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Hank fills us in on the endocrine system - the system of glands that produce and secrete different types of hormones directly into the bloodstream to regulate the body's growth, metabolism, and sexual development & function.


Table of Contents
1) Signalling Systems 2:07:0
2) Pituitary 3:19:1
3) Hypothalamus 4:17:1
4) Thyroid 4:52:1
5) Adrenal 5:38:1
6) Pancreas 6:51:1
7) Biolography 8:49:2

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CC Kids:

Hank: Hormones! Those things that make teenagers moody, miserable, and they cause growth spurts and acne, they make a perfectly normal student totally obsessed with his algebra teacher. Not that I have any, you know, boots-on-the-ground experience with that last one.

But all that mayhem is just the handiwork of your sex hormones. The fact is, that there are more than 50 different kinds of hormones coursing through you right now, and all multicellular organisms produce one kind or another.

For instance, hormones regulate the process of metamorphosis in insects, they're what stimulate plants to grow and fruits to ripen. In animals, the network that makes and releases hormones, your endocrine system, is one of the two ways, along with the nervous system, that important information is communicated from one part of your body to another.

Right now, your endocrine system is spraying hormones into your bloodstream, that are doing all kinds of things all over your body, giving instructions to other glands, regulating the levels of salt and sugar and water in your blood, telling your heart to beat faster.

And yes, they're partly responsible for that daydream you may or may not be having about Taylor Lautner right now. But keep your eye on the prize here, we're doing science, pay attention!


The endocrine system and the nervous system both carry information around the body, but while the nervous system carries information really quickly, and the responses are usually short-lived, endocrine responses take a while to get going but their effects can last for hours or even weeks.

The word hormone comes from the Greek word for "to arouse activity" and they're secreted by endocrine glands, the series of organs that also manufacture them.

In addition to endocrine glands, you also have exocrine glands like salivary glands and sweat glands. As you can tell by the name, they send stuff outside of the body, whereas endocrines keep the crines, which is Greek for "secretions", in.

And your glands are all over the frickin' place. Some of the heaviest hitters are in your brain, but you also have them in your throat, right over your kidneys, right below your stomach, and of course, in your baby making areas. All glands have blood vessels coming from them so that the hormones that they release can get into the bloodstream fast.

And many of your hormones circulate through your whole body, only binding to the cells that have the right receptor proteins that fit them. But there are some hormone driven messaging systems that are more localized, for instance, paracrine signalling releases hormone molecules that degrade really quickly and are only received in a small region of the body.

Example: testosterone! Manufactured by the testes, tells the testes how many sperm they need to be making right this second. To see hormones work on an even smaller scale, get a load of autocrine signalling, which sends chemical signals within a cell or from one cell to the adjacent cell at most. This is what happens in your immune system when a single t-cell realizes it needs to start cloning itself so it can fight off a virus.

Your cells receive hormones through signal receptors, but how and where a hormone binds to its receptor depends on what kind of hormone it is. There are three different types. There's the steroids, which do a lot more than make your muscles big and get you all angry and stuff. Steroids are derived from cholesterol and there's a bunch of different types of them. There are peptides which are just chains of amino acids and monoamines which are based on a single amino acid.

The only really important thing we need to keep straight about these is that peptide and amine-hormones are water-soluble and don't dissolve in lipids. And since cell-membranes are made of lipids, those hormones can't pass into a cell. Instead they bind with receptors that are on the surface of the cell. But steroids are lipid-soluble, so they are able to to penetrate the membrane and bind with receptors in the cell's nucleus.

Using these methods, the endocrine system sends out all kinds of important chemical bulletins, many of which start up in the brain in a tiny, tiny gland about the size of a pea: the pituitary gland. 

The pituitary gland, it's the master gland, the Napoleon of the endocrine system. Except that Napoleon wasn't very small, that's a myth, but you get what I'm saying.

The pituitary gland makes hormones that instructs other glands to make other hormones, and those hormones actually get the real leg-work done.

The pituitary is connected to the hypothalamus, the part of the brain that acts as a liaison between the nervous system and the endocrine system. So a big part of its job is to tell your glands what to do based on information it gets from your senses and other nerve functions. 

For example, breast-feeding women will start releasing milk when their baby starts crying. Sensory information, in this case, auditory, comes to the hypothalamus from the nervous system telling it that there is a little snuggle of baby nearby that might be hungry. This causes the hypothalamus to nudge the pituitary gland, which in turn releases hormones that stimulate milk production and secretion. Pretty cool.

The pituitary gland sits directly underneath the hypothalamus and has two lobes, which are actually two different glands fused together. The posterior pituitary is an extension of the hypothalamus and it secretes two hormones that are actually made by the hypothalamus.

One of them is oxytocin, which stimulates contraction of the uterus during childbirth and helps with breast-feeding. But it probably also has a role in things like social recognition, pair-bonding, orgasms, and anxiety, which is interesting and weird. And the other hormone secreted by the posterior pituitary is anti-diuretic hormone which tells the kidneys to retain water.

The anterior pituitary, on the other hand, both manufactures and secretes a whole battery of hormones, and one of the places these hormones end up is the thyroid.

The thyroid regulates your metabolism, your appetite, muscle function, blood pressure, heart rate, among other things. And a way that it interacts with the pituitary is a good example of a negative feedback loop, a method of communication that's common all over the body and especially in the endocrine system. 

Basically, the pituitary is like the thyroid's thermostat. It can read how much thyroid hormone is in your blood stream and when its levels are low, it spits out a tiny bit of thyroid stimulating hormone, or TSH, which travels to the thyroid.

The thyroid, in turn, secretes thyroid hormone, which boosts our metabolism, and that increase in metabolism tells the pituitary to stop sending out TSH. So the effect of the pituitary secretion is a signal to secrete less of it, and that's the negative feedback. 

Other glands that are controlled by his royal highness, the pituitary gland, include adrenal glands. These guys sit right on top of the kidneys and are in charge of making hormones that help the kidneys maintain the level of salt and water in your body. But they also, you may have heard, respond to stress.

Want to see how it works? Well, let's say you are walking down the street, minding your own business, and you get hit int he face by an angry duck. Let's say this is unusual for you and you don't know what is going on, just that you are being attacked by something. As soon as the sympathetic nervous system senses that something potentially dangerous is happening, the hypothalamus tells the pituitary gland to secrete adrenocorticotropic hormone, or ACTH for those of us who don't have all frickin' day.

This stimulates the adrenal glands to make epinephrine, also known as adrenaline. Now the epinephrine in your bloodstream will tell a bunch of different organs to do a bunch of different things all at once. Cut off blood supply to your digestive system, send a bunch of blood to your lungs and muscles, and speed up your heart rate. All to help you on your quest to vanquish this dastardly drake. 

Unlike pretty much every other muscle contraction in your body, your heart is controlled by the endocrine system as well as your nervous system. You may have noticed that after a scare, your heart races for a couple minutes afterwards. That's because the epinephrine is still in your bloodstream, telling your heart to race like crazy, even after you are no longer in mortal danger or whatever.

Alright, I know you are wondering when we're going to get to the gonads, but let me warm you up first with the function of your pancreas, super sexy gland, the biggest in the body. I've mentioned a couple times that glands regulate the balance of solutes in your blood. This is one of the most important things that the endocrine system does, and no one does it better than your pancreas. 'Cause its job is to regulate the levels of glucose in your blood, and since glucose is what makes cellular respiration, and therefore your life, possible, this is important.

When the concentration of blood glucose rises, say after you ate a couple of Ho Hos, the pancreas secretes insulin into the blood. The insulin then travels around your body and stimulates pretty much every type of body cell to absorb glucose.

Liver and muscle cells convert the glucose to glycogen for storage, and other cells in the connective tissue, called adipose cells, convert the glucose into fat.

But if your blood sugar is too low, your pancreas has got your back there too. Say you're in a push-up contest with Christian Bale. You're going to lose, but you're going to try. And the trying is going to require quite a lot of energy. Your friendly pancreas will release another hormone, glucagon, which stimulates the liver and muscles to start the process that breaks up the glycogen and fat to release the glucose. So that you can, you know, lose to Christian Bale. But you know, losing to Christian Bale is better than winning against most people.

Alright, so now that we are back to, you know, muscular men, let's uh, let's get back to everybody's favorite topic: the gonads. Sex glands come in two fla- flav-, that's not the right word. Flavors? That's bad. Now that, okay, whatever, we're just going to go with it.

There's the testes and there's the ovaries. They get instructions from the pituitary gland to make sex hormones. The testes make androgens, the main one of these being testosterone, which helps with sperm-making, among other things. 

Ovaries make estrogens and progestins which stimulates the growth of the uterine lining and the, some other stuff.

Like what other stuff? Well, you might think that your biological sex is determined, like, by the parts you have, but that's only kinda true. It turns out that why we're either male or female has a lot to do with hormones. And someone get me a chair, so I can tell you how we know that.

Back in the 1940s, a French embryologist, Alfred Jost, was studying sex differentiation in bunnies, because that's what you do when you are a French embryologist in the 1940s, apparently. 

He wondered whether the hormones secreted by the gonads during embryonic development had anything to do with whether a bunny embryo would turn out to be a boy or a girl.

So he, uh, very carefully, very very carefully, and this is a little disturbing, removed bunny embryos from their mother, and then, also very carefully, removed the part that would become the ovaries or the testes from the bunny embryos, and then, also very carefully, he put the embryos back in the mamma rabbit.

What Jost found, after the bunnies were born, was that the ones that he performed the surgery on, turned out to be girls. So, in the absence of gonads, and therefore hormones that specifically instructed the development of testes and the growth of a pee-pee and a deep bunny voice, he discovered that the default setting for mammalian embryos is 'make it female'.

So sex hormones are hard at work, even during fetal development, to make us who we are, but they're super hard at work during puberty when the pituitary gland puts the gonads on red-alert. In boys, telling the testes to make a whole lot of androgens like testosterone, that lower the voice and make a bunch of hair, increase muscle and bone mass, and encourage people to do stupid stunts and post them on Youtube.

In girls, estrogens, the most important ones being estradiol and progestins, like progesterone, kick off the process of menstruation and breast growth and all that good stuff. Largely, helping the female body get ready to grow and nurse a body.

But what we still don't understand very well is how sex hormones affect our emotions. We do know, for example, that estrogen is required for the manufacture of serotonin, the neurotransmitter that gives us a sense of calm and well-being. So, when estrogen levels drop quickly during a woman's menstrual cycle, it can make her feel off-kilter.

But the effects of sex hormones not just on our bodies, but our minds remains a significant mystery, which is good, because I don't want to even go there.

Thank you for watching this episode of Crash Course Biology! Table of contents over there, if you want to revisit anything.

Thanks to everybody who helped to put this episode together.

And if you have any questions for us, there's Facebook, there's twitter, and of course, there's the comments below.