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let's meet testosterone we produce chemical signals in our bodies that allow one part of our body to communicate with other parts of our bodies and these signals are called hormones testosterone is one of these hormones so testosterone is produced by our testes here so let me draw a zoomed in view of one testes and after the testes make testosterone in cells that they have called Leydig cells that testosterone is transported to other parts of the body via the bloodstream so I'm drawing red blood cells in red and I'll show you the little molecules of testosterone in green and these blood vessels carry testosterone all over the body so that it can carry out all of its biological functions so let me just point out that testosterone isn't only produced in men it's actually produced in men and women but males past the age of puberty about 12 or 13 years old have about seven or eight times the testosterone that women of the same age as them do so we still refer to it as the primary male sex hormone and to clarify I'm just calling it a sex hormone because it's produced primarily by some of the male sexual organs ie the testes so I've told you that it's made in the testes and then it travels around in the blood vessels to every part of the body but once it gets to its target what exactly happens how does it work so to show you this I'll blow up a little cell here in the thigh so here you have a cell and up here sort of lining the cell or just beside the cell you have that bloodstream that the testosterone is floating around in and so I'll draw back in those red blood cells let's just say the blood is going that way and here's our testosterone so when testosterone sees a cell that it wants to enter it crosses through the cell membrane which is sort of the cells barrier to things outside the cell almost like a selectively permeable gate then once the testosterone gets inside the cell it sort of meets this carrier protein and the carrier protein is something that will bind the testosterone and take it to its next destination within the cell so we'll draw that carrier protein as a taxi you have testosterone sitting right here in the back seat of the taxi and then that carrier protein just drives it a little short way to something called the nucleus and the nucleus contains our DNA and those are the blueprints of how we're made so in red I drew a specific part of our DNA called a gene and our DNA is made up of millions of genes so genes are little segments of the DNA and and some hormones interact with our genes to change our characteristics so that's exactly what testosterone does it jumps out of the cab once it gets into the nucleus and it sort of binds onto that gene I'll just label that that's a gene and when testosterone binds the gene it's able to influence its function so for example if the gene told the body to make more muscle than testosterone and interacting with that gene would increase the rate of muscle production and in fact that is one of testosterones functions and and we'll touch on that a little bit later now one more note on this topic depending on the type of cell that testosterone enters it can be converted into a different hormone before going to the nucleus to interact with a gene so the two possibilities are dihydrotestosterone or DHT and estrogen and that may sound a little counterintuitive because I think most people think of estrogen as a female only hormone but actually males need estrogen as well and we have a little bit of it and it comes from testosterone and even in the female testosterone is converted to estrogen and that's how estrogen is made it just happens a lot more often in the female having said that in men only about 7% of testosterone is converted to DHT and less than 1% is converted to estrogen so it's primarily testosterone that's that's that's exerting all the hormonal effects you're probably wondering what some of the roles of testosterone are so let's cover those so testosterone actually starts working when you're really young when you're still in development in your mother's womb and what it does there is it induces your reproductive organs to differentiate into male reproductive organs because when males and females start out in development and in the womb they have the same precursor reproductive structures and so the presence of testosterone actually pushes the pushes the reproductive organs to turn into masculine ones another really important thing testosterone does you remember how we said that testosterone is made in the laid egg cells here and the Ladak cells are one of the cells in the testes when testosterone starts to be released in higher quantities from those Leydig cells once you hit puberty around age 12 or 13 that testosterone signals other cells in the testes to start the process of making sperm a process called sperm and Isis after sperm production starts having a baseline level of testosterone in the testes keeps sperm production going so these are the major functions that testosterone has on the male reproductive tract the testosterone actually has a lot of other functions around the body it's responsible for what we call secondary sex characteristics and these are just physical traits that we typically think of of being masculine or feminine for example it stimulates the growth of facial hair armpit air pubic hair and hair on your arms and legs and your chest just to name a few areas it's also responsible for the deepening of your voice that happens as you develop as a male and that happens because testosterone induces growth of the voice box or the larynx and some people know that as the Adam's apple that's why you see a prominent sort of bulge in in the throats of males another thing it does is it induces male patterns fat distribution and so that's sort of fat distribution around these central part of the body it also has some structural effects on our bodies so it stimulates the process of anabolism and what that means is taking smaller components in our bodies like proteins and building them up to bigger components or aggregates of those components like muscles so muscles are basically a big aggregate of proteins that eg just means for example testosterone also stimulates bone growth so it'll make your bones bigger in diameter and longer but it also stimulates the termination of bone growth once your bones can't grow anymore so those are the major secondary sex characteristics that testosterone stimulates it affects your behavior increasing your sex drive and has been thought to possibly cause higher levels of aggression testosterone can increase the number of red blood cells that we have and it does that by stimulating our kidneys to produce another type of hormone called erythropoietin or EPO and EPA's role is to cause the creation of more red blood cells so that all sounds pretty awesome right well except for me the possible link to aggression but you know it turns out there's a limit to the testosterone that you want floating around in your blood at any given time if you have too much bad things can start to happen it's been theorized that the prostate has cells that are stimulated by DHT or dihydrotestosterone do you remember that metabolite of testosterone we talked about earlier and so the idea is that they grow bigger when there's too much DHT around and increases the risk of developing prostate cancer the jury's still out on that one though but one thing we do know is that male pattern baldness that is baldness on the top of the scalp is actually promoted by excess DHT in the blood so this tells us that we don't want too little or too much testosterone traveling around in our blood and and affecting ourselves so how do we control how much testosterone is in our body well it turns out that testosterone to a large extent self regulates how much is produced but how can it do that it actually does that through something called a feedback loop and this is actually this concept of the feedback loop is by far how most hormones work so again a feedback loop is a method of self-regulation so an example of a feedback loop would be something like a thermostat let's say you set the temperature in your room to a nice 25 degrees Celsius so let's say it's 20 degrees in the room now the thermostat will sense the temperature of the current air in the room and heat the room another five degrees until it gets to 25 degrees and once it's sensed that 25 degrees has been breached it will turn off the thermostat and testosterone in the body's regulated in a similar way there's a part of the brain called the hypothalamus that acts similar to that thermostat it's actually sometimes referred to as the body's thermostat because it regulates most of our feedback loops and the testosterone feedback loop is one of them so the hypothalamus will sense the amount of testosterone that's floating through your blood if it's not enough testosterone the hypothalamus will send a signal to another gland in the brain called the anterior pituitary and that actually just sits right underneath the hypothalamus and then the anterior pituitary sends more hormones to the testes that increases their testosterone production conversely if there's too much testosterone in the blood the hypothalamus will sense that and it'll stop sending signals to the anterior pituitary and then the anterior pituitary will stop sending signals to the testes and so when that happen you actually get a decrease in testosterone so overall this negative feedback loop is used to control the amount of testosterone in the blood and that control of blood testosterone levels is actually referred to as testosterone homeostasis homeostasis meaning to remain constant and that's testosterone