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Basic overview of hormones and the endocrine system. Created by Sal Khan.
Video transcript
I'm here at Stanford Medical School with Neil Gesundheit, who is a faculty member here at the Med School. NG: Hi. SK: So, what are we going to talk about? NG: Well, the topic for today is Endocrinology, which is the study of hormones. NG: The word hormone is derived from the Greek word which means "arouse the activity." N: 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 and, as suggested [by the name], "arouse the activity" and give function to another organ. S: So they essentially are a kind of signalling, a way to communicate between one part of the body and the other. N: Exactly. They are very sophisticated communicators, I think that's a perfect term. Another way to think of it is that our body can communicate directly--e.g., 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 are wireless. They are made in one place and go into the bloodstream-- --which are like the airwaves, if you will-- --and [the hormones] work on another part of the body at a distance, without directly, mechanically connecting to that part of the body. S: And hormones, are they a specific type of protein or a specific type of chemical, or are they pretty much anything that will do what you described? N: They are pretty much anything but they fall into two major categories: There are small molecules typically derived from amino acids, and those molecules are 300-500 daltons--aka, mmu-- up to large proteins which can be hundreds and hundreds of amino acids in size. S: I see, so anything that has a signaling function... N: That's right, would be considered a hormone. N: And the other thing is, when we talk about hormones, is the three subcategories, we call some of them 'endocrine' hormones, where they get in the bloodstream and work at a far distance. And we'll give some examples with your diagram 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, say, in one part of the body and work within a small distance of that site. N: And then the third category, which would be less common, is the autocrine hormones. And the autocrine are made directly in one cell and work on that same or the cell right next door, at a very small distance. S: I see. Are these things--I think I have a mental model for it--endocrine hormones are released, and faraway in the body they are picked up by some receptor, the paracrine hormones, is their effect small because they are only able to travel small distance? N: Typically the paracrine hormones do get into the bloodstream, but the concentration of the receptor--the receiving end, is right close by-- so it tends to make the paracrine hormones work regionally, is the high concentration of the receptors is very close to the site of synthesis. S: I see. N: And the same with autocrine hormones, they are made and there is a very high concentration of the receiving end right at that cell. S: And, this might be a silly question, there are endocrinologists, are there paracrinologists? N: Well, that's a good point, I don't think so. I think we just--because the paracrine function of hormones was discovered later-- we still carry this all under the umbrella of endocrinology. S: So all hormones are in endocrinology, the endocrine hormones are those working at far distances. N: That's right. I think that's a good way to summarize it. I like the diagram you have here because it illustrates some of the major endocrine ogans, the ones we'll be focusing on in later lectures. So the first one you showed very nicely, in the head, at the base of the brain, is that orange structure--and that would be the pituitary gland. The pituitary gland is called the "master gland" because from the pituitary gland we make hormones that work on yet other organs. So, I'll give you an example, one of the hormones made by the pituitary is called the thyroid-stimulating hormone, or the TSH. And after it leaves the pituitary, it goes into the circulation, and it acts on the thyroid 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 triiodothyronine (T3). Those are the two main circulating thyroid hormones. S: And what do those do? N: Those regulate metabolism, they regulate appetite, they regulate thermogenesis, they regulate muscle function--they have widespread activities on other parts of the body. S: They are kinda up-regulates or down-regulates the metabolism of your entire body N: That's right. So someone with hyperthyroidism will have very high metabolism, you may know the classic picture of someone with the high heart rate, rapid metabolism, weight loss-- that would be someone with excessive amounts of thyroid hormone. And then you see pretty much the inverse picture when someone has a deficiency of thyroid hormone-- hypothyroidism. So, it's critical to maintain just the right amount of all of these hormones, and thyroid hormones is a good example. But the ultimate regulation is from that pituitary gland. S: Right, this is kinda the master one, it sends a signal there and... N: That's right. And we'll talk later about feedback loops, because how does the pituitary know when to stop making TSH? Basically, like a thermostat, it can sense the level of thyroid hormone, and when those hormones are at just the right level, and not too high, it will decrease the amount of TSH it makes, if the levels are too low, it will stimulate production of TSH to make the thyroid gland make more thyroid hormone. S: Very cool. What else do we have here? N: The other hormones, some of the major ones, the pituitary, in addition to making TSH, it makes a hormone called ACTH, adrenocorticotrophic hormone, which acts on the adrenal cortex, and the adrenal is the gland that sits right on top of the kidney, and the outer layers of the adrenal gland are the adrenal cortex, and those are stimulated by ACTH. S: They are not related to the kidney, they just sit on top there? N: Right. They're related only in the sense that the blood supply is rich, like the kidneys' blood supply, and they happen to sit above the kidney. They are called 'ADrenal' because they are adjacent to the kidney, which is the 'renal' part, S: OH! That should've been obvious. X.x N: But they don't per se, filter blood or do any of the key functions of the kidneys. S: I see 8-) So what's their role? N: The adrenal glands make the adrenal hormones like cortisol, which regulates glucose metabolism, which maintains blood pressure and well-being, and then it makes mineralocorticoids like aldosterone which is important in regulating salt-water balance. You also have adrenal androgens, which are somewhat important. And those three hormones are the main hormones made by the adrenal cortex. S: I see 8-) N: 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. S: Okay. And we have a few more organs here? :-9 N: Also out of the pituitary we make luteinizing hormone and follicle-stimulating hormone. Those are abbreviated LH and FSH. And those act on the gonads, so in the male on the testis, and in the female on the ovaries, to stimulate development of sperm in the male, and oocytes (eggs) in the female, and also the production of gonadosteroids, predominantly testosterone in the male, and estradiol in the female. S: many more...are we missing any thing? N: Well, there are two other hormones that are derived from the anterior pituitary, and those are the Growth Hormone (GH) which is critical for optimal growth (e.g., of long bones) S: Pituitary really does do a whole lot X.X N: It does! MUAHAHA!! S: hGH, human Growth Hormone? N: Right. and that works e.g., on long bones, and we would have prolactin (LTH), which in women is important in lactation, being able to breast feed after delivering a child. S: And insulin is... N: Insulin is key, but it doesn't come from the pituitary. So now we're gonna work our way down a bit, we talked about the thyroid gland making the thyroid hormones, and when you get to the pancreas, which is that yellow structure right in the middle, inside of 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. And the absence of insulin can produce all the symptoms of diabetes that we'll talk about later [?! :( S: And it seems, just structurally, you have the pancreas right here, you have the adrenal glands right there, they're all kinda near that interchange...because they're all so important... N: Yeah, that's a good observation. They all have a lot venous drainage from them, so when they make their hormones, it gets into the bloodstream rather quickly, because they are vital structures. S: Very cool 8-). I think we can leave it there, and for the next video you have some pictures which look pretty interesting =-D N: Okay, in the next video we'll talk about how you have to have the right amount of a hormone, or else things go awry.