- Hematologic system questions
- Mini MCAT passage: Symptoms of low platelet counts
- What's inside of blood?
- Hemoglobin moves O2 and CO2
- Bohr effect vs. Haldane effect
- Blood types
- How do we make blood clots?
- Coagulation cascade
- Life and times of RBCs and platelets
- Blood cell lineages
This video explains the body's clotting process, focusing on the role of fibrinogen and fibrin in forming a solid plug at the site of a blood vessel injury. It further delves into the intrinsic and extrinsic pathways of the coagulation cascade, highlighting the importance of thrombin in both the clot formation and the negative feedback mechanisms. Created by Patrick van Nieuwenhuizen.
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- For the MCAT, do we seriously need to know the Roman numeral cascade sequence? If you mentioned in the video that we don't need to know this, I apologize in advance. I just zipped through the video to see if you were explaining all those numeral factors (and you were). This is a topic that I can't see the MCAT testing you on through rote memorization. Medical school is a different story perhaps. For the MCAT, I can see the whole factor-numeral system being fully explained and/or displayed within the passage itself, but that's as far as it should go.(30 votes)
- I have studied and taken the MCAT and I am quite sure you do not need to have this cascade sequence memorized (the study materials I used did not even mention it).(27 votes)
- Is this clothing process a permanent solution or will the cells surrounding the wound eventually repair it? If it doesn't repair the hole, wouldn't all our bloodvessels be full of small plugs? If it does repair the hole, how does it clear the plugs and what would that process be called? Thanks!(12 votes)
- I believe the process by which the platelet plugs dissolve is called fibrinolysis? That's how clots dissolve. I know scabs, which are external, eventually fall off, but for internal clots (and perhaps external ones to an extent) are cleared by fibrinolysis.(9 votes)
- How does the thrombin know when to produce anti-thrombin and catalyze the plasminogen to plasmin reaction? Does the fibrin mesh feedback to the thrombin to initiate this process?(2 votes)
- Thrombin is active in promoting further activation of the coagulation cascade, but is also part of its own feedback loop. Thrombin, once cleaved from prothrombin, will bind to an enzyme thrombomodulin. The complex of thrombomodulin + thrombin activates a protein called protein C, which forms a complex with Protein S. The levels of activated Protein C and ProteinS increase at the site of coagulation. This increase causes the prevention of activation of F V as well as F VIII. This shuts down the intrinsic pathway, as well as the extrinsic pathway.
Antithrombin, freely floating in the blood, is activated by heparin, made in mast cells. This prevents F II (Thrombin) and F X from being activated. Antithrombin is not produced from thrombin. Coagulation is activated by damaged endothelial cells releasing chemical messengers to initiate the cascade. The reason "normal" people don't clot all the time is because the undamaged, healthy endothelial cells express factors on their surface that inhibit and prevent clotting. Thrombomodulin, PAI, heparin, TFPI, and tPA are just a few.
tPA is found on healthy cells. It activates plasminogen to plasmin, which in turn initiates fibrinolysis. Plasmin will cleave away the fibrin cross linked with F XIIIa. This system is initiated any time there is bleeding by the surrounding healthy tissue cells. However, if the number of damaged tissue outweighs the amount of healthy tissue, there will not be enough negative signal to stop clotting. This concept, of cells that express + and - factors for coagulation, allows the body to target the specific site of damage in order to prevent damage to healthy cells via coagulation.
The coagulation cascade is complex. There are many graphical representations online for you to see a better flow. Its one of those things that words really can't help with. The physiology, however, is incredibly interesting. Good question!(12 votes)
- How was this insanely complex system discovered? I never knew we had such an amazingly complex clotting mechanism.(2 votes)
- This complex system was discovered by many people, but most people say that the father of medicine; Hippocrates; was the first one who contributed to the coagulation cascade discovery . Other scientists such as Mercurial, Hew-son, and Owren had contributed as well. I hope this helps!
- What is the difference between hemophilia A, B and C except of the coagulation cascade? And how does the coagulation cascade affect he quality of the coagulation?(5 votes)
- All types of hemophilia are linked to the X chromosome.
Type A (defective Factor VII) is inherited, but it may be caused by spontaneous genetic mutation. It affects all ethnic groups equally and it can be mild, moderate or severe, depending on ratio of Factor VIII. In severe cases, the bleeding can occur spontaneously in the joints and muscles.
Hemophilia B (defective Factor IX) is also inherited and it can be caused by spontaneous genetic mutation as well. The severity levels and symptoms are the same as hemophilia A.
Hemophilia C (missing or defective Factor XI). It occurs in up to 8% in Israel, in Ashkenazi Jews (mainly due to intermarriage). Factor XI deficiency is inherited in an autosomal recessive pattern, meaning both parents must carry the gene to pass it on to their children. Unlike hemophilia A and B, men and women are affected equally.
Unlike hemophilia A and B, symptoms don’t correlate with Factor XI levels in the blood. People with lower levels may bleed less than those with higher levels of Factor XI(1 vote)
- What is the "extra piece" that is attached to fibrin (making it fibrinogen) in the blood?(4 votes)
- Where did fibrinogens in the blood come from?(3 votes)
- What does Prothrombin have to do with roman numeral I'm confused cause there suppose to be like numbers like 1 I 2 II 3 III 4 IV 5 V 6 VI 7 VII 8 VIII 9 IX 10 X 11 XI 12 XII 13 XIII 14 XIV 15 XV 16 XVI 17 XVII 18 XVIII 19 XIX, but what does this have to do with clotting ?(3 votes)
- What's the difference between the intrinsic and the extrinsic pathways? Are they both active simultaneously?(2 votes)
- One is started by cells damaged inside capillaries, the other is started by cells damaged in tissue. Yes, they are both initiating a blood clot at the same time to ensure that we do not bleed to death. Both pathways s result in the formation of thrombin and the production of fibrin from fibrinogen as well as causing platelets to clump at the injury so that the clot forms.(3 votes)
- The kind of exam I'm preparing for requires memorising entire coagulation cascade. Can anyone suggest any trick?(1 vote)
- Write out the cascade repeatedly from a trusted source. Unfortunately, it seems to be a brute memorization type thing.(3 votes)
- Let's say this is a blood vessel and it's made up of these endothelial cells, the same way that all blood vessels are. Let's say that this blood vessel gets into a fight and it gets a little bit hurt. So, the walls break open and the cells now no longer seal the vessel from what's around it. When this happens, we wouldn't want all the blood in this vessel to come pouring out of the vessel because we'd lose way too much blood. So the body has a method of containing the blood. The first thing that's going to happen is that little platelets, which are circulating in the body, are going to come and deposit there and form an initial plug. So these little things are platelets. But it turns out that this plug is not quite solid enough, and so the body needs a second mechanism to solidify the plug. And that mechanism is what we're going to talk about most in this video. I'm drawing that mechanism here. What I'm drawing is little fibrin strands, which will come and act as a kind of mesh to hold the platelet plug together solidly. So what do these fibrin strands look like? Well, they're made up of little fibrin sub-units. These are actually what we refer to as the fibrin molecules or proteins. These, it turns out, have a natural affinity for each other, so that when you bring them together, they form a polymer. They join end to end and they create a fibrin strand. So the question then becomes, how do you make sure that these fibrin units only join together and make a strand at the site of injury? So for example, if you had a bunch of little fibrin molecules over here, how would you keep them from joining together? The answer is that you can't, which is why you don't have fibrin circulating in your blood. You actually have something else, which I'm going to draw here. That something else is fibrin, but plus an extra piece. That extra piece, as you can see, covers one of the active sites of the fibrin and therefore prevents it from joining to itself. So the name of this thing is fibrinogen, probably because it will form fibrin, hence the fibrinogen. It's only when you convert fibrinogen to actual fibrin that the units can join together and form a strand. So again, to repeat, you don't have fibrin circulating in your blood, you actually have fibrinogen. So the question then becomes, how does your body know to convert fibrinogen into fibrin at the site of injury? The answer is that when you injure your endothelium here, you're going to expose your blood to new proteins. And maybe your actual endothelial cells will release some proteins because they're damaged. So basically, you have new proteins that weren't seen before and that are seen now. Those proteins will eventually cause fibrinogen to turn into fibrin. So while evolution was designing us, it could've said, let's use these little yellow guys to convert fibrinogen to fibrin. That might've worked, but it's actually not the most efficient way to do things. The reason is, imagine if you and a couple of friends have a huge amount of work to do. Let's say you need to convert a million fibrinogen to fibrin. Is the best way to do it, to actually sit down and crank it out? Or would it be more efficient to have each of you call five friends and invite those friends to come work. And ask those friends to each call five friends. And ask those friends to each call five friends. Well, obviously that would get the job done much faster assuming you had those friends. That's also what your body does. So actually, it doesn't use these yellow guys to convert fibrinogen to fibrin. There's another player which does that, and it's an important one, so we'll give it a little drawing like that, and it is called thrombin. Thrombin, just like fibrinogen, is activated from an inactive form, which we call prothrombin. The prothrombin has a little piece on the end that prevents it from working, so this is prothrombin. That piece is removed when you want to get to work. So is prothrombin activated by the little yellow guys? Well, actually, he's not either because the chain of amplification is much longer than that. To draw the actual amplification cascade, you just need to see it and practice. But there is an easier way to draw it than it usually is drawn, so we'll do that now. So let's say you were counting down from XII. Normally you would start at XII. You'd go to XI. You'd go to X, and then you'd go to IX. But let's say that you weren't very good at counting. You would start with XII. You'd go to XI. Then you'd make a little mistake - you'd go to IX. Then you'd realize you forgot X, so you'd go to X. Now it's good that you remembered X because X is a big deal, and he's going to help bring us thrombin. Turns out that thrombin is also known as II. We know very well that thrombin helps give us fibrin, which is known as I, and that's no surprise because fibrin's the most important guy. He's the ultimate goal, and thrombin is the guy that helps us get the ultimate goal, so he should be number II. Now unfortunately, it's not quite that easy. X likes V because they're both multiples of five, so they work together and IX likes VIII because they're right next to each other and so they work together. So this is the first part of our clotting cascade and it turns out that we call this part here, the intrinsic pathway and we can talk about what that means later, but for now let's just give it its name. But what's perhaps more important to be clear about is that in this drawing, XII is not actually becoming XI, and XI is not actually becoming IX. What's happening is that XII, when it's activated, is a catalyst to convert XI from it's inactive form into its activated form, which we'll draw XIa. And then once XI is activated, it serves as a catalyst to convert IX from its inactivated form into its activated form. So you see that these arrows are actually more about catalyzing. Now I said that this was an amplification sequence, so I just wanted to share a little data to show that that's true. It turns out that this guy, factor I, or fibrin in its inactivated form, which is fibrinogen, has about 3,000 micro-grams per milliliter in blood, while this guy has about 100 micro-grams per milliliter in blood. Meanwhile, X has about 10 micro-grams per milliliter in blood and factor IX has about five micro-grams of milliliter in blood. So you can really see that as you go down this thing, you're increasing your amounts in your blood, which reflects the fact that you're also going to increase the number of active forms of these when you have a clot. But anyway, I said that this was the intrinsic pathway because there is another pathway, which also leads to an activated X, but in this other pathway, what activates the X is an activated VII, which is activated by III, also known as tissue factor. I'll just write TF for tissue factor. In this pathway, which I'll circle here - and I apologize for the poor organization - this one is known as the extrinsic pathway, extrinsic. So what's the difference between these two pathways? Well, it turns out that the extrinsic pathway is the spark. It's the one that gets activated by the original insult over here, whereas the intrinsic pathway is kind of like the workhorse that really gets most of the coagulation done. So how does that work? Well, you first get this tissue factor, which is actually one of these little yellow guys. And that tissue factor activates VII, which activates X, so you get a shot - a spark that shoots down this way and activates a little bit of X. And then X will activate a little bit of thrombin, and then thrombin will get the intrisic workhorse going. And how will thrombin do that? Well thrombin actually activates a whole bunch of these guys, and to remember the ones that it activates, you just need to take the five odd numbers starting at five. So what is that? That's V, VII, IX, XI, and XIII. Actually, this is just almost right, but it actually turns out that it's not IX, it's VIII because it couldn't be quite that easy. So those are the five that it activates. So let's draw that in here in our drawing. So let's draw that in the form of blue arrows because thrombin is blue. We said it's going to activate V. We said it's going to activate VII. We said it's going to activate not IX, but VIII, so this will be an awkward arrow to draw. We said it's going to activate XI, and we said it's going to activate XIII. Where's our XIII? Well, we haven't actually drawn it in yet, so let's quickly chat about that. The end goal of this whole cascade is to get these fibrin molecules, and these fibrin molecules together will form some strands. It actually turns out that there's one more step, which is to connect these strands together. So we're going to want to connect these strands together with some cross links. These cross links will just hold them together so that they actually form a tight mesh. It turns out that it's this step right here, which is enabled by factor XIII. So let's draw the final thrombin activity, which is to activate XIII. So you can see that once you activate a little thrombin, it's going to activate all the necessary things in this intrinsic pathway to get it going. You might actually be wondering about XII up there because thrombin is not hitting him, and actually it turns out that if you remove a person's factor XII, they can still clot pretty well. So it's clear that XII is not a totally necessary part of this intrinsic pathway. And to be clear again, with our use of arrows, this green arrow here is different from these white arrows in the sense that here, we are saying that fibrin is going to become fibrin strands, which is going to become interlaced fibrin strands. So if this was all there was to the story, then every time you had a little bit of damage to your endothelium, you would cause the extrinsic pathway to fire. So you'd create a little activated VII. You would activate some X, which would activate some II, which is thrombin, which would start to create fibrin from fibrinogen. And moreover, the thrombin would have this positive feedback, which would cause more and more thrombin to be produced, which would cause more and more fibrin to be produced. And basically, this system would just spiral out of control and you would become one large walking clot. So to keep that from happening, there are some negative feedback loops. And like much of the other steps in this picture, they are governed by thrombin. So one thing that happens is that if thrombin helps create plasmin from plasminogen, much the way it helps create fibrin from fibrinogen. And this plasmin acts directly on these mesh networks of fibrin and breaks them apart. So that's one helpful factor, but that's not really the example of negative feedback because it won't prevent the continued production of all this. So another example is that thrombin actually stimulates the production of anti-thrombin, which is kind of counterintuitive, but that's sort of classic negative feedback. And what anti-thrombin is going to do, as you could guess, is it's going to decrease the amount of thrombin that's being produced from prothrombin, and it's also going to impede the production of activated X from X. So what if this whole system didn't work? I don't mean the negative feedback, but I mean the whole clotting system? Well, if it didn't work, then you wouldn't form a stable plug here, and you would get lots of blood pouring out of your damaged endothelium. So what we call that is hemophelia. Hemo refers to blood and phelia means love. So people who have hemophelia, love to bleed. There are three different kinds of hemophelia. There is A, there's B, and there's C. It's easy to remember the causes of these, because A is associated with VIII. B is associated with what comes after VIII? IX. And C is associated with what comes after IX? Not X, actually, but XI. So we can draw those into our clotting cascade over here to see that a factor VIII deficiency will give you hemophelia A. A factor IX deficiency will give you hemophelia B. Whereas a factor XI deficiency will give you hemophelia C. So we see that these guys target the intrinsic pathway.