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so we know that a female's eggs develop in her ovaries and and that as they as I sort of developed we get these fluctuations in in females sex hormones released from the ovaries so to be more specific we get estrogen we get progesterone and we get inhibin released from the ovaries while while the eggs are developing so that's all fine and good but why exactly is this happening what exactly are these hormones doing in the female body and and why do their levels change well there's this handy graph that that will just refer to as the ovarian cycle graph that I guarantee you'll see if you're studying female reproductive physiology that's it's actually really quite helpful in understanding and visualizing what exactly is going on in the body during during each reproductive cycle so so this is sort of the skeleton of the graph here just the axes and and I'll orient you to the axes first and then we'll look at what information the graph actually contains so the x axis here is time and time in this situation is sort of constricted to 28 days because that's how long each reproductive cycle is and and by the way it says 28 / 0 here because the 28th day is the same day as the zeroeth day if that makes sense in other words once you reach day 28 of one cycle you're on day zero of the next cycle there's no sort of gap in between and remember ovulation happens here at day 14 so that's the x axis and before we talk about the Y axis I'll just I'll just quickly mention that we're gonna split the reproductive cycle into two main phases the follicular phase and the luteal phase and you'll see why they're called that soon so on the y axis there's a few different things that will sort of will sort of track at the same time and the reason they're all here on the y axis at the same time is because they're all related they happen at the same time in the body so we want to see them all at once on one graph they're even listed in a sort of order on this graph so first at the top we've got the gun at atrophic hormone levels FSH and LH and remember these are released from the edge your pituitary gland in the brain and these hormones affect the development of follicles in the ovarian cycle and these hormones affect the development of follicles in the ovarian cycle I will actually look at the ovarian cycle just below here and as the follicles develop they cause the release of hormones from the ovaries so the hormone levels are here below and last we have stages of the uterine cycle which are influenced by the levels of sex hormones released from the ovaries and and broadly the stages of the uterine cycle are menses or menstruation where the endometrial lining is shed the proliferative phase where a new layer of endometrium forms and grows or or proliferate and the last phase is the secretory phase where the endometrium becomes ready for implantation by a fertilized egg so even if there's no fertilization of the egg the the endometrium still gets ready just in case and we'll talk about these phases a little bit more later on and let me just quickly say that that in in pink appear in the ovarian hormone levels the the pink here is estrogen the blue line underneath it is inhibin and the orange line is progesterone so those are the three ovarian hormones that we're going to be concerned with so we've got this sort of logical stepwise set up here and and hopefully that makes it easier to remember and what's going on so for now we'll just look at the first half of the graph the the follicular phase part of the graph and we won't really worried about the luteal phase part of the graph just yet we'll just get rid of that so on day zero here the anterior pituitary gland is releasing some FSH and some LH and you can see those baseline levels here and we know that the FSH is stimulating growth of the follicle here and you can see it growing as the days go by and while it grows it's a number of granulosa cells is increasing right the granulosa cells are represented by this purple color here and we know that granulosa cells secrete estrogen so the amount of estrogen and the blood is going up and up and up as these follicles grow and to add to that besides what FSH is doing luteinizing hormone is making the thecal cells that surround the follicle produce a hormone called Andres teen diode Andres teen diode is really really similar in structure to estrogen and actually the granulosa cells get ahold of that and wresting die own and convert it to actual estrogen so the estrogen levels are just going way up and and you can see that reflected here so as a follicles grow the the estrogen level is just going way up and by the way if we look down here at what's happening in the endometrium of the uterus that's the inner lining of the uterus we can see that we're in the proliferative phase and it's called the proliferative phase because the increasing estrogen levels that we see here are inducing a new layer of endometrium to form since the old one was shed and menstruation in the previous week so that's what this proliferation phase is all about so at this point some really interesting stuff starts to happen so when the hypothalamus and the anterior pituitary gland in the brain start to sense that the levels of estrogen are super-high like this they begin to release less FSH and LH and you can see them dipping here and that sort of makes sense right because the point of releasing FSH and LH in the first place was to cause development of the follicles and the follicles make the estrogen so when the brain senses lots of estrogen it must mean that the follicles are developing right so it doesn't actually have to continue to release so much FSH and LH that makes sense so that's why we see these dips here in FSH and LH levels in the blood because the the highest Riggin levels tell the brain to sort of reduce their production and and release of these gonadotrophins but then it starts to get even more interesting our granulosa cells are just cranking out estrogen at this point and and they actually start to produce two more hormones and higher amounts they start to produce a bit of progesterone and they start to produce a hormone called inhibit and let me just say that there's two types of inhibin in heaven a and inhibin b but we're just going to consider them as one thing for now inhibit and and inhibits role to inhibit FSH release from the anterior pituitary so you can kind of see here that as inhibin starts to increase FSH in blue here starts to decrease and again that's because inhibin is stopping the anterior pituitary from releasing FSH you might think that's the end of the interesting stuff it gets even more interesting do you remember how we said that as estrogen gets higher and higher it stops the hypothalamus and the anterior pituitary from making more FSH and LH by a bit of negative feedback well it turns out that if estrogen reaches a super-high level like up here we'll say it reaches that super-high level up here it actually causes the brain to want to release more FSH and LH it's sort of a paradoxical sounding event so we reach such a high level of estrogen that the brain tries to release this really really high amount of FSH and LH but on the graph here we only really see a high release of LH and not FSH so why is that aha remember earlier we said that our granulosa cells were releasing inhibin which reduces FSH release from the anterior pituitary well look here our inhibit amount is pretty high now and that inhibin sort of curtails the amount of FSH released from the from the anterior pituitary but it doesn't really affect the LH that gets released so the net effect is a huge release of LH from the anterior pituitary in an event called the luteal surge and this LH that gets released plus the the still reasonably high amount of FSH that gets released that sort of pushes development of the follicle to its final step ovulation and the egg you can see sort of popping out of the follicle here in the process of ovulation and remember that happens at day 14 here so that's the follicular phase and ovulation on the graph