, sizes of the two molecules. I'm just going to keep them all together to make it simple. So these are going to be, generally larger hormones and polar because they're, made of amino acids. And that's going to actually mean that they can't cross the cell membrane, but they're going to be water-soluble. Now this is important because, whether or not a hormone can cross the cell membrane, as we're going to see shortly, will affect how that, hormone, is you know going to bind to receptors and act on a cell. And additionally, the fact that it's water-soluble means that it can diffuse into the bloodstream and move through the bloodstream unassisted. Now amine hormones, sometimes called amino acid-derived hormones, are going to be synthesized from the amino acid Tyrosine, and these tend to be fairly polar. You can see an example of an amine hormone right here. Here we have an amine hormone, and, behind me, much larger we have a polypeptide. Now in the middle here, what we have is a steroid hormone. And these, hopefully, you'll recognize the structure of cholesterol in this, hormone. These are lipid-soluble hormones, meaning that they'll be able to cross that cell membrane unassisted. They'll just diffuse through. But it also means they won't be soluble in water which is why they have to be transported by proteins in the blood. And, here again, this is a steroid hormone. Now, in addition to these classifications on structure, hormones are sometimes also classified based on whether they act directly on tissues or whether they stimulate some other gland to secrete hormones. And we call, these two classes direct hormones, those that act directly on tissues to produce some result, and tropic hormones that stimulate other glands to secrete hormones. Because as you'll see in the endocrine system, it's quite common for, one thing to chemically signal another thing to have that chemically signal another thing, and basically develop these, signaling cascades of different glands with different hormones. So with that, let's go ahead and turn the page.

Hormone Signaling

As I said previously, steroid hormones can readily cross the membrane because they're lipid-soluble. In fact, cholesterol is a major important component of the cell membrane. And because steroid hormones will readily move through the membrane, they actually tend to have intracellular receptors or just receptors that are found inside the cell. And they tend to act in ways that modify gene expression. They'll do this either by binding to a receptor and having that receptor hormone complex act as some type of transcription factor, or they will trigger some signal in the cell that will activate other transcription factors. Now, the elements that a hormone receptor complex will bind to help initiate transcription is known as a hormone response element. You can see that whole process playing out in this image behind me, where we have a steroid hormone that would have had to be transported through the blood, assisted by proteins because it's not water-soluble but is going to easily diffuse through the cell membrane, bind to this intracellular receptor, and that's going to actually move into the nucleus where this is going to find its hormone response element and it will modify transcription. You can see in this case it's going to lead to the translation of some new protein.

Now, it's worth noting that although thyroid hormones are amine hormones, they behave like steroid hormones because they are non-polar. This is due to the iodine atoms found in thyroid hormones. We'll talk more about thyroid hormones in just a bit. I do just want to point out this exception because thyroid hormones are going to behave similarly to what we see happening in this image as opposed to how water-soluble hormones will behave.

Now, water-soluble hormones are going to bind to cell surface receptors because they can't cross the membrane. And what they'll end up doing is activating some type of signal transduction pathway that's going to communicate that hormone signal within the cell. Often they're going to be using G-protein coupled receptors as well as second messengers to transmit these signals. You can see a little model of that happening here where this hormone is going to go ahead and bind to this G-protein coupled receptor. Here you can see the GTP being exchanged for the GDP. This is going to activate this protein here, which is adenylyl cyclase. Adenylyl cyclase is going to take ATP and turn it into cyclic AMP. Cyclic AMP is a very common second messenger used, and will frequently be part of these signal transduction pathways. Now, a second messenger, to be clear, is just any non-protein intracellular signaling molecule. Often these signal transduction cascades will involve a series of activations or inactivations, or both, of various molecules, frequently involving protein kinases and phosphorylases and dephosphorylases, phosphorylating and dephosphorylating each other in a series of steps that will transmit the signal. What's cool about that is along the way, the signal can get amplified. Let's say one signaling molecule can actually activate two other downstream signaling molecules. If those guys can both activate two, then you can see how over time, or rather, over the course of the transmission of the signal, it gets amplified. More and more molecules are communicating that signal. These are just common features of these water-soluble hormones.

With that, let's flip the page.

Receptor Specificity

Hello, everyone. In this lesson, we are going to be talking about how the different types of hormones actually get into the cell and do the jobs that they need to do. Okay. So let's go into our lesson, and we're going to be talking about the 2 major classes of hormones. They're going to be steroid hormones or lipid-soluble hormones, and they're going to be water-soluble hormones. So basically, there's a set of hormones that are hydrophobic and a set that are hydrophilic, and this is going to determine how they are going to get into the cell and how they are going to bind with their receptor. Okay, everyone? So we know that hormones are going to be long-distance signals that travel through the bloodstream. And they're going to be made by very specific glands, and they're going to be taken up by very specific cells. So we're talking about steroid hormones and water-soluble hormones, and steroid hormones and water-soluble hormones are different because of their composition, but they're also going to be different based on how they affect the cell. Steroid hormones generally have very long-term effects and it's going to be a very slow cellular response. It's going to take a long time for that cell to respond to a steroid hormone. Now, water-soluble hormones actually have very quick effects, but they don't last terribly long. So there are 2 different types of hormones that are going to have 2 different types of effects. So whenever you're talking about a steroid hormone, this is going to be one that's derived from lipids. It's going to be something that's hydrophobic. And if you guys were wondering, examples of steroid hormones are going to be the sex hormones, and the thyroid hormones. They are all made out of lipids. So, estrogen, testosterone, thyroxin. These are all going to be lipid-soluble hormones or steroid hormones. Now, water-soluble hormones are going to be a little bit different. You guys may recognize one of these, and that is Epinephrine. Epinephrine and other water-soluble hormones are going to be composed of hydrophilic components, like proteins, or amino acids, and things like that. Okay? Alright. So, let's have a look at our diagram here, because it's going to be depicting how these two different types of hormones are going to get into the cell. So this is a steroid hormone right here, meaning that it's lipid-soluble. It is hydrophobic. And then, we're going to have a water-soluble hormone right here. So it's hydrophilic. It is not lipid-soluble. It's water-soluble. So first off, let's start with our steroid hormone. We can see that it has this line where the steroid hormone just simply goes through the plasma membrane. And that's exactly what happens with steroid hormones. Because they're hydrophobic and they're lipid-soluble, they can easily travel through the hydrophobic plasma membrane. So they just go right through the plasma membrane. They don't need any membrane-bound protein receptor. They do bind to a receptor, but it's going to be in the cytoplasm, as you guys can see. Cytoplasm as you guys can see. Oh, sorry. As you guys can see right here, this is going to be its cytoplasmic receptor. And then, once the steroid hormone, maybe it's estrogen, maybe it's testosterone, binds with its receptor in the cytoplasm, it is then going to travel into the nucleus. And, this is really interesting, steroid hormones, their main job is to alter genetic expression of that particular cell. So the complex of the steroid hormone and its receptor are actually going to determine which genes are going to be transcribed, which ones are not. So which genes are going to be expressed in which cells, and that is going to create these newly created mRNAs, which will become these new proteins or new gene products. So, for example, a hormone that does this would be estradiol, which is a form of estrogen, and it is going to enter liver cells, and it's going to alter the genetic expression of liver cells. And this genetic expression is going to be making these products that the female will use to build eggs. So you guys can think about this, building eggs and forming eggs is a very long-term process. It takes a lot of energy and a lot of effort and takes a long time, and steroid hormones are going to trigger that beginning of that long process of the liver creating these different components of the egg cells. Specifically, the liver is going to be important for creating the yolk of the egg cell. But, I told you steroid hormones take a little while to have their effects, and that one does as well. So now, let's move over to this one right here. I'm going to go out of the picture, so you guys can actually see the rest of this image. Okay. So we have this particular hormone called ACTH. You guys can look it up if you want to. You don't particularly have to know that at this moment. You'll learn more about that, I believe, in osmoregulation and excretion, but you don't have to know that exactly at this moment. So, ACTH is very interesting because it is water-soluble. Water-soluble. So what does that tell us? That tells us that it is a hydrophilic molecule, meaning that it can't simply diffuse through the hydrophobic cellular membrane like our steroid hormone did. So it's going to have these receptors. You guys can see them here in the cell membrane, and ACTH has actually bound to one of them. This is going to trigger a response. This is going to trigger a transduction cascade, or a cascade of these secondary messengers. So, you guys see it's cyclic AMP is here. Cyclic AMP is a very common secondary messenger. So, basically what's going on here is the ACTH cannot get into the cell itself because it cannot get past the plasma membrane. So it's going to bind to the receptor, and then the receptor is going to send off all of these intracellular signals or secondary messenger signals. And those secondary messengers are then going to cause this cascade to happen. So as you guys can see, we have these cyclic AMP molecules here. They're going to be activating whatever these particular molecules are here, and then those molecules are going to activate these. And then, you guys can see that Cortisol, which is a different type of signal, was actually made. That is going to be the cellular response. So this process here of creating or activating all of these different messengers and these different proteins is going to be called a signal cascade or a transduction cascade. It's something happening where it's triggering all of these different signals, and then they are going to make the gene product, which is going to be cortisol. And this is actually a relatively short process, and it's not going to involve the changing of the genetic expression. So it's going to happen very quickly and it's not going to last a very long time. Cortisol is involved in stress, so it will only stay around as long as you are stressed. So it's a short-term period kind of thing. Okay, guys. So, now, we're going to scroll down and we're going to look at a little bit more stuff. We're going to look at how these different hormones can actually trigger different responses in the cell. Okay. Alright. So we're told that a hormone binds to the cell receptor, and then it's going to cause a particular cellular response. Well, it's interesting to know that depending on which cell that hormone is triggering, it's going to depend the type of response that cell is going to have. So the effect of a hormone depends on the presence of specific receptors. So unique receptors for the same hormone will cause different cellular responses. So for example, one hormone can have many receptors. So epinephrine, which is also called adrenaline, is going to have many different types of receptors. Epinephrine is going to be a water-soluble hormone, so you're going to have many different cell membrane receptors. And depending on which receptor the cell has, it's going to depend how it's going to react. So in some cells, it can increase blood flow to the muscles. So in particular, muscle cells it will increase the blood flow. In the digestive system, it will decrease the blood flow. In the liver, it will tell the liver to start breaking down glycogen so it can put glucose into the blood so your body has fuel. So epinephrine has many different jobs depending on which particular cellular receptor that cell has. So these are some examples of the ones for epinephrine. You have alpha 1, alpha 2, and beta receptors. These are very common receptors for epinephrine, and they're going to cause different things to happen. So you guys can see down here that the first alpha one is going to lead to a cellular response that is smooth muscle contraction. And whenever you're talking about epinephrine, smooth muscle contraction is generally going to be dealing with the blood vessels. Constricting the blood blood vessels, raising the blood pressure, and diverting blood to the areas of the body that need it, like your muscles and your brain. Okay? And you guys can see that it also does smooth muscle contraction here. It's going to inhibit certain other molecules from transmitting because of the stress response, and very interestingly, it will cause the heart muscles to contract because they're pumping harder, pumping the blood. Because epinephrine, if you guys don't know, epinephrine is used in your fight or flight response, which is going to be your emergency response. So if you're being chased by a bear, you're being attacked, or you forgot something and you're panicking, epinephrine is going to kick in and it's going to cause your muscles to actually get more blood so you can run away from the dangerous thing. It's going to cause your heart muscles to contract, so you pump all that blood to your muscles that need it. And it's going to cause the glycogenolysis, which is the breaking down of glycogen in your liver to fill your blood with glucose, so your muscles have energy to actually run or do whatever you need. So, everyone, in this lesson, it was very important to realize there are 2 different types of hormones, hydrophobic or steroid hormones and hydrophilic or water-soluble hormones, and they're going to have different receptors in different areas of the cell, and one hormone can have many different receptors, which will lead to many different outcomes or cellular responses. Okay, everyone. Let's go on to our next topic.

Hypothalamus and Pituitary

The hypothalamus is that brain structure that bridges the nervous and the endocrine systems. It interacts directly with a gland called the pituitary gland. The pituitary gland actually has two lobes, and the hypothalamus has two different interactions with these two lobes. It's worth noting that the hypothalamus is the homeostatic center for many regulatory systems, including body temperature and blood pressure. The way in which it interacts with these two lobes of the pituitary gland is through linked blood vessels, which you can see over on this side.

These are blood vessels that are directly linked to blood vessels on the anterior pituitary gland, as it's called. On the posterior side, or the posterior pituitary gland, the hypothalamus is actually going to have these nerves. So, neurons that are from the hypothalamus up here actually extend what's called their axons down to the posterior pituitary, and then they synapse, or connect, to the posterior pituitary. They'll actually release neurohormones there to the posterior pituitary. Remember, neurohormones are going to be hormones that get released by neurons. These two different interactions are significant in terms of what hormones are being delivered to which part of the pituitary. We'll get into that in just a little bit.

Now, the other structure I want to mention is the pineal gland, a small endocrine gland in the brain that produces this hormone called melatonin, which is involved in circadian rhythms and makes you sleepy. The thing I want you to take note of is that the hypothalamus sits right on top of the pituitary gland. In terms of the anterior pituitary, a lot of what the hypothalamus is going to be doing is releasing hormones that are tropic hormones causing the pituitary to release some other hormones.

We're not going to focus too much on the hormones the hypothalamus releases, because they're really just there to stimulate the pituitary to release its hormones. So, we're mainly going to look at the hormones of the anterior pituitary, but I just want to give you a quick example of how the hypothalamus will actually release something called thyrotropin-releasing hormone, and that's going to go to the anterior pituitary and cause the release of thyroid-stimulating hormone. Likewise, you have corticotropin-releasing hormone that's going to stimulate the release of adrenocorticotropic hormone. The hypothalamus, a lot of what it releases, are just these releasing hormones that are going to stimulate something in the anterior pituitary to be released.

With that, let's flip the page.

Anterior and Posterior Pituitary

Okay, everyone. In this lesson, we are going to be learning about specific glands in the endocrine system. And specifically, we're going to be talking about the pituitary gland. Okay. So let's get into the lesson, and we're going to talk about the pituitary glands. But it's hard to talk about the pituitary gland without talking about the hypothalamus gland because they are very, very strongly linked. So, we have the pituitary gland, and it's going to be composed of 2 sections. The Anterior Pituitary Gland and the Posterior Pituitary Gland. Now, the hypothalamus, which we learned about in our last lesson, is also going to be part of the endocrine system. But remember, the hypothalamus' main job is to connect the nervous system with the endocrine system. The nervous system is going to send the hypothalamus signals about the exterior world or what the brain is feeling or what the body is feeling. And then the endocrine system will or, sorry. The hypothalamus will process those signals and tell the rest of the endocrine system what it needs to do. So, for example, if the nervous system detects seasonal changes like drops in temperature or less sunlight, then it will tell the hypothalamus that information, and perhaps the hypothalamus will tell the rest of the body that it needs to start making sex hormones because perhaps it's breeding season for that particular animal. That's going to be one way that the hypothalamus is going to work. But the hypothalamus doesn't really send out these hormones on its own. What it does is it tells the pituitary gland to send out these hormones. And there are 2 parts, right, the anterior and the posterior pituitary, and they're going to do different things. So, first, let's talk about the Anterior Pituitary. Now, the Anterior Pituitary is going to secrete hormones that are going to be hormones that affect other Endocrine Glands. Direct hormones are going to affect other body parts that are not endocrine glands. Maybe it has something to do with hormone signaling to the muscles or the liver. But if it's a hormone that signals another endocrine gland, like perhaps the testes or the ovaries that do make these different hormones, then it's a tropic hormone. Okay? So, tropic hormones signal to other endocrine glands. And the anterior pituitary is going to make these tropic and direct signals, and it is going to be linked to the hypothalamus via blood vessels. And that is because the hypothalamus is going to direct what the anterior pituitary is going to do. The anterior pituitary is its own gland. It makes its own hormones, but it can't release them or stop releasing them until the hypothalamus tells it that it can. And the way that the hypothalamus does this is the hypothalamus is in the brain and so is the pituitary gland, and they're very close together. And in fact, they're going to have these blood vessels that connect them. And these are generally called the portal vessels or the portal system, and these are going to be very short blood vessels that transfer the signals from the hypothalamus to the pituitary gland. And the anterior pituitary is going to get its signals from the portal system. Now, the hypothalamus secretes hormones into this portal system, and they're going to tell the Anterior Pituitary what to do. Now, this list of hormones, Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH), Adrenocorticotropic Hormone (ACTH), Thyroid Stimulating Hormone (TSH), Prolactin, and Growth Hormone (GH) are all going to be the different types of hormones that the anterior pituitary makes and can secrete. So FSH stimulates follicle maturation and spermatogenesis. Follicles are going to be things that actually make the gametes. For example, follicles in ovaries make the eggs. So FSH is very important for the production of gametes. Luteinizing hormone stimulates ovulation and testosterone synthesis. Ovulation is going to be the releasing of the egg from the ovaries into the fallopian tubes, and testosterone synthesis is simply the creation of the male sex hormone. So these are going to be mostly important for sexual habits. Okay? Now, we have these different hormones. ACTH is going to stimulate the Adrenal Cortex to secrete other hormones or Glucocorticoids, which is hard to say, like Cortisol. So this one right here is going to be a tropic hormone because it is going to signal to another endocrine gland to create more hormones. Kind of like a stepwise process. And then you're going to have the thyroid stimulating hormone, which is another tropic hormone because your thyroid is going to be another endocrine gland, a very important one, which we'll learn in our next lesson. And the thyroid stimulating hormone does exactly what it sounds like. It's going to stimulate the thyroid to create those thyroid hormones. And then you're going to have prolactin. Prolactin is very important for the secretion of milk in the mammary glands of mammals, and it is going to be secreted in the production of milk after the baby is born. But I also want you guys to know it is also very important for your immune system, your metabolism, and your pancreas actions. Okay? So it does more than just produce milk, but that's its main function. Then you have the human growth hormone or simply growth hormone. And it's going to stimulate the regeneration and the creation of new cells. Now, down here is going to be a diagram of how the Anterior Pituitary and the hypothalamus are going to interact with one another. So, these are going to be the hypothalamic hormones that it creates by the hypothalamus. Okay? And these are going to be delivered via the portal system or the portal vessels to the Anterior Pituitary. And then these particular hormones or hypothalamic hormones are going to be telling the anterior pituitary what to make. So Gonadotropin Releasing Hormone (GnRH) is going to tell the Anterior Pituitary to release the gonad hormones, or the FSH and LH hormones, which go to the gonads. So you'll find that the naming of the hormones that come from the hypothalamus are pretty self-explanatory because they are the releasing hormone or the inhibiting hormone. Because the hypothalamus can tell the anterior pituitary to release something or inhibit something. So the gonadotropin releasing hormone tells it to release the gonad hormones. Corticotropin Releasing Hormone (CRH) is going to tell it to release the corticotropin hormones. And Thyrotropin Releasing Hormone (TRH) is the thyroid hormones. It's going to tell the anterior pituitary to release those thyroid stimulating hormones. Now, the only one that's a little different is going to be dopamine (DA). So it doesn't really fit its name all that well, but dopamine is actually going to tell prolactin to stop being released at a particular time. Okay? While the thyrotropin releasing hormone is going to tell prolactin to be made. Now, the Growth Hormone Releasing Hormone (GHRH) is simply the Growth Hormone Releasing Hormone. Pretty self-explanatory. It's going to tell the Anterior Pituitary to release the Growth Hormone, which you guys can see as the plus. Just so you guys know the plus signs here are telling the Anterior Pituitary to secrete that hormone. Now, the minuses are telling the Anterior Pituitary to stop releasing that hormone. So, GHRH is telling the Anterior Pituitary to make Growth Hormone, but Somatostatin (SST) is telling it to not make growth hormone. Do you guys know what SST actually is? SST is going to be somatostatin. Do you guys know what this is? Somatostatin is actually another word for growth hormone. So the growth hormone is actually going to be a negative feedback system. Whenever there's too much growth hormone or too much somatostatin, it is going to go back to the hypothalamus, and the hypothalamus is going to tell the anterior pituitary to stop making growth hormone. So growth hormone basically tells itself to turn off, which is kind of interesting. So I hope this makes sense. This is going to be how the Anterior Pituitary interacts with the hypothalamus. The hypothalamus doesn't make any of the Anterior Pituitary hormones, but it does tell the Anterior Pituitary to either release or inhibit those hormones that it makes on its own. But the posterior pituitary is quite different. The posterior pituitary does not make its own hormones. It is going to release the hormones that the hypothalamus makes. So the hypothalamus is basically going to make these 2 hormones, and those are going to be the oxytocin hormone and the antidiuretic hormone, and it's going to store them in the posterior pituitary gland. And then it will tell the posterior pituitary gland when it is allowed to release these particular hormones. So the posterior pituitary secretes direct hormones. These are not going to be hormones that interact with other endocrine glands. They're going to affect tissues directly, and it is linked to the hypothalamus by these neuron axons. And these neuron axons are going to be the pathway that oxytocin and ADH actually travel to get to the posterior pituitary. So the hypothalamus produces these neurohormones, and it stores them in the neurons that connect to the posterior pituitary. So, simply, the posterior pituitary is the storage location. And then the hypothalamic axons synapse on these capillaries, and tell the posterior pituitary when it is allowed to release oxytocin or ADH. Okay, guys? You can actually see these structures here. This is going to be up here in blue. This whole thing right here is the hypothalamus. And it is actually connected to the posterior pituitary via these axons, which you guys can see here. So let me highlight that a little bit better for you guys. These are going to be the axons, and they're going to have or the, neurons, and they're going to have their axons that go all the way down into the posterior pituitary. And then whenever the hypothalamus tells the axons, the axons tell the posterior pituitary to release oxytocin or ADH. Now, oxytocin and ADH are very important hormones whenever you're talking about the survival of your body. ADH, you guys will learn more about whenever we learn about osmoregulation and excretion, but just know that it's used greatly whenever you are dehydrated or you have blood loss. Because ADH, also called vasopressin, increases your water reabsorption in your kidneys, and you don't lose as much water in your urine. So it builds water back up in your blood. Now, oxytocin has a huge range of these different functions that it can do. It is used in females to signal lactation. After the baby's born, it will cause milk to be lactated. It also stimulates contractions and labor, And really cool, this one right here, it plays an important role in social bonding. They have done studies that show that oxytocin is greatly linked to maternal behavior. So whenever a mother has a baby and then she holds the baby, her oxytocin levels go up, and she feels more maternal, has more maternal instincts. Also, oxytocin is used when partners interact or loved ones interact. Your oxytocin levels go up, so it's correlated with bonding. They've even done studies on pets and seen that animals, such as dogs, whenever they interact with you, like, you pet them, their oxytocin levels go up. So that's really neat. It's basically the bonding hormone, and it's used for reproduction as well in the form of labor contractions and lactation. But that's all the information I have for you guys on the pituitary gland. Remember, it has 2 components, the anterior pituitary gland, which makes a whole bunch of hormones on its own and is told by the hypothalamus when to release those hormones, and the posterior pituitary gland, which is given hormones by the hypothalamus and then secretes those hormones when the hypothalamus tells it to. But in our next lesson, we are going to learn about the thyroid and how it plays a part in the endocrine system.

Thyroid and Parathyroid Glands

The thyroid gland is an endocrine gland that's involved in the regulation of metabolic rate and calcium homeostasis. Now thyroid hormones, of which there are 2 commonly referred to as T₃ and T₄, are synthesized from Tyrosine, making them amine hormones. However, they act like steroid hormones because of the iodines attached to their structures, which essentially shield the molecule. And these two different thyroid hormones are, for our purposes, going to be treated the same. I mean, they largely have the same effects. However, there are 2 different types and you should be aware there are 2 different structures. And their structures differ in that T₃ has 3 iodine atoms and T₄ has 4 iodine atoms, so pretty easy to remember there. Now, the thyroid hormones T₃ and T₄ have a wide range of effects. But, generally, they're going to affect metabolic rates, heart rate, and heat production in the body, which is actually pretty intricately linked to these other rates. Now, thyroid hormones are going to act in a negative feedback loop, and they're going to block the release of thyrotropin-releasing hormone from the hypothalamus and thyroid-stimulating hormone from the anterior pituitary. So, a really nice negative feedback loop here, where the downstream product, so to speak, of this chain of events will feed back and shut off the release of the hormones that stimulate its release. Very clean negative feedback loop there.

Now, the parathyroid gland is going to be involved in calcium homeostasis along with the thyroid. They each kind of secrete a hormone that is counterbalanced to the other. So, the parathyroid glands actually kind of sit on the thyroid more or less; they're not as distinct as the thyroid. So, they're kind of hard to picture. But they're going to release parathyroid hormone, which is going to act in opposition to calcitonin. So, first, let me talk about parathyroid hormone, and then I'll get back to calcitonin. So, parathyroid hormone is going to essentially try to increase calcium levels in the blood. When it did, when the body detects low calcium levels, it's going to secrete this hormone and that's going to cause bones to get reabsorbed, which is going to extract calcium from the bone and put it in the bloodstream. It's also going to decrease calcium excretion in the kidneys, meaning that the body is going to retain more calcium. Additionally, it increases calcium absorption in the gut. So, your body is going to get more efficient at taking in the calcium from the food you eat. So, this all serves to raise blood calcium levels. However, calcitonin works in opposition to that. This is a peptide hormone that's secreted by the thyroid, and it's going to be secreted in response to high levels of calcium in the blood. And what it's going to do is essentially increase calcium storage in the bone, so opposite effect from that. Right? It's instead of pulling calcium out of the bone, it's going to start depositing calcium into the bone. And it's also going to increase, calcium excretion in the kidneys. So, again, opposite of that, it's going to make sure that the body gets rid of more calcium because it has too much. Lastly, it's going to decrease calcium absorption in the gut. So, opposite effect of that, these two hormones work to balance each other out. Calcitonin, the way to remember what it does is it tones down calcium. So, tones down calcium. And remember that calcitonin is secreted by the thyroid, and parathyroid hormone is secreted by the parathyroid. With that, let's flip the page.

Adrenal Glands

Before getting to the adrenal glands that sit on top of the kidneys, I want to mention the kidney's own hormone, erythropoietin. This hormone is secreted by the kidneys to stimulate red blood cell production in bone marrow. Now the adrenal glands sit on top of the kidneys, and like the kidneys, they have a two-layer structure, where they have an outer cortex and an inner medulla. The outer cortex is an endocrine gland and the inner medulla is actually a neuroendocrine gland. The adrenal cortex, as it's called, secretes mineralocorticoids and glucocorticoids. Now the hypothalamus is going to secrete CRH, and that stimulates the pituitary to secrete ACTH, and that's going to stimulate the release of these glucocorticoids. These glucocorticoids are named because they're involved in glucose metabolism, but don't let that fool you. They do lots of other stuff too. The one to know is cortisol. Cortisol is definitely the most important glucocorticoid. It's a steroid hormone involved in long-term stress responses and fight-or-flight responses. Now the mineralocorticoids are steroid hormones that are going to help the kidney regulate water balance and electrolyte balance. And this is going to be, you know, the main hormone to know is aldosterone. And if you want to know more about that, check out our lesson on osmoregulation in the kidneys.

Now the adrenal medulla is the inner gland that's going to secrete epinephrine and norepinephrine in response to synaptic signals because this is a neuroendocrine gland. Now epinephrine, you may have heard called adrenaline, but it's the same hormone. And the same thing with norepinephrine, which is sometimes referred to as noradrenaline. Norepinephrine is an amine hormone, and it's going to be involved in the stress response, and we're going to talk about the special stress response right now. The fight or flight response is a short-term stress response, and it's going to be triggered by a division of the peripheral nervous system called the sympathetic nervous system. Now the hypothalamus is essentially going to send synaptic and endocrine signals, to the adrenal gland in response to some type of perceived threat, like you see happening here. Where this dog and this cat are kind of wigging out when they see each other. Right? They're either about to throw down and scrap right now or probably run away in either direction. That's, you know, a typical fight or flight response. Now, the synaptic signals are going to cause the adrenal medulla to secrete epinephrine and norepinephrine. And these are going to lead to a variety of physiological changes. And the endocrine signals are going to cause the adrenal cortex to secrete cortisol. Now these are going to affect the body in many different ways, but the main idea is that they're going to lead the body to basically prepare to either fight for your life or run like heck. And it, you know, the effects are going to involve things like increased breathing rate, you know, to get more oxygen in your blood, to get more oxygen to your muscles so that you can either fight for your life literally or run as fast as you can. It's also going to do things like increase blood pressure. It's going to dilate your pupils, and it's going to increase your blood glucose levels. And that is going to help provide more glucose for your brain and provide more glucose, especially to your muscles, which are going to need it. Now, the stress response is controlled by this famous negative feedback loop, known as the HPA axis or hypothalamic-pituitary-adrenal axis. And this is a negative feedback loop that's going to control stress levels, particularly with cortisol. You know, think cortisol when you're thinking about these stress levels. So we know the hypothalamus secretes CRH, and that causes the anterior pituitary to secrete ACTH, which causes the adrenal cortex to secrete that stress hormone, cortisol. So I'm going to jump out of the way here, so you can see this image better. And when cortisol is secreted by the adrenal cortex, it's actually going to feed back negatively to both the hypothalamus and the pituitary gland and shut off the release of CRH and ACTH. Now this is important because overdoing it on the stress response can lead to some negative consequences for the body. For example, it can lead to suppression of the immune system, which is obviously very bad. Now, if your ACTH secretion really gets out of control, you can have this persistent stress response known as Cushing's disease. And this is bad news. It has a bunch of different negative effects on the body. For example, high blood pressure. You know, just all-around not what you want. Stress response is something that's good in the short term, but needs to be controlled very tightly so it doesn't run amok in the body. With that, let's go ahead and flip the page.

Ovaries and Testes

Gonads are endocrine glands that produce gametes, like sperm and egg, and they're also going to be responsible for secreting sex hormones. They're going to be regulated by luteinizing hormone and follicle-stimulating hormone from the pituitary. Now the testes are going to be male gonads, and these are going to produce steroid hormones called androgens, which are male sex hormones. The most famous and the main one is testosterone. This is going to regulate the development and maintenance of male sexual characteristics. For example, the deepening of the voice. Now, Malarian inhibitory substance is also going to be secreted by the testes, and this is going to prevent female reproductive anatomy development. There's this saying that nature's default is a female. So essentially, without the testes producing this hormone, the body would default and sort of, you know, develop female reproductive anatomy. Now, the ovaries are female gonads, and these are going to produce estrogens, which are female sex hormones. The main estrogen is estradiol, and this regulates the development and maintenance of female sexual characteristics, like breast development. Progesterone is another estrogen, and this is a steroid hormone that's going to be involved in the menstrual cycle and pregnancy. Now, these days, we have a lot of what are known as xenoestrogens in the environment. These are foreign substances that can bind estrogen receptors and disrupt endocrine function. It's especially frightening because we're seeing a huge rise in the development of secondary sexual characteristics in very young girls due to all these xenoestrogens that are out in the environment. Now, normally, these secondary sexual characteristics aren't initiated until puberty, which is this series of physical changes that are initiated by hormonal signals and prepare a child's body for sexual reproduction. So it's going to develop the sexual characteristics necessary. And you can see a nice outline of the connection between the pituitary gland and the gonads here, and notice that testosterone and estrogen will have a negative feedback effect on GnRH release, which is going to come from the hypothalamus to stimulate the pituitary gland. So these sex hormones are going to cause negative feedback at that point. Alright. With that let's go ahead and flip the page.

Growth Hormone and Pancreas

Just a few more hormones to cover here. First up, growth hormone, which is a peptide hormone secreted by the anterior pituitary that generally leads to increased growth. Behind me, you can see one of the tallest people in history, Robert Wadlow, who had an excess of human growth hormone that led to his massive size. Now the pancreas is both an endocrine gland and an exocrine gland. It's going to have exocrine functions in the digestive system, and endocrine functions that include the secretion of somatostatin, which is a peptide hormone that inhibits the effects of growth hormone. Now the pancreas is better known for its endocrine involvement in blood sugar homeostasis, by secreting the hormones insulin and glucagon. The pancreas is going to produce its hormones in these special clusters of cells called islets of Langerhans. Now, if we look at the human body here, you can see the pancreas, sort of, nestled on top of the intestines. We can see that it has these cell clusters like you see here. Those are islets of Langerhans, and they're going to contain alpha cells, beta cells, and delta cells. Now alpha cells produce glucagon, beta cells produce insulin, and delta cells produce somatostatin. The pancreas is actually going to receive hormonal signals from the duodenum during digestion, in the form of secreting. This hormone released by the duodenum to the pancreas is going to stimulate bicarbonate secretion from the pancreas, which is important when the chyme, during digestion, hits the duodenum. You know, all that acid needs to be neutralized. Now, hunger and satiation, or sort of the feelings of being hungry and being full, are controlled by a pair of hormones that have antagonistic effects. Hopefully, you're noticing that pattern thus far. There are lots of hormones that act in pairs and have effects that counter each other. Now leptin is going to be the hormone produced by adipocytes, or fat cells, and it has receptors in the hypothalamus that inhibit appetite. So, you know, if you think about it in terms of, you know, eating a big meal, taking in lots of nutrients, including some fats, those adipocytes are going to stimulate satiation. Ghrelin is a hormone that works opposite to leptin and actually stimulates appetite. And here you can see a pair of mice, and what's interesting is this giant mouse over here has actually been manipulated so that it does not respond to leptin. And because it doesn't respond to leptin, it doesn't get that appetite inhibition and will overeat. Alright. That's all I have for this one. I'll see you guys next time.