 Let's summarize the female reproductive system. It starts with the ovaries, which creates the excels. We have two of them. And when we are young, we have a lot of immature excels, but once we become old enough, once we hit puberty, then these excels start maturing and we get about one excel every month. Of course, the ovaries are also responsible for the secondary sex hormones. All right, once the excels are created, where does it go? It goes into the fallopian tube. The fallopian tube's job is to start pushing the egg all the way into this sac-like structure, which is called the uterus. This is where the baby usually grows and gets all the nutrition and everything that is necessary. And then once the baby is finally ready to be born, it comes out through this particular hole, which we call the cervix, and finally it comes out through the vagina. So these are the major parts of the reproductive system. All right, now let's look at what happens once the eggs are formed inside the ovaries. Once the eggs are formed, once it becomes fully mature, it goes into the fallopian tube and the fallopian tube starts pushing it towards the uterus. And guess what? The uterus also starts preparing for a fertilization. And the way it does that is that its lining starts becoming thicker with a lot of blood over here. The whole idea is that if fertilization happens, then it requires a lot of nutrition. The baby that will eventually grow requires a lot of nutrition and therefore it's preparing for it. What happens next? Well, what happens next really depends upon whether fertilization happens or not. So it really depends upon whether there was sexual intercourse and whether there were sperms over here. So let's first look at what happens when there aren't any sperms over here. If there aren't any sperms, then this particular Excel cannot travel a lot. It will disintegrate after a few days. So that Excel will be gone. And now since the body understands that there was no fertilization, there is no point of having this thick blood lining. And so the body gets rid of it. And this is basically your menstrual cycle. And that's why you have a menstrual cycle pretty much every month because you have one Excel happening, one Excel is being pushed into the fallopian tube every single month. And whenever there are no fertilizations, you have this thing, this lining gets shed off. But now let's think about the other case. What happens if there are sperms over here? Then if the sperms travel all the way up over here, then their fertilization can happen. Then what happens is the sperm unites with the Excel's to form what we call a zygote. A zygote is a cell that is formed after the unification of the sperm cell and the Excel. Once the zygote is formed, no other sperm cells will be able to penetrate into any other egg. I mean, sometimes it's possible that you would have two eggs and twins are possible. But in general, usually nothing else happens so all the other sperms die off. And now what happens is that that zygote starts getting multiplied. So the cells, one cell becomes two, two becomes four, four becomes eight and so on and so forth and so forth. And eventually by the time it comes into the uterus, you have like lots and lots of cells and it gets implanted over here. This ball of cells is now what we call an embryo. Think of embryo as your, you know, yet to be a baby. This is the cell that if this is the ball of cells that eventually becomes a baby. But here's a question. For this embryo to start growing into a baby like a full blown human being, it requires a lot of things like oxygen, it requires nutrition, it's blood needs to get filtered and all of that. How does your body take care of it? Well, this is where a specialized tissue comes into the picture. So here's an example of what happens maybe after a few months once the baby has grown. A specialized tissue is formed inside the uterus which is called the placenta. Now it looks a little complicated but here's how to think about the placenta. The placenta has two sets of blood vessels. The one blood vessels that you can see over here are the maternal blood vessels that comes from the mother. And what her blood vessels does is that it, you know, puts the blood into this pool over here. You can kind of see there's a pool of blood over here. So it puts like rich oxygen, rich nutrition, rich blood over here. And this is the baby's blood vessels. The baby's blood vessels will simply start, you know, using diffusion, start collecting the oxygen, starts dumping or carbon dioxide into it, starts collecting the nutrition, starts dumping all the waste products into it. And that's how the baby starts getting nutrition from her mother or his mother and all the oxygen from the mother. And then once this pool of blood is deoxygenated, it is sucked back and then new blood is replenished. And that's how the maternal blood vessels keeps on recycling and giving this fresh blood. And the baby's blood vessels starts sucking all the nutrients and all the oxygen. But remember, the blood never mixes, right? The blood from the mother doesn't go into the baby's blood vessels. It doesn't. It just comes very close for that diffusion can take place. And for the sake of increasing the diffusion rate, you can see the villi-like structures are over here. So that increases the surface area. So this is how placenta is a pretty cool organ, which makes sure that the two bloods, maternal blood and the baby bloods do not mix, they do not come into contact with each other, but ensures that it comes very close to each other so that diffusion can happen and so the mother can provide all the nutrition and all the stuff that is needed for her baby.