So now that we've covered the three steps of hemostasis in our previous lesson videos, in this video, we're going to talk about clot retraction and fibrinolysis. And so recall from our previous lesson videos that after hemostasis is complete and the blood clot has been formed, there are still two more steps that help complete the healing process. And again, these two steps are clot retraction and fibrinolysis. Now, clot retraction is actually a platelet-induced process that helps to further stabilize the blood clot, and it helps to promote the overall healing process in order to allow the damaged blood vessels and the damaged tissues to regenerate and resume back to their normal states. Now, in this process of clot retraction, the coagulated platelets that are in the blood clot will actually contract. In fact, these coagulated platelets contain actin and myosin protein filaments, and so their contraction is similar to the contraction of smooth muscles, which we covered in previous lesson videos. And so in this process of clot retraction, the coagulated platelets, when they actually contract, the entire blood clot will retract, and that will actually squeeze out fluids from the blood clot, and that will stabilize the blood clot. And also, as the blood clot retracts, it's also going to pull the ruptured edges of the blood vessel closer together, which is going to promote the overall healing process. Now, in addition to this, during the process of clot retraction, the platelets are also going to secrete a hormone called platelet-derived growth factor, which can be abbreviated as PDGF. And so this platelet-derived growth factor or PDGF is actually going to help to promote the overall healing process by triggering mitosis of cells to allow for the regeneration of the damaged tissues. And so if we take a look at our image down below, notice on the left-hand side, we're focusing in on clot retraction. And so notice that we have our damaged blood vessel down below, and you can see that hemostasis has successfully produced a blood clot that is reinforced with fibrin. And again, after hemostasis is complete and the blood clot has formed, clot retraction is going to occur. And clot retraction entails the platelets actually contracting, which causes the overall blood clot to retract in this fashion. And that again, that's going to squeeze out fluids from the blood clot to stabilize the blood clot, and again, it's going to pull the edges, the ruptured edges of the damaged blood vessels closer together, which is going to promote the healing process. And what you'll notice is that zooming into these platelets, you can see that they contain these actin and myosin protein filaments, and so again, their contraction is similar to the contraction of smooth muscles. Now, fibrinolysis is a process that occurs after clot retraction is complete and after the blood vessel healing process is complete as well. And so if we take a closer look at the roots within the word fibrinolysis, you'll notice that it has the word fibrin embedded in it, and it also has the root lysis embedded in it. And that root lysis means to break down or break apart. And so in this process of fibrinolysis, the fibrin protein that is serving as the molecular glue to stabilize the blood clot is actually going to be broken down. And through the breakdown of fibrin, it's also going to dissolve and remove the entire blood clot. And so in this process of fibrinolysis, again, it is going to break down the fibrin protein in order to remove the now unneeded blood clot. And the reason that the blood clot is now unneeded is because, again, the blood vessel healing process has already been completed, and so removing that blood clot is going to help to resume the normal blood flow through that blood vessel. Now the fibrinolysis process is going to involve an inactive plasma protein called plasminogen, which circulates through the blood in its inactive form. And this plasminogen inactive plasma protein is actually going to be incorporated into the blood clot as the blood clot is forming. And so during this process of fibrinolysis, the plasminogen inactive plasma protein is going to be converted into plasmin, which is an active enzyme, specifically a fibrin-digesting enzyme, and so its role is to break down fibrin, Again, that molecular glue that stabilizes the blood clot, and so breaking down the fibrin is going to help to remove the unneeded blood clot. So let's take a look at our image down below over here on the right-hand side for fibrinolysis, and what you'll notice is that the blood vessel that was initially damaged has now completed the healing process. And so notice that the walls of the blood vessel are no longer ruptured, and, the healing process is complete. And so what this means is that the blood clot that was formed is now unneeded. It's no longer needed, and so it actually needs to be broken down and removed so that the normal blood flow can resume through this blood vessel. And so this is going to involve taking this plasminogen protein, this inactive protein, and converting it into its active form that we call plasmin. And plasmin is going to be an active fibrin-digesting enzyme. And so notice in this image, you can actually see that plasminogen is a plasma protein that is dissolved in the plasma and circulates in the blood in its inactive form. And, again, it will be incorporated into the blood clot as the blood clot is forming. And during fibrinolysis, the plasminogen that's incorporated into the blood clot is gonna be converted into plasmin. So you can now see that the plasmin is active inside of the blood clot, and it is digesting the fibrin protein in order to, again, break down the blood clot, dissolve it, and remove it. And so after that, the entire healing process has been completed. And so this here concludes our lesson on clot retraction and fibrinolysis. Moving forward, we'll be able to apply these concepts and learn more. So I'll see you all in our next video.
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17. The Blood
Platelets: Hemostasis
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