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