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- [Voiceover] We're gonna talk about the transport of sperm. I just mean how it gets transported out of the male reproductive tract, and into the female reproductive tract, where it can hopefully fertilize an egg, and result in a pregnancy. To do this, we're gonna first look at a sagittal view of the male reproductive system. I'll just show you what a sagittal view is. Basically, if we were to look at this blue guy over here, and if we said this is his left side and this is his right side, we would make a cut down his midline, like so, and we would sort of cut away everything on one of the sides. In this case, we're gonna cut away everything on his left side. Then we're gonna look, here's an eye here, we're gonna look at his right side of his cut surface on the right side here. When we do that, we find that we see something like that. This is what we call a sagittal view. Just to remind you, this would be his right leg. The bottom line is that sperm needs to be transported out of our bodies. We do that, in males, via a two-step process. The first step is called erection. An erection is basically when the penis goes from having very little blood within its core, a state called flaccidity, and I'll write that out, to a state where it's filled with blood, and we call that an erection, when it's filled with blood. The second step in our two-step process is called ejaculation. Ejaculation is basically the expulsion of sperm out of the reproductive tract. Let's go into the mechanisms of how this happens. Before we talk about how an erection happens, let's briefly discuss why an erection happens. Let's just clear off some of this stuff here. Believe it or not, an erection actually starts in your brain. Here we have a brain. When you're physically or mentally sexually stimulated by sights or sounds, or smells, or even thoughts, your brain sends signals to your penis, and it can cause an erection. It does that by sending signals to the blood vessels in the penis, and those signals cause those blood vessels to open up, and allow blood into the penis. So it's this filling of the penis with blood which results in an erection. We'll expand on that in a moment. First, we'll talk about some regional anatomy of the penis. The base of the penis, here, that's called the base, or the root. This area here, is known as the body. This area here is called the head, or, the glans. What we'll do next is look at a more detailed view of the anatomy of the inside of the penis. The best way to visualize that is if we did something called a transverse view. Transverse view is basically a cut that goes this way across the penis. We'll put our little eye here and we'll look up at this cut surface here. When we do that, you'll find that it looks something like this. Again, this is called a transverse view. Just to orient you, this transverse view is of two different penises, side by side. The one on the left is a flaccid penis, It's not filled with blood. You can tell because it's got a lot more visible blue veins, which you don't see in erection. Over on the right we have an engorged, erect penis. You can see the veins have been compressed to the sides. So that one's erect. I'll explain all that in a minute. In this view, it's quite easy to see the three cylindrical vascular compartments that get filled with blood during an erection. These vascular compartments, or chambers, are called the corpora cavernosae, and you have two of those, one on each side here, and the corpus spongiosum, which you have down here. By the way, this is the bottom of the penis, the underside, and this is the top of the penis. Also, this structure here in the center, this tube, is the urethra. Blood normally flows into these vascular chambers via both dorsal arteries and cavernosal arteries. Here's a dorsal artery, and here's a cavernosal artery here. When the penis is filling with blood, blood actually leaks out of these cavernosal arteries and into these purple circles that you see called lacunar spaces. These lacuner spaces run the entire length of these vascular chambers, so there are a lot of spaces there for blood to leak out into. That's what causes the penis to get engorged. Blood is normally drained out of these chambers by veins with the same name. So, dorsal veins and cavernous veins. But under normal conditions, i.e, when you do not have an erection, the blood flow into the penis equals the blood flow out of the penis, so there's no actual net change in erection status. Let's just label these here. This one on the left here is flaccid. This one over here on the right is erect. The reason why the one on the left is flaccid, is because it has arterials that are constricted. These red arterials, when they're constricted, they don't actually allow much blood into the penis. Over here on the right, the erect penis is sexually excited. So you can see visually that the arterials on this side are much more dilated, that is, they let in a lot more blood, thus can cause an erection. What's keeping this penis flaccid? What is keeping these arterials from opening up? Well, It turns out that it has to do with your brain. Normally, little chemical signals from the brain, called norepinephrine, cause the arterials to stay constricted. We can see a neuron here, leaving the spinal cord, and sending a signal to the arterial, to keep it small. By the way, a neuron is a cell of the nervous system responsible for sending messages. Over on the other side, on the erect side, there's another neuron, actually a different type of neuron, that actually sends a different signal to the arterials, and results in them opening up and allowing blood into the penis. The signal that this neuron is sending, it's called norepinephrine. What norepinephrine is, it's a little chemical signal from a division of the nervous system called the sympathetic nervous sytem, or the SNS. That'll become important later on. The signal that this neuron is sending is NO, or nitric oxide. This neuron is actually from a division of the nervous system called the parasympathetic nervous system, or the PSNS. As these arterials dilate and allow lots of blood into these sinuses here, they actually fill so much that they push outward against the edges and compress the veinous drainage of the penis. That basically prohibits flaccidity, thus, it results in an erection. Just to be complete, an erection is reversed when the inflow of blood is stopped, and the veins are then allowed to open up and allow the blood back out of the sinuses. Then you go flaccid again. That might happen when the nitric oxide from those green parasympathetic neurons stop being produced. One last thing before we move on. You might be thinking, "Vish, you told us that "these arterials dilate and allow blood "into the vascular chambers." Well, what's to stop the penis from filling up indefinitely? We actually have, if you look at these yellow circles here, the three of them, they surround our vascular chambers and they prevent them from over-expanding. These wrappings are called the tunica albuginea, and they're just a supportive structural tissue. Let's just clear off some of this stuff and move on to the second phase, which is ejaculation. We'll start with a definition. What is ejaculation? Ejaculation is the discharge of semen from the penis. You normally discharge about three to five milliliters per ejaculation. In that three to five milliliters, you actually pack about 300 million sperm. An ejaculation happens when, basically, a critical level of sexual excitement has been reached. Sexual stimulation actually causes nerves in the penis to send chemical signals to the spinal cord and brain. The brain and the spinal cord send messages back to the penis to cause ejaculation. There's two phases to ejaculation in itself. We'll start with our erect penis because now we're sexually excited. Remember, now that we're erect, we've filled these vascular chambers here with blood. The first part is sympathetic nervous system stimulated. Remember, in red we drew these neurons, the sympathetic nervous system neurons. They're gonna release norephinephrine, which I'll abbreviate as NE, onto all of the following structures: the epididymis, the vas deferens, the accessory glands, and the ejaculatory duct, which is here sitting inside the prostate gland. To respond to that norepinephrine, these structures that I mentioned here, actually contract and emit semen into the beginning part of the urethra. In the second phase, the semen is now sitting here at the beginning part of the urethra. The smooth muscle of the urethra itself, all along its length, and this muscle at the base of the penis called the bulbospongiosum muscle, they then contract and expel the semen from the urethra out of the tip of the urethra, called the meatus. That's basically the process of ejaculation. All of these muscular contractions are associated with a feeling of extreme pleasure. You also get full body physiological changes. For example, you see a decrease in heart rate and blood pressure after ejaculation. In some, the process of ejaculation and the whole body physiological changes, is called an orgasm.