breastfeeding growth spurt

hormones! those things that maketeenagers moody and miserable, and they cause growth spurtsand acne, and they can make a perfectly normal student totallyobsessed with his algebra teacher. not that i have any real, boots-on-the-groundexperience with that last one. but all that mayhem is justthe handiwork of your sex hormones. the fact is that thereare more than 50 different kinds of hormones coursingthrough you right now, and all multicellular organismsproduce one kind or another.

for instance, hormones regulatethe process of metamorphosis in insects, and they're whatstimulate plants to grow and fruits to ripen. in animals, the network thatmakes and releases hormones, your endocrine system,is one of the two ways, along with the nervous system,that important information is communicated from onepart of your body to another. right now, your endocrinesystem is spraying hormones into your bloodstream thatare 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 partlyresponsible for that daydream you may or may not be havingabout taylor lautner right now. but keep your eye on the prize here! we're doing science! pay attention! the endocrine system and thenervous systems both carry information around the body,but while the nervous system

carries information reallyquickly and the responses are usually short-lived,endocrine responses take a while to get going, but their effectscan last for hours or even weeks. the word hormone comes from thegreek for "to arouse activity," and they're secretedby endocrine glands, the series of organsthat also manufacture them. in addition to endocrine glands,you also have exocrine glands like salivary glandsand sweat glands, and as you can tell by the name,they send stuff outside the body,

whereas endocrineskeep the "-crines," which is greek for "secretions," in. and your glands are allover the freakin' place: some of the heaviest hittersare in your brain, but you also have them in yourthroat, right over your kidneys, right below your stomach, and ofcourse in your baby-making areas. all glands have blood vesselscoming from them so that hormones that they release can getinto the bloodstream fast. and many of your hormonescirculate through your whole body,

only binding to thecells that have the right receptor proteins that fit them. but there are somehormone-driven messaging systems that are more localized. for instance, paracrine signalingreleases hormone molecules that degrade really quicklyand are only received within a small region in the body. example: testosterone, manufacturedby the testes, tells the testes how many sperm they need tobe making right this second.

and to see hormones workon an even smaller scale, get a load of autocrine signaling, which sends chemicalmessages within a cell, or from one cell to theadjacent cell, at most. this is what happens in yourimmune system when a single t-cell realizes it needs tostart cloning itself so it can fight off a virus. your cells receive hormonesthrough signal receptors, but how and where ahormone binds to its receptor

depends on what kindof hormone it is. there are three different types:steroids, which do a lot more than make your muscles big andget you all angry and stuff. steroids are derived from cholesterol and there's a bunch ofdifferent types of them. there are peptides, which arejust chains of amino acids. and monoamines, which are basedon a single amino acid. the only really important thing weneed to keep straight about these is that peptide and aminehormones are water-soluble

and don't dissolve in lipids. and since cell membranesare made of lipids, those hormones can'tpass 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 withreceptors in the cell's nucleus. using these methods, the endocrinesystem sends out all kinds of important chemical bulletins,many of which start up in the brain,

in a tiny gland aboutthe size of a pea. the pituitary gland. the pituitary gland,it's the master gland, the napoleon of the endocrine system. except that napoleon actuallywasn't very small, that's a myth, but you get what i'm saying. the pituitary gland makeshormones that instruct other glands to make other hormones, and those hormones actuallyget the real leg work done.

the pituitary is connectedto the hypothalamus, the part of the brainthat acts as a liaison between the nervous systemand the endocrine system. so a big part of its job is totell your glands what to do based on information it gets from yoursenses and other nerve functions. for example, breastfeeding womenwill start releasing milk when their baby starts crying. sensory information,in this case auditory, comes to the hypothalamus fromthe nervous system telling it

that there's a little snuggle ofbaby nearby that might be hungry. this causes the hypothalamusto nudge the pituitary gland, which in turn releases hormones that stimulate milk productionand secretion. pretty cool! the pituitary gland sits directlyunderneath the hypothalamus and has 2 lobes, which are actually twodifferent glands fused together. the posterior pituitary is anextension of the hypothalamus and it secretes two hormones that areactually made by the hypothalamus. on of them is oxytocin, whichstimulates contraction of the uterus

during childbirth andhelps with breastfeeding, but it probably also has a rolein things like social recognition, pair bonding, orgasms, and anxiety.which is interesting and weird. and the other hormone secreted bythe posterior pituitary is antidiuretic hormone, which tellsthe kidneys to retain water. the anterior pituitary on theother hand both manufactures and secretes awhole battery of hormones and one of the places thesehormones end up is the thyroid. the thyroid regulates yourmetabolism, your appetite,

muscle function, blood pressure,heart rate, among other things, and the way it interacts withthe pituitary is a good example of a negative feedback loop, a methodof communication that's common all over the body, and especiallyin the endocrine system. basically, the pituitary islike the thyroid's thermostat. it can read how much thyroidhormone 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 inmetabolism tells the pituitary to stop sending out tsh. so the effect of thepituitary's secretion is a signal to secrete less of it,and that's the negative feedback. other glands that arecontrolled by his royal highness the pituitary glandinclude adrenal glands. these guys sit right on top ofthe kidneys and are in charge of making hormones that help thekidneys maintain the level of salt

and water in yourbody, but they also, you may have heard,respond to stress. wanna see how it works? well, let's say you're walkingdown the street minding your own business and you gethit in the face by an angry duck. let's say that thisis unusual for you, and you don't know what's going on, just that you're beingattacked by something. as soon as the sympatheticnervous system senses

that something potentiallydangerous is happening, the hypothalamus tells thepituitary gland to secrete adrenocorticotropic hormone,or acth for those of us who don't have all freakin' day. this stimulates the adrenalglands to make epinephrine, also known as adrenaline. now, the epinephrine in yourbloodstream will tell a bunch of different organs to do a bunchof different things, all at once: cut off blood supply to yourdigestive system, send a bunch

of blood to your lungs and muscles,and speed up your heart rate. all to help you on your questto vanquish this dastardly drake. unlike pretty much every othermuscle contraction in your body, your heart is controlledby the endocrine system, as well as your nervous system! you may have noticedthat after a scare, your heart races for acouple of minutes afterwards? that's because the epinephrineis still in your bloodstream, telling your heartto race like crazy,

even after you're no longerin mortal danger or whatever. alright, i know you're wondering whenwe're going to get to the gonads. but let me warm you up firstwith the function of your pancreas. super sexy gland,the biggest in the body. i've mentioned a couple of timesthat glands regulate the balance of solutes in your blood: this is one of the mostimportant things that the endocrine system does, and no onedoes it better than your pancreas. because its job is to regulate thelevels of glucose in your blood,

and since glucose iswhat makes cellular respiration, and therefore, your life,possible, this is important. when the concentrationof blood glucose rises, say after you eat acouple of ho ho's, the pancreas secretesinsulin into the blood. the insulin thentravels around your body and stimulates pretty much everytype of body cell to absorb glucose. liver and muscle cells convertthe glucose to glycogen for storage, and other cellsin the connective tissue

called adipose cellsconvert the glucose into fat. but if your blood sugar is too low, your pancreas has gotyour back there, too. say you're in a pushupcontest with, christian bale. you're going to lose,but you're going to try. and trying, is going torequire quite a lot of energy. your friendly pancreas willrelease another hormone, glucagon, which stimulates the liverand muscles to start the process that breaks up the glycogenand fat to release the glucose,

so that you can loseto christian bale. but you know, losing tochristian bale is better than winning against most people. alright, so now thatwe're back to muscular men, let's get back to everybody'sfavorite topic. the gonads! sex glands come intwo 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 andthere's the ovaries. they get instructions from thepituitary gland to make sex hormones. the testes make androgens, the mainone of these being testosterone, which helps with sperm-making,among other things. ovaries make estrogens andprogestins which stimulate the growth of the uterine liningand some other stuff. like what other stuff? well, you might think that yourbiological sex is determined by the parts that you have.but that's only kind of true.

it turns out that why we'reeither 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 frenchembryologist alfred jost was studying sex differentiation in bunniesbecause that's what you do when you're a french embryologistin the 1940's, apparently. he wondered whether thehormones secreted by the gonads during embryonic developmenthad anything to do with whether a bunny embryo turned outto be a boy or a girl.

so he very carefully,very, very carefully, and this is a little disturbing, removed bunny embryosfrom their mother, and then, also very carefully, removed the part that wouldbecome the ovaries or the testes from the bunny embryos,and then, also very carefully, he put the embryos backin the mama rabbit. what jost found after thebunnies were born was that the ones that he performed thesurgery on turned out to be girls.

so, in the absence of gonads,and therefore hormones that specifically instructedthe development of testes and the growth of a peepeeand a deep bunny voice, he discovered thatthe default setting for mammalian embryosis make it female. so, sex hormones are hard atwork even during fetal development to make us who we are. but they're super hard atwork during puberty, when the pituitary gland putsthe gonads on red alert,

in boys telling the testes tomake a whole lot of androgens like testosterone that lower thevoice, make a bunch of hair, increase muscle and bone mass, and encourage people to do stupidstunts and post them on youtube. in girls, estrogens, the mostimportant one being estradiol and progestins, like progesterone,kick off the process of menstruation and breast growthand all that good stuff, largely helping the female body getready to grow and nurse a baby. but what we still don'tunderstand very well is

how sex hormones affect our emotions. we do know that, for instance,that estrogen is required for the manufacture of serotonin,the neurotransmitter that gives us a sense of calm and well-being. so when estrogen levelsdrop 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 remainsa significant mystery. which is good, because idon't want to even go there.

thank you for watching thisepisode of crash course biology. table of contents over thereif you want to revisit anything. thanks to everyone who helpedput this episode together. and if you have any questionsfor us, there's facebook, there's twitter, and of course,there's the comments below.