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 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 eat a couple of Ho Ho's,

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 pushup contest with, Christian Bale.

You're going to lose, but you're going to try.

And 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 lose to Christian Bale.

But you know, losing to Christian Bale is better

than winning against most people.

Alright, so now that we're back to muscular men,

let's get back to everybody's favorite topic. The gonads!

Sex glands come in two different flav-

That's not the right word.

Flavors? That's bad.

Ok. 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 stimulate the growth

of the uterine lining and some other stuff.

Like what other stuff?

Well, you might think that your biological sex is determined

by the parts that you have. But that's only kind of true.

It turns out that why we're either male or female

has a lot to do with hormones.

Someone get me a chair. so I can tell you how we know that.

Back in the 1940's French embryologist Alfred Jost was studying

sex differentiation in bunnies because that's what you do

when you're a French embryologist in the 1940's, apparently.

He wondered whether the hormones secreted by the gonads

during embryonic development had anything to do with whether

a bunny embryo turned out to be a boy or a girl.

So he 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 mama 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