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Voiceover: I want to start with an overview of hemostasis first. Here, we have our blood vessel and it's been damaged. And we want to make sure that we stop any bleeding or minimize it. And the way we do this is through hemostasis. The first part is making this platelet plug. And that happens in primary hemostasis. But it's weak, and we need to make it stronger. And we make it stronger with this fibrin mesh. And that happens during secondary hemostasis. And that's what I'll be focusing on right now. So, let me erase everything. And I'm going to put this blood vessel off to the side in the corner so that you'll remember that we're focusing on secondary hemostasis. Here, we have another blood vessel and it's been damaged. But we've already gone through primary hemostasis. We have our platelet plug, but it's weak. And we need to make it stronger. The way we do this is with fibrin. Fibrin is able to link up and aggregate on top of the platelet plug and stabilize it. Sort of like tying a knot. Fibrin comes in and double knots it and triple knots it so that it's sturdy. But fibrin isn't floating around in our blood as fibrin. It floats around with this extra piece of protein. And when it has this extra piece of protein, it's called fibrinogen. So, we need to wait to get fibrin from fibrinogen. And we get fibrin through the coagulation cascade. In our blood, we have this family of proteins called coagulation factors. And I'm drawing them all as one color right now because they're family of proteins. They're similar, but there are different kinds. But I don't want to go into those details here. And what happens is a coagulation factor will get activated. And then it will activate another and activate another and it will set off this cascade. And ultimately what we end up with is we get fibrin from fibrinogen. And then that fibrin is able to link up on top of that platelet plug and make it stronger. There are two ways that we can activate the coagulation cascade. One way is through a protein called subendothelial collagen. This protein is not normally exposed to the blood. It's underneath the endothelial cells. But when a blood vessel has been injured and the endothelial cells damaged, then that protein is exposed to the blood. Once it's exposed to the blood, then it triggers and activates its set of coagulation factors. That set of coagulation factors is part of the intrinsic pathway of the coagulation cascade. The second way that we can activate the coagulation cascade is through another protein that is secreted by the endothelial cells whenever we have damage to the blood vessels from outside of our bodies. This protein is called tissue factor or it's also called thromboplastin. And when this protein gets released into the blood, it activates its own set of coagulation factors. And these set of coagulation factors are part of the extrinsic pathway of the coagulation cascade. So now that we have fibrin. Before we can make the fibrin mesh and stabilize that platelet plug, we need more two things. One of them is calcium. And the second is a phospholipid surface. And both of these are provided by the platelets. Calcium is released during primary hemostasis when platelets get activated. And the phospholipid surface is provided by the platelet itself. And this is important because it links making the fibrin mesh with the platelet plug. And this is what we call the stable clot.