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Basic overview of hormones and the endocrine system. Created by Sal Khan.
Video transcript
SALMAN KHAN: I'm here at Stanford Medical School with Neil Gesundheit, who's a faculty member here at the med school. NEIL GESUNDHEIT: Hi, Sal. SALMAN KHAN: So, what are we going to talk about? NEIL GESUNDHEIT: Well, the topic for today is endocrinology, which is the study of hormones. And the word hormone is derived from the Greek word which means arouse the activity. And what hormones do is they're chemical messengers that are made at one part of the body and typically go to another part of the body to, as suggested, arouse the activity and give function to another organ. SALMAN KHAN: So they're essentially kind of signaling, a way to communicate between one part of the body and the other. NEIL GESUNDHEIT: Exactly. They're very sophisticated communicators. I think that's a perfect term. And I think the other way to think of it is our body communicates in some ways directly. For instance, nerves innervate muscle. And when you want to contract your muscle, you give a signal from your brain, it goes down the nerve, and it directly attaches to the muscle and causes it to contract. Whereas, hormones are more like the Wi-Fi of the human body. They're wireless. They are made at one place. They go into the bloodstream, which is like the airwaves, if you will. And then they work on another part of the body at a distance, without directly connecting to that part of the body mechanically. SALMAN KHAN: And hormones, are they a specific type of protein or a specific type of chemical? Or are they really anything that does what you just described? NEIL GESUNDHEIT: It's pretty much anything, but they fall into two major categories. There are small molecules that typically derive from amino acids. And those molecules are just 300 to 500, at most, daltons, which are molecular mass units, up to large proteins that can be hundreds and hundreds of amino acids in size. SALMAN KHAN: I see. So anything, anything that really has this signaling function. NEIL GESUNDHEIT: That's right, would be considered a hormone. And the other thing is we talk about hormones in three sort of subcategories. We call some of them endocrine hormones, where they really get into the bloodstream and work at a far distance. And we'll give some examples with your diagram right there in just a minute. But there are others that are called paracrine hormones. And paracrine hormones are more regionally active. So they might be made, let's say, in one part of the body and work within a small distance of that site of synthesis. And then the third category, which is less common, would be autocrine hormones. And the autocrine hormones are actually made directly at one cell and work on that same cell or in this cell right next door, at a very, very small distance. SALMAN KHAN: I see. So, the endocrine hormones, I think I have a mental model for it. They're kind of released far away in the body someplace. If they're picked up by the right receptor, they'll have the right function. The paracrine hormones, is their effect small because they only are able to travel a small distance? Or is it something else? NEIL GESUNDHEIT: Typically the paracrine hormones do get into the bloodstream, but the concentration of the receptor, the receiving end, as you suggested, is right close by. So what tends to make a paracrine hormone work regionally is that the high concentration of the receptors are very close to the site of synthesis. And the same with autocrine, is often they're made, and there's a very high concentration of the receiving end right at that cell, right next to that cell. SALMAN KHAN: And this might be a silly question, but it's called endocrinology. Are there paracrinologists? NEIL GESUNDHEIT: Well, it's a good point. I don't think so. I think we just, perhaps because the paracrine function of hormones was discovered later, we still carry this all under the umbrella of endocrinology. SALMAN KHAN: Right. So all of hormones is endocrinology, even though endocrine hormones are the ones that act at far distances. NEIL GESUNDHEIT: That's right. I think that's a good way to summarize it. Now I like the diagram that you created here because it illustrates some of the major endocrine organs, the ones we'll be focusing on in later lectures. So the first one that you showed very nicely in the head, at the base of the brain, is that orange structure. And that would be the pituitary gland. That's right. And the pituitary gland is called the master gland because from the pituitary, we make hormones that work on yet other organs. So I'll give you an example. One of the hormones that's made by the pituitary is called thyroid stimulating hormone, or TSH. And after it leaves the pituitary, it goes into the circulation and it acts on the thyroid gland, where there are high receptors for TSH on the surface of the thyroid cells. And it stimulates the thyroid gland to make thyroid hormone, typically thyroxine T4 or triodothyronine, T3. Those would be the two main circulating thyroid hormones. SALMAN KHAN: And what do those do? NEIL GESUNDHEIT: Those regulate metabolism, they regulate appetite, they regulate thermogenesis, they regulate muscle function. They have widespread activities on other parts of the body. SALMAN KHAN: But it kind of upregulates or downregulates the entire body and the metabolism. NEIL GESUNDHEIT: That's right. So someone with hyperthyroidism would have very high metabolism. You may know the classic picture someone with a high heart rate, rapid metabolism, weight loss. That would be someone with excess amounts of thyroid hormone. And then you see pretty much the inverse picture when someone has a deficiency of thyroid hormone and someone with hypothyroidism. So it's critical to maintain just the right amount of almost all of these hormones, and the thyroid hormones are good examples of this. But the ultimate regulation is from that pituitary gland. SALMAN KHAN: This is kind of the master one. It sends a signal there, and then that kind of does the-- NEIL GESUNDHEIT: That's right. And we'll talk later about feedback loops, because how does the pituitary know when to stop making TSH? And basically, like a thermostat, it can sense the levels of thyroid hormone. And when those levels are just at the right level, and not too high, it'll decrease the amount of TSH it makes. If the levels are too low, it'll increase TSH to try to stimulate the thyroid gland to make yet more thyroid hormone. SALMAN KHAN: Very cool. And what else do we have here? NEIL GESUNDHEIT: OK. So the other hormones, some of the major ones. The pituitary, in addition to making the thyroid stimulating hormones, it makes a hormone called ACTH, adrenal corticotrophic hormone, which acts on the adrenal cortex. And the adrenal is that gland exactly that sits on top of the kidney. And the outer layers of the adrenal gland are the adrenal cortex, and those are stimulated by ACTH. SALMAN KHAN: And they're not related to the kidney. They just sit on top there. They're structurally there. NEIL GESUNDHEIT: Right They're related only in that sense that the blood supply is rich like the kidneys' blood supply, and they happen to sit above the kidney. And they're called adrenal because they're adjacent to the kidney, which is the renal part. SALMAN KHAN: That should have been obvious. I never realized that. NEIL GESUNDHEIT: But they don't per se filter blood or do any of the key functions that the kidney serves. SALMAN KHAN: I see. And what's their role? NEIL GESUNDHEIT: So the adrenal glands make the adrenal hormones like cortisol, which regulates glucose metabolism and is important to maintaining blood pressure and well-being. And then it makes mineralocorticoids like aldosterone, which is important for regulating salt and water balance. You also have adrenal androgens, which are somewhat important. And those three hormones are the main hormones made by the adrenal cortex. The ACTH primarily regulates the cortisol and the adrenal androgens. And there's another system that regulates the mineralocorticoids that we'll talk about later. SALMAN KHAN: OK And we have a few more organs here. NEIL GESUNDHEIT: Yeah. So Also add out of the pituitary, we make luteinizing hormone and follicle stimulating hormone. Those would be abbreviated LH and FSH. And those act on the gonads. So in the male, it'll act on the testes, and in the female, it'll act on the ovaries to stimulate the development of sperm in the male and oocytes, or eggs in the female, and also the production of gonadal steroids, primarily testosterone in the male and estradiol in the female. SALMAN KHAN: Right And are we missing anything? NEIL GESUNDHEIT: Well, there are two other hormones that also derive from the anterior pituitary. And those would be growth hormone that's critical for optimal growth of long bones. SALMAN KHAN: The pituitary really does do a lot. NEIL GESUNDHEIT: It does. Yeah. SALMAN KHAN: So it's HGH, human growth hormone. NEIL GESUNDHEIT: Yeah. Human growth hormone, and that would act on long bones, for instance. And then we would have prolactin, which is important in women for lactation, being able to breastfeed after delivering a child. SALMAN KHAN: And insulin is? NEIL GESUNDHEIT: Insulin is key, but it doesn't come from the pituitary. So now we're going to work our way down a little bit. We talked about the thyroid gland making thyroid hormone. And then when you get to the pancreas, which is that yellow structure right in the middle, inside the pancreas, there are small islands called the islets of Langerhans. And the islets within the pancreas make endocrine hormones like insulin and glucagon. But insulin is vital. Without insulin, you have diabetes. And without insulin, you don't transport glucose into muscle and remove glucose from the bloodstream normally. The absence of insulin can produce all of the symptoms of diabetes that we'll talk about later. SALMAN KHAN: It seems just structurally, you have the pancreas right here. You have the adrenal glands right there, that they're all near kind of that interchange on the-- because they're all so important to get to where they need to get to. NEIL GESUNDHEIT: That's a good observation. They all have a lot of venous drainage from them so that when they make their hormone, it gets into the bloodstream rather quickly because they are vital structures. SALMAN KHAN: Very cool. So I think we can leave it there. And in the next video, you have some pictures that I think will be pretty interesting. NEIL GESUNDHEIT: OK. Yeah. In the next video, we'll talk about how you have to the right amount of the hormone or else things go awry. SALMAN KHAN: Very cool. Thanks a bunch. NEIL GESUNDHEIT: OK. Thank you.