凝血是如何做到的？

let's look at a blood vessel a blood vessel is kind of like a tube I think you'll agree with me it's a tube through which blood travels so here I'm drawing a tube and I'm gonna ask you a question which is what makes up the walls of this tube now for a blood vessel what makes it up is something called an endothelial cell so actually the walls are made up of these kind of gooey and öthey lleol cells that are stuck together tightly and that together form a tube through which the blood will travel so here I'm drawing a bunch of cells tightly stuck together they're stuck together tightly to prevent blood from coming out of course so this is more or less what it looks like each of these is a cell and so of course each one has a nucleus which I'll just quickly draw like that now let's delete that and let's change views to something a little bit easier so now here is the same blood vessel in cross-section and so these are the endothelial cells which we're seeing in cross-section and maybe here we can draw the nucleus of this one maybe it's visible right there so now we have blood moving through this blood vessel here are some red blood cells and they're moving along providing the body with oxygen but a very important question to ask is what happens if this blood vessel gets damaged so let's say that those two cells right there split open and they break open what's gonna happen if we don't fix this is that all of our blood is just going to rush out here and we're gonna lose blood so what is your body gonna do about this well it's gonna use a special player that we haven't talked about yet which is the platelet so I'm drawing a platelet here and a platelet is basically a tiny piece of a cell it doesn't have a nucleus or anything it's a tiny little piece of a cell that your body uses to block up holes like this so you have them floating around in your blood all the time here I'm drawing a bunch and what happens is when you have a hole in your blood vessel they're going to come together they're gonna stick together and they're going to clog up this hole and so basically they've built a little barrier so that we won't keep losing all our blood so are you satisfied well you shouldn't really be satisfied because there's a big question here which is why are these platelets clumping here and why aren't they clumping you know for example up here why don't they clump there or maybe even just in circulation why don't they clump up like this because if what they do is to clump up how would they know to clump up here and not here what's telling them these are things that we don't want to happen so I'm gonna put in a little X through them and then the solution to this problem is actually quite quite simple and beautiful the point is that the environment in the blood vessel is different from the environment outside of the blood vessel so outside we have some things that we don't have inside and one of those things I'm going to draw here I'm drawing it here and it has a name it's called collagen so I'll write that down here it's called collagen you don't have to worry too much about what it is collagen is kind of a structural protein that your body uses to to give structure to things and so the important thing is you have collagen down here and you don't have it here you don't have it here you don't have it here and it turns out that collagen chemically interacts with the platelets and maybe will draw a little a little spark there to show that they're chemically interacting it chemically interacts with them and causes them to stick together and form this plug that we're talking about so we can call this by the way a platelet plug because it's plugging the hole and by the way just to be clear you also have collagen up here and over here so it's basically everywhere outside the blood vessels now it turns out that this is only step one of the clotting mechanism of your body so up here we'll put number one platelet plug and there's actually two steps because the platelet plug itself is not quite as strong as we would like it to be so there's a stack a second step which makes this plug stronger and that second step involves something called fibrin so we'll write that here fibrin and fibrinogen cell like platelets are a fragment of a cell like platelets are fibrin is just a protein and what fibrin is gonna do and it's gonna come here and it's gonna try to strengthen this plug by forming this mesh of protein that's gonna hold all these platelets together and form a very tight object now these fibrin strands that's with each of these you know little squiggles is it it's a fibrin strand these fibrin strands are made up of little fibrin subunits which I'll draw here and it turns out that these subunits naturally like to come and stick together and I guess the technical word for this is they polymerize they form a polymer basically they just stick together and lots of them will stick together and align to give you this fibrin strands a year and where does this fibrin come from does it come from down here no it actually also circulates in the blood so let's draw some little fibrin molecules up here so they're circulating the blood is this right well this actually can't be exactly right because I just told you that these fibrin molecules naturally stick together and so if we had these fibrin molecules circulating in the blood like this what would happen well they would actually stick together in the blood and they would form these long strands in the blood that we didn't want because we only want the strands here at the platelet plug so let me remove those strands so it turns out that we don't have fibrin circulating in the blood what we have is something slightly different we have fibrinogen fibrinogen and so I'll draw a fibrinogen down here of course keep in mind these are all my little cartoons and probably it doesn't look like this so these guys we said we're vibrant vibrant but now this molecule is a fibrinogen and you'll notice it's the same as a fibrin except it has an added little piece to it and that little piece as you can tell is gonna keep it from sticking to itself these fibrinogen are not going to be able to stick together the way that these fibrin are so what do we need to do then well of course we need to turn this fibrinogen into fibrin but where are we gonna do that only at the site where we want fibrin strands to gather so only here where we have that damage and so again we face the same question that we faced with the platelets which was how do they know how do the platelets know to aggregate here well likewise we want to know how do the the fibrinogen know to turn into fibrin here so that they can then stick together and form the strands and the answer basically is luckily the same so we have some some chemicals down down here now we're not talking about collagen anymore we're talking about something else and if you really want to know what it is I'll write the name here but it's not really the name that's important that's the principle here the principle is you have these little proteins down here called tissue factor and they're normally not in the blood they're only down here outside of the endothelial cells lining the blood vessel and so these little proteins tissue factors are gonna be right here in this little wound and around the wound and they're they'll cause these fibrinogen to become little fibrin protein molecules and then those fibrin will be able to stick together and they also stick to of the platelets by the way I hope I was clear about that and form the mesh that we see there now if we want to get really fancy we can ask how this tissue factor turned fibrinogen into fibrin and you can ask is tissue factor kind of like a little knife is it and by knife I really need enzyme is it a little knife or enzyme that causes this piece to break off of the fibrin so that we're left with just fibrin and actually unfortunately the story is much more complicated than that and there's actually a good reason for it because let me propose to you a situation let's say you have to turn a million fibrinogen into fibrin because each of these fibrinogen is a very small protein so we need about a million of them to make up this clot so you are tissue factor and you want to turn a million fibrinogen into fibrin is the best way to do it for you to actually sit down and chop off this piece off of all these fibrinogen one-by-one that would take you a long time is that better or is it better for you as tissue factor to call up five friends and have each of those friends call up five friends and have each of those friends call up five friends and keep doing that until you have a huge number of people there ready to help you and now get all of those people to help you convert fibrinogen to fibrin so obviously that latter scenario is a more efficient mechanism and so that's what your body does and that's what tissue factor does so tissue factor and by the way I know this is getting convoluted but try stick with me tissue factor is going to turn another protein that we haven't talked about into its active form and then that other active form of that protein that we haven't talked about is going to activate another protein that we haven't talked about into its active form and then that one is gonna do the same and then that one is gonna do the same and so basically every time you introduce a new protein there you're ramping up the total number of activated proteins of that type and so finally by the time you get to the one that's going to break the fibrinogen apart and create fibrin and by the way I'll draw him and he has a special name called thrombin again I don't think froggen is necessarily a word you need to know but just in case so by the time you get to thrombin you have a lot more activated thrombin then you had tissue factor in the beginning but the tissue factor here is the spark and that's the guy that that really got us going so that's basically it this is how your body plugs holes and blood vessels and keeps you from losing too much blood and if you want to know more about that kind of complicated stuff I said at the end about you know all these different levels of activated proteins you can look it up it's called the coagulation cascade it's called a cascade because you're cascading through all these different levels of proteins