 Please note that this video has sexual health content. Let's look at the female reproductive system in humans. So on the right you can see where the reproductive system is located. And on the left we have a zoomed in version of that. So let's look at the different parts of it. Let's start by these overlooking things. These are called the ovaries. What do ovaries do? Well, the ovaries, one of their main jobs is to create egg cells. Create egg cells, which are basically the female gametes. And it turns out that once girls obtain puberty, basically when their sexual organs start maturing, the ovaries will release roughly on an average about one egg per month. And just to be more clear, you see, whenever girls are born, they already have lots and lots of immature eggs in their ovaries. There are lots of them, but it's only when they hit puberty. That's when those eggs start maturing. And then we find that about one egg is released per month. And if you're wondering, well, we have two ovaries, so will that be two eggs released per month? No. In general, it will be still one egg per month. You can kind of imagine one month, this ovary will release the egg, the next month, this ovary will release the egg. So they will alternate. But of course, there will be, there could be times at which, you know, both the ovaries might release the egg. So you might have two eggs. That's totally possible. But in general, it's usually one egg per month. All right. What else do they do? Well, they also secrete some very important sex hormones like progesterone. And estrogen. This is something we've talked about in great detail in our videos on endocrine system. But it's basically these hormones that help in the development of the secondary sexual characteristics in girls during puberty. And it's also these hormones that actually regulate the number of eggs that get released from the ovaries. Okay. So let's say this is that mature egg which has been released. Where does it go now? Well, it enters into this tube. It's a very, very thin tube. And it's also given a name. This tube is called the oviduct. Ovi-duct. And if you look at the name carefully, the duct means a passage, kind of like a passage. And ovi means ovum, meaning egg. Egg is called ovum, right? So it's a passage for egg. That's what it literally means. It also has another famous name. It's called fallopian tube. Fallopian tube. Both names are very commonly used. So we should be familiar with both of them. So what does the oviduct do? Well, it basically pushes the egg all the way into this bag-like structure. An elastic bag-like structure which is called the uterus. In common language, it's also called the womb. This is where the baby will be during pregnancy. And if you look below over here, you can see the opening to the uterus. This opening is also given a name. It's called the cervix. During the childbirth, when the baby has to come out, it's the cervix that needs to open up. And finally, we see the vagina. This is where the penis is supposed to enter during sexual intercourse. So roughly, these are the parts of the reproductive system. But let's come back to that egg and see what happens once the egg enters into the fallopian tube. What happens over there? Well, as I said, the fallopian tube starts pushing the egg, starts pushing the egg, and starts moving it all the way towards the uterus. Now, once the ovaries release the egg, they will signal the uterus and tell the uterus, hey, I have released the egg. That egg might get fertilized anytime. We need to prepare for it. The uterus says, alright, I'm going to prepare for it. And as a result, you will see that the lining of the walls of the uterus start getting to become thicker. It starts to become rich with blood. Well, why does that happen? Because as we will see that if the egg gets fertilized, then it requires nutrition. And the egg will get the nutrition from this blood. And that's why the uterus is basically preparing itself for fertilization. Alright, what happens next? What happens next really depends upon whether there are sperms present here or not. So let's say there are no sperms present over here. Alright, then what will happen is that this egg, which has been now unfertilized for quite a while, that egg cannot survive for too long. After a couple of days of release, we will see that that egg will disintegrate. So it has a pretty short lifetime after it has been released. It needs to get fertilized or it will just get disintegrated. Now, once it gets disintegrated, the ovaries will tell, okay, that egg is not fertilized. It tells the uterus, you know what? Uterus, you can get rid of that lining. We don't need that anymore. So then the uterus will now start getting rid of it. And the way it does that is that it starts shedding it out of the system. And so as a result, you will now see blood coming out from the vagina. And this is what we call menstruation. And so during this time of menstruation, that bleeding might last for a long time, it might last from two to four to five days. It depends on the individual until all of that is gotten rid of. And then the whole cycle can continue again. And therefore, menstruation happens almost every month. All right, but now let's consider what if there were sperms present? So let us go back a little bit. Let's keep that egg again. All right, so let's say this time there was sexual intercourse and there are sperms present. What happens then? Then you can see lots and lots of sperms will enter into the vagina. They will come through the cervix. They will enter into the uterus. In the process, a lot of them will die. And eventually they may divide into two halves. One will try to go this way in search of the egg. Another will try to go this way in search of the egg. Again, a lot of sperms will die over here. This is not an easy journey. But eventually the fittest of the fittest sperms will be able to finally go and meet with that egg. And fertilize that egg. So let's say one of that sperm undergoes and fertilizes the egg. Meaning what's going to happen is that the sperm is going to release its genetic material into the egg. And once that happens, we will say the egg is fertilized. After that, no more penetration can happen into the egg. Almost all of the other sperms will die out. And as a result, we'll now end up with a fertilized egg which is called the zygote. The zygote is basically the fused egg and the sperm. And then as the obiduct pushes the zygote towards the uterus, we will see the zygote starts dividing. From one cell to two, from two to four, from four to eight, and so on and so forth. Until they reach the uterus, we will find that the zygote now has converted into a ball of many, many cells. At this point, we will no longer call this as the zygote. Now we will call this an embryo. So let me write that. Now this ball of cells is the embryo. And of course, over here, the eggs are shown to be much bigger than the width of the Philippian tube, just so that we can see it. Obviously, the eggs are smaller than the width of the Philippian tube. Anyways, after this, we will now find that the embryo will firmly attach itself by deeply embedding itself into the thick wall of the uterus. This is what we call as the implantation because it's implanting itself and then it starts getting nourished from this particular blood. And now the embryo can keep on growing and growing and growing. And for the next nine months, it'll stay there until it grows into a big baby, big enough to finally come out from the cervix, come out from the vagina, and then we'll have a baby. And you know, for me, when I studied this, what blew my mind is that this means that all babies and all of us, including you and me, we at one point of time were a single-celled zygote. That is something truly hard for me to comprehend, truly mind-boggling. Anyways, before we wind up the video, a couple of interesting tidbits. One is that when a zygote converts into an embryo, sometimes it converts into two embryos, splits into two embryos instead of one. And he guesses what happens because of that. That's when we get identical twins. And nobody knows, even till date, when or how that happens. There are some theories, there are some hypotheses, but so far we have no clue why certain, sometimes zygotes do that. So that gives us identical twins when we come from a single zygote. On the other hand, sometimes what can happen, as I already told, sometimes you can have two eggs present in the ovary, that's totally possible. And if both the eggs get fertilized by two different sperms, then again we will have twins, but they will be non-identical twins because they came from two different sperms and different eggs. They have a little different genetic makeup. They will be just like any other siblings, just that they will be born at the same time. So non-identical twins. And the last curious question you could have is as this embryo starts growing into a bigger and bigger baby, its energy demands, its nutrition demands, its oxygen demands, they all start increasing. How does the mother take care of that? Well, this is where we will see slowly the uterus start developing a very specialized tissue to take care of all the baby's demands, which is what we call the placenta. But of course, we will talk about the placenta in more detail in a future video altogether. So let's quickly go ahead and summarize what we learned in this video. The ovaries create egg cells about one mature egg per month after puberty. Once the eggs are released, they are collected by the fallopian tube which pushes the egg towards the uterus. After the release of the egg, the ovaries signal the uterus to prepare for a fertilization happens. And so it prepares by thickening the lining of its walls, richly supplied by blood. And if the egg doesn't get fertilized, then after a couple of days, the egg disintegrates and this lining is shed off over a span of few days where she will start bleeding and that's what we call the menstruation. And since the cycle repeats every month, we call it the menstrual cycle. And on the other hand, if it does get fertilized, then we call that a zygote. When the zygote moves towards the uterus, it starts dividing into many, many cells and by the time it comes to the uterus, it becomes a ball of cells which we call now an embryo which firmly implants itself into the wall of the uterus and starts feeding off from the blood and that nourishes the embryo and the embryo starts growing.