Hi, everyone. Behind me is our lesson on blood coagulation or blood clotting, which is the third and final step of hemostasis, the process that helps to prevent and control bleeding after an injury. Now, there is a lot of information here in this lesson, so we're going to break it down and approach it one step at a time starting with this top section that you can see boxed in red. So let's zoom in and get started. Now, recall from our last lesson video that the second step of hemostasis resulted in the formation of a relatively unstable platelet plug. And so, in this third step of hemostasis, blood coagulation or blood clotting, it actually reinforces that unstable platelet plug from step number 2 to make it more stable and more effective at preventing and controlling blood loss. And it does this reinforcement by using a protein called fibrin as a molecular glue to stabilize that platelet plug. And so, it's really important to remember moving forward that really the main goal of blood coagulation is to generate this fibrin protein. Now it turns out that fibrin forms via a very complex enzyme cascade, as you can see by this complex diagram behind me. In fact, there are over 30 chemical reactions and over a dozen clotting factors. And many of these clotting or coagulation factors involved are numbered with Roman numerals in terms of the order of their discovery, not necessarily the order that they're involved in the pathway. Now that is just super complex. But the good thing is that this is really the medical school level of detail, and this process can be simplified into just 3 phases that focus on some of the most important coagulation factors, just like what you can see here. Now that is just so much better. And note that we've strategically implemented these interactive blanks for the first letters of many of these coagulation factors. And that's because we're going to utilize them in memory tools in our next lesson video.
So stay tuned for that. So let's shrink this image down, so I can show you the text in our lesson for each phase. Phase 1, phase 2, and phase 3. So we're going to cover each of these 3 phases 1 by 1, starting with phase number 1. So let's re-enlarge this image and push phase number 1 up to the top. And there's still quite a lot of information being shown here. So let's wipe this slate clean, so we can take this step by step. So phase 1 of blood coagulation can occur via 1 of 2 different pathways, and those are the extrinsic pathway and the intrinsic pathway. And as you can see in this diagram down below, regardless of how phase 1 is initiated, both of these pathways will ultimately lead to the formation of prothrombin activator, which is an enzyme that's also sometimes referred to as prothrombinase. And so down below in our image, we can fill in the interactive blank for the first letter of prothrombin activator. And again, although both of these pathways ultimately lead to the formation of prothrombin activator, we can see in this diagram that technically these two pathways meet at an earlier step here at this intersecting point, which is factor 10 activation. And so the Roman numeral 10 is represented as the letter X, and so we can remember this detail by remembering that X marks the spot where these two pathways technically meet. Now the extrinsic pathway, as its name implies, is initiated by factors that are extrinsic to or that are outside of the blood itself. And so these factors are not a component of the blood. And more specifically, the extrinsic pathway is initiated by a protein called tissue factor or factor 3, which is a protein that is expressed on damaged tissues. And so down below in our image, we can fill in the interactive blank for the first letter of tissue factor here. And notice that in this image on the left, we're showing you some tissue immediately outside of the blood vessel, and notice that it is damaged right here in this region. And this damaged tissue is expressing some tissue factor, which are these blue little dots. And so when the damaged blood vessel releases blood and when the blood comes into contact with that tissue factor, it will actually initiate the extrinsic pathway, which again will ultimately lead to the formation of prothrombin activator.
Now the intrinsic pathway, on the other hand, is initiated by factors that are intrinsic to the blood, or that are inside of the blood itself. And so these factors are a component of the blood or the blood plasma. And more specifically, the intrinsic pathway is initiated by a plasma protein called Hagemann factor or factor 12. And so down below in our image, we can fill in the interactive blank for the first letter of Hagemann factor here, and notice that inside of the blood vessel in the image in the bottom left, we are abbreviating Hageman Factor as HF. And so it's important to note that Hageman Factor must first become activated before it can initiate the intrinsic pathway. And so Hageman factor is activated by negatively charged surfaces, which is why we have this negatively charged symbol here inside of the blood vessel. And so this negatively charged surface could, for example, be the negatively charged surface of activated platelets, as we discussed in some of our previous lesson videos, or it could be the negatively charged surface of the glass in a test tube, which is why our blood clots inside of a test tube. Now it's also important to note that the extrinsic pathway is a relatively fast pathway that will form a prothrombin activator and ultimately lead to a blood clot in a matter of seconds. Whereas the intrinsic pathway is a relatively slow pathway that will typically take several minutes to activate prothrombin act