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  • Hormones! Those things that make teenagers moody and miserable,

  • and they cause growth spurts and acne, and they can make

  • a perfectly normal student totally obsessed with his algebra teacher.

  • Not that I have any real,

  • 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, and 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, 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 systems 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 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,

  • and as you can tell by the name, they send stuff outside the body,

  • whereas endocrines keep the "-crines,"

  • which is Greek for "secretions," in.

  • And your glands are all over the freakin' 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 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 signaling releases hormone molecules

  • that degrade really quickly and are only received

  • within a small region in the body.

  • Example: testosterone, manufactured by the testes, tells the testes

  • how many sperm they need to be making right this second.

  • And to see hormones work on an even smaller scale,

  • get a load of autocrine signaling,

  • which sends chemical messages 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: 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're able 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 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 actually wasn't very small, that's a myth,

  • but you get what I'm saying.

  • The pituitary gland makes hormones that instruct

  • 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, breastfeeding 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's 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 2 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.

  • On of them is oxytocin, which stimulates contraction of the uterus

  • during childbirth and helps with breastfeeding,

  • 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

  • antidiuretic 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 the way 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 bloodstream,

  • 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's 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.

  • Wanna see how it works?

  • Well, let's say you're walking down the street minding

  • your own business and you get hit in the face by an angry duck.

  • Let's say that this is unusual for you,

  • and you don't know what's going on,

  • just that you're 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 freakin' 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 of minutes afterwards?

  • That's because the epinephrine is still in your bloodstream,

  • telling your heart to race like crazy,

  • even after you're no longer in mortal danger or whatever.

  • Alright, I know you're 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 of 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.

  • Because its job