通过勃起和射精传播的精子

Oh so we're gonna talk about the transport of sperm and 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 and so to do this we're gonna first look at a sagittal view of the male reproductive system and and I'll just show you what a sagittal view is basically it's 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 we would make a cut down his midline like so and we would sort of cut away everything on one of his side's and in this case we're gonna cut away everything on his left side and then we're gonna look here's an eye here we're gonna sort of look at his his right side of this cut surface on the right side here and so when we do that we find that we see something something like that and so this is what we call a sagittal view and just to remind you this would be is right leg and so the bottom line is that sperm needs to be transported out of our bodies and so we do that in males via a two-step process and 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 and the second step in our two-step process is called ejaculation and so ejaculation is basically the expulsion of sperm out of the reproductive tract so let's go into the mechanisms of how this happens but before we talk about how an erection happens let's let's briefly discuss why an erection happens actually let's just clear off some of this stuff here so believe it or not an erection actually starts in your brain and so here we have a brain and so when you're physically or mentally sexually stimulated by sights or sounds or or smells or even thoughts your brain sends signals to your penis and it can cause an erection so it does that by sending signals to the blood vessels in the penis and those signals causes those blood vessels to open up and allow blood into the penis so it's the stilling of the penis with blood which results in an erection and will expand on that in a moment but first we'll talk about some regional anatomy of the penis so the base of the penis here that's called the base or the root this area here is known as the body and this area here is called the head or the glans and so 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 and so when we do that you'll find that it looks something like this and so again this is called a transverse view so just to orient you this transverse view is of two different penises side by side so the one on the left is a flaccid penis it's not filled with blood and you can tell because it's got a lot more visible blue veins which you don't see in erection and over on the right we have an engorged erect penis and you can see the veins have sort of been compressed to the sides so that one's erect and I'll explain all that in a minute so in this view it's quite easy to see the three cylindrical vascular compartments that get filled with blood during an erection and so these vascular compartments or chambers are called the corpora cavernosa 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 and also this structure here in the center this tube is the urethra so blood normally flows into these vascular chambers via both dorsal arteries and cavernosa arteries so so here's a dorsal artery and here's a cavernosa artery here and so when the penis is filling with blood blood actually drains out it leaks out of these cavernosa arteries and into these purple circles that you see called lacunar spaces and these lacunar spaces run the the entire length of these vascular chambers so there are a lot of spaces there for blood to leak out into and that's what causes the penis to get engorged and blood is normally drained out of these chambers by veins with the same name so dorsal veins and cavernous veins but under normal conditions ie when you do not have an erection the the blood flow into the penis equals the blood flow out of the penis so there's no actual net change in erection status and let's just label these here this this one on the left here is flaccid and this one over here on the right is erect and the reason why the one on the left is flaccid is because it has arterioles that are constricted and these red arterioles when they're constricted they don't actually allow much blood into the penis but over here on the right the erect penis is sexually excited and so you can see visually that the arterioles on this side are much more dilated that as they let in a lot more blood and the scan can cause an erection and so what's keeping this penis flaccid what is keeping these arterioles from opening up well it turns out that it has to do with your brain and so normally little chemical signals from the brain called norepinephrine cause the arterioles to stay constricted and so we can see a neuron here leaving the spinal cord and sort of sending a signal to the arteriole to keep it small by the way in neuron is a cell of the nervous system responsible for sending messages but over on the other side on the erect side there's a another neuron actually a different type of neuron that actually sends a different signal to the arterioles and results in them opening up and allowing blood into the penis and so the signal that this neuron is sending it's called norepinephrine and what norepinephrine is it's a little chemical signal from a division of the nervous system called the sympathetic nervous system or the SNS and that'll become important later on and the signal that this neuron is sending is n-o or nitric oxide and this neuron is actually from a division of the nervous system called the parasympathetic nervous system or the PS and and so as these arterioles dilate and allow lots of blood into these into these sinuses here they actually feel so much that they push outward against the edges and compress the venous drainage of the penis and that basically prohibits flaccidity thus it results in an erection and just to be complete an erection is reversed when the inflow of blood is stopped and then the veins are then allowed to open up and allow the blood back out of the sinuses and then you go flaccid again and that might happen when the nitric oxide from those green parasympathetic neurons stop being produced and one last thing before we move on you might be thinking this you told us that these arterioles dilate and allow blood into the vascular chambers well what's to stop the penis from sort of filling up indefinitely and we actually have if you look at these yellow circles here the three of them they surround our vascular chambers and they sort of prevent them from over expanding and these wrappings are called the tunica albuginea and they're just sort of a supportive structural tissue okay let's just clear off some of this stuff and move on to the second phase which is ejaculation so we'll start with a definition what is ejaculation ejaculation is the discharge of semen from the penis and you normally discharge about 3 to 5 milliliters per ejaculation and in that 3 to 5 milliliters you actually about 300 million sperm an ejaculation happens when basically a critical level of sexual excitement has been reached so sexual stimulation actually causes nerves in the penis to send chemical signals to the spinal cord and brain and then basically the brain and the spinal cord send messages back to the penis to cause ejaculation and there's two phases to ejaculation in itself so we'll start with our erect penis here because now we're sexually excited and remember now that we're erect we've filled these vascular chambers here with blood so the first part is sympathetic nervous system stimulated remember in red we drew these neurons as sympathetic nervous system neurons and they're gonna release norepinephrine which I'll abbreviate as n e 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 and so to respond to that norepinephrine these structures that I mentioned here actually contract and emit semen into the beginning part of the urethra now in the second phase the semen is now sitting here at the beginning part of the urethra and the smooth muscle of the urethra itself all along its length and this muscle at the base of the penis called the bulb of spongy owesome muscle they then contract and sort of expel the semen from the urethra out of the tip of the urethra called the me aidid and that's basically the process of ejaculation now all of these muscular contractions are associated with a feeling of extreme pleasure and actually you also get full-body physiological changes for example you see a decrease in heart rate and blood pressure after ejaculation and in some the process of ejaculation and the whole body physiological changes is called an orgasm