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MLA Full: "Reproductive System, Part 3 - Sex & Fertilization: Crash Course Anatomy & Physiology #42." YouTube, uploaded by CrashCourse, 16 November 2015,
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After weeks of discussion of human reproduction, today we arrive at the main event: THE SEX. Hank explains the four phases of the human sexual response, how fertilization happens, and how different contraceptive methods work.

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Introduction 00:00
Four Phases of Human Sexual Response Cycles 0:53
Fertilization 3:01
Contraceptive Methods 6:59
Review 8:58

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CC Kids:
Welcome! To the main event. After weeks of talking about our reproductive parts and how they work, today we finally discuss what happens when those parts come together to what they were born to do: The Sex. 

But first, let's talk about what we won't be talking about. We're not gonna give any advice, we're not gonna talk about STIs, even though some STIs can affect conception. We're also not gonna talk about sexual behavior or mate selection, and there will be no discussion of birds OR bees. And while people have sex in lots of different ways and for lots of different reasons, we're only going to be talking about the kind of sex that could lead to fertilization. 

We are going to talk about contraception though, in terms how and where and when different kinds can prevent fertilization. And we're going to look at what exactly is going on in your body before, during, and after sex - in the most very safe-for-work clinical terms possible. 


Humans have been experiencing coitus for as long as they could be called humans, and actually before that, because obviously there would be no humans without The Sex. And although it's something that generally comes pretty naturally, it wasn't until the late 1950s and 60s that anyone thought to look at the physiological science behind the act of sex. So when American gynecologist William Masters and sexologist Virginia Johnson kicked off their unprecedented and controversial research into the so-called human sexual response cycle, they began by inviting hundreds of male and female volunteers to come into the lab and have sex - in the name of science. 

Though it wasn't as voyeuristic as it might sound. Outfitted with loads of wires and heart monitors, volunteers were periodically interrupted in the middle of the act to be measured. Ten thousand trials or so later, Masters and Johnson found out some interesting stuff. Namely, that a typical complete sexual cycle runs through four distinct phases: excitement, plateau, orgasm, and resolution. 

During the excitement phase, things are just getting going. Sexual arousal triggers the parasympathetic reflex that causes arterioles in the external genitalia to dilate and fill with blood - which is actually one of the only times the parasympathetic system controls any arterial action. The plateau phase is marked by increased muscle tension, heart rate, blood pressure, and breathing rates. Here the male urethral sphincter contracts  to make sure urine doesn't mix with semen. During orgasm, pelvic and other muscles around the body contract rhythmically as an anatomical male ejaculates, releasing the sperm that might just get a chance to fertilize an egg, depending on the situation. 

Female orgasm is not required for conception and some may never experience one, and some may never experience one, but if it does happen the resulting uterine contractions and increased lubrication can help draw up and hold sperm in the uterus. As the body relaxes after all that excitement, it enters the resolution phase. Here we see males entering a refractory period that may last several minutes to a day or two, during which they are usually unable to reach orgasm again. Anatomical females are thought to not need this time out physiologically, though they may need it physically. Instead, some females are capable of serial orgasms, one after the other. 

Masters and Johnson's work has its share of  critics, many of whom point out that not everyone fits into their four-stage mold, and that things are not always quite so linear in the heat of the moment. But overall, their core physiological framework is still accepted. Now, regardless of exactly how sex happens, if it involves a fertile anatomical female and male and the timing is right, it could ultimately end with fertilization. 

So how do a single little ovum and an even tinier sperm come together to make a fertilized egg or zygote that divides and multiplies and grows into the trillions of cells that make up the glorious complex individual that is you? You'll remember that during ovulation, about once a month, an ovary releases a secondary oocyte that heads down the Fallopian tube and towards the uterus. That oocyte has a window of about a day or less to meet a sperm before it's no longer viable. But, that being said, sperm can persist in the female reproductive tract for as much as 3-5 days. Which means, for fertilization to occur, they have to either be in place during ovulation, or they have to arrive no more than a day after. But now I feel like I'm getting a little ahead of myself. 

Let's go back to how the sperm gets into the anatomical female body in the first place. During sex, if a male ejaculates into a female's vaginal canal, millions of hopeful sperm will be on their way. Despite the fact that they only have about a dozen or so centimeters to go, their journey isn't for the faint of heart. Luckily, they don't have hearts. The truth, of course, is that most sperm never reach an oocyte. Some just go in the wrong direction, others are killed in the strange new acidic environment of the vaginal canal, and millions more fail to penetrate the gauntlet of mucus that surrounds the cervix. Even the ones that swim hard enough in the right direction and manage to reach the uterus may then be culled by defending resident white blood cells. In fact, sometimes only a dozen or so of the original millions make it through all those trials to actually reach an oocyte. But even if they do, it's still not over for them. 

The sperm that do make it to the uterus and then the Fallopian tube have another hurdle to overcome because they can't actually penetrate an oocyte, not yet. They could literally hang out right next to one for hours and be unable to make a move until they are what's known as capacitated, and go through one last set of changes. As they swim through the cervix, uterus and Fallopian tube, female secretions start to degrade some of the sperm's protective proteins, until the cap on the tip of its little head, called the acrosome, is fragile enough that it can leak special hydrolytic enzymes. Those enzymes are the key to getting through the oocyte's protective outer layers. But before that, it has to weave through the granulosa cells that form the ooycte's outermost layer, known as the corona radiata. Once it's past that, it runs into the zona pellucida, a layer of glycoprotein that covers the oocyte's plasma membrane. And this is where it starts to get a warmer reception. This layer has specific sperm receptors waiting for it to bind to, and when it does, it opens a bunch of calcium channels which flood the sperm with calcium ions. These ions trigger the final acrosomal reactions that release the enzymes the sperm needs to work through the zona pellucida.

Now, a number of sperm make it to this phase, and in terms of strategy or luck, it's best to arrive on the scene after your compadres have already been hard at work digesting holes through the pellucida. Because, once a path is cleared, a lucky sperm can wiggle against the now-exposed oocyte membrane until it locks onto a nother set of sperm-binding receptors. And when a sperm docks into one of those, the membrane of the sperm fuses with the innermost membrane of the oocyte and the contents of the sperm enter the oocyte proper. Then, something pretty cool happens. 

Remember that the egg is still just a secondary ooycte here and still hasn't completed meiosis. As the sperm delivers its payload, it causes a flash of calcium ions from the oocyte’s endoplasmic reticulum. This tells the secondary oocyte to get ready to complete its second meiotic division. But it also triggers a reaction that suddenly seals the egg by destroying all the leftover sperm receptors in the membrane, preventing any other sperm from weaseling in. The oocyte then quickly completes meiosis II, leaving two daughter cells: the large ovum and a puny second polar body.

Meanwhile, the sperm’s detached nucleus swells up to five times its original size, forming the male pronucleus, one of two sort of proto-nuclei that contain each gamete’s genetic information. The other one, the female pronucleus, takes shape from the nucleus of the ovum, and then the two then start to approach each other, pulled along a protein scaffolding, or mitotic spindle, between the two. When the membranes of the two pronuclei rupture, chromosomes go flying, and boom, you officially have a diploid zygote, which almost immediately starts mitosis. And that, folks, is how you fertilize an egg.

But even though our bodies are made to have lots of sex and babies, there are plenty of people who are interested in enjoying the sex part, without enjoying the baby part. And given all of the things that usually need to go right to achieve fertilization and implantation, a little disruption at some stage along they way is usually all you need to remain baby-free. So folks have been experimenting with various herbal, chemical, physical, and behavioral methods of contraception for centuries -- for millennia. Some of which have been pretty nutty, like drinking cocktails laced with lead, or wearing weasel testicle amulets or rabbit butts, or shoving herbs in places where herbs should not go.

But, of course, contraception has come a long long way in recent years. In cultures that provide access to it, folks have wide range of contraception options -- you know, in addition to abstinence. First, if you’re really sure you don’t want kids, or more kids, sterilization is an option. During a tubal ligation or vasectomy procedure, a doctor snips, blocks, burns, or ties up the Fallopian tubes or vas deferens, effectively ending the gamete express by permanently preventing sperm and egg from ever getting to the penis or uterus.

But those who prefer a less permanent option also have lots of choices. We’ve got barrier methods that work mostly by physically keeping sperm and egg on either side of a wall, which is usually made of latex. Condoms, of course, cover the penis and catch ejaculated sperm before it can go anywhere, while diaphragms, sponges, cervical caps, and female condoms are inserted into the vagina where they form a wall over the cervix, turning sperm away.

Hormonal methods of contraception -- like the pill, or any number of newer shots, patches, and rings -- use synthetic estrogen and progestin to prevent the progression of the ovarian cycle to ovulation, and sometimes make changes to the functional uterine layer, the endometrium, and cervical mucus to help deter sperm.

An IUD, or intrauterine device, is a small T-shaped object a medical provider inserts into the uterus to prevent zygotes from implanting. And the implantation is key. Because, even if a sperm and egg manage to complete fertilization, in order for gestation to take place, the zygote has to find a home for itself in the uterus pretty quickly, so it gets the nutrients it needs to continue developing.

So that’s what we’re gonna be talking about next time. It takes a long time to talk about all the ways to make a baby -- but it’s important! But for now you learned about the four phases of the human sexual response, how a sperm finds and fertilizes an egg, creating a zygote, and how different types of contraception work to prevent that from happening.

Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon patrons whose monthly contributions help make Crash Course possible, not only for themselves, but for everybody, everywhere. If you like Crash Course and you want to help us keep making videos like this one, you can go to

This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written by Kathleen Yale. The script was edited by Blake de Pastino. Our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael Aranda, and the Graphics team is Thought Cafe.