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so we kind of looked at the first half of this graph the follicular phase and we kind of understood that the follicular phase is all about stimulating growth of the follicles in the ovaries to eventually cause the ovulation of an egg and this stimulation of the follicles comes from the release of various hormones FSH LH estrogen and so on so the second half after ovulation is called the luteal phase and it's called the luteal phase because it's about the development of this this yellow structure here called the corpus luteum so let's talk about it so here after ovulation we have an egg that's been expelled from the follicle so so we therefore have the remainder of the follicle and at this point luteinizing hormone LH is really high and together LH and FSH are going to induce the old follicle here to turn into a structure called the corpus luteum and and that just means yellow body in latin it's actually a bit yellowy because of some yellow pigments it hasn't it and so remember the follicle was what released estrogen from its granulosa cells and since this is not a follicle anymore it greatly reduces the amount of estrogen it makes and and it actually begins to mass-produce progesterone so just to clarify though it still does make some estrogen it's just not really its primary product progesterone is and so that's why you see this this dip in estrogen here and this this almost uptick in progesterone that gets produced so let's think about this for a second we've all ated so there's a chance our egg could get fertilized by a sperm and once it gets fertilized it's it's going to need a place to hang out and grow it's gonna have to implant into the lining of the uterus called the endometrium and so what would be really helpful right about now is if we had a nice vascular hospitable uterine lining that our fertilized egg could implant into - to support gestation of our new embryo and by the way at the stage of implantation our embryo is actually called a blastocyst and I'll just write that down over here so in comes progesterone and let's just look at the word for a second Pro meaning for and and just referring to gestation and this last part just kind of Clues you in that it's a hormone so progesterone is a pro gestation hormone and it's what mainly stimulates the uterine lining to prepare for implantation and gestation during during this phase called the secretory phase and in this secretory phase progesterone does a few things it increases blood flow to the endometrium by by sort of stimulating the development of special arteries and the endometrium called spiral arteries and so you can see them here in rent and these spiral arteries allow the embryo to have to eventually have good access to nutrients from the from the mothers bloodstream progesterone also increases uterine secretions from special glands in the endometrium and these secretions are important for nourishment to the embryo and finally progesterone actually reduces the contractility of the muscles of the uterus remember the the uterus has a lot of smooth muscle in its walls so the progesterone actually reduces the contractility of those muscles so that the growing embryo doesn't doesn't really get too disturbed and doesn't get expelled out of the the mother's body too early by those muscles contracting so since we really want that nice hospitable environment for gestation the corpus luteum produces lots and lots and lots of progesterone and by the way there's still a reasonable amount of estrogen kicking around and and it's really both a little bit of estrogen plus the lots and lots and lots of progesterone that helps to to ready the endometrium for pregnancy the corpus luteum hormones they're doing other things too though so the progesterone and the little bit of estrogen produced by the corpus luteum they're going to suppress the FSH and LH production by the anterior pituitary by by that process of negative feedback so you can see their levels dipping pretty low here and on top of the estrogen and progesterone negatively feeding back on FSH and LH release the corpus luteum is also producing inhibin and you can see the amount of inhibin peak here when the corpus luteum reaches around its maximum size and that inhibin actively inhibits FSH released from the anterior pituitary and so unfortunately for the corpus luteum it kind of needs FSH and LH to survive and since they're being suppressed by the corpus luteum own hormone release the the corpus luteum starts to atrophy it starts to wither away and die off and when it dies off progesterone and estrogen levels drop and when progesterone and estrogen begin to drop two things happen the first thing is that the end of the luteal phase is triggered so you can see we've sort of reached the end of the graph here and so basically we've sort of triggered the end of the luteal phase but the start of the next follicular phase and so at this point menstruation begins to occur and the endometrial lining that is that has built itself up and prepared itself for implantation starts to shed and and it'll be lost through the vaginal canal and what we what we commonly call the menstrual period or menses and notice that that sort of takes place at the beginning of each reproductive cycle so generally speaking menstruation is a sign that pregnancy has not occurred and this period can last anywhere from two to seven days and fYI women lose usually around 40 milliliters of blood per menstrual phase the second thing that happens when estrogen and progesterone levels drop is that they stop exerting their negative feedback effects on FSH and LH released from the anterior pituitary so the FSH and LH levels begin to go back up again and this increasing FSH then goes on to stimulate more follicular development in the ovaries to start the whole cycle all over again over the course of another 28 days now what I told you about the corpus luteum withering away and dying is only actually true when no pregnancy is occurring and that's sort of the case in most reproductive cycles and that's why I covered that first but it's important to know that things are a little different if pregnancy does occur and the fertilized egg is indeed implanted into the endometrium so we said that the corpus luteum estrogen and progesterone release suppressed FSH and LH and suppression of FSH and LH in turn caused the corpus luteum to atrophy right and that's because the corpus luteum needs luteinizing hormone to survive well when a blastocyst implants into the endometrium and gets established there the resulting embryo that develops from the blastocyst starts to produce a special hormone called human chorionic gonadotropin or HCG and this HCG is structurally really really similar to luteinizing hormone so much so that that levels of HCG produced by the embryo are enough to keep the corpus luteum alive because remember we said that the corpus luteum relies on LH to stay alive and so now with the corpus luteum remaining alive it can continue to produce that estrogen and progesterone that's that's necessary to maintain the endometrial lining and keep it nice and supportive of the pregnancy so there's a couple implications of this first because the HCG production is unique to the embryo most pregnancy tests work by checking for the presence of HCG in the blood or or in the urine second the corpus luteum doesn't produce progesterone for the entirety of the pregnancy it does most of the progesterone production for about the first two to three months and after that a joint organ of exchange shared between the mother and the fetus called the placenta that starts to take over as the as the major producer of progesterone third the corpus luteum being rescued or kept alive means that its continued hormone release maintains that endometrial lining so the lining isn't lost in menstruation it's kept in order to support the pregnancy so just to recap if no pregnancy occurs then the corpus luteum withers away and the reproductive cycle just continues to repeat itself about 28 daily and if pregnancy does occur the implanted embryo starts to produce HCG which rescues the corpus luteum from atrophying and the endometrium is thus maintained so no menstruation happens you're not losing the endometrium and the reproductive cycle is put on hold for the duration of the pregnancy