This video, we're going to talk about the second step of hemostasis, which is platelet plug formation. As its name implies, platelet plug formation is when platelets aggregate or clump together to plug a hole in a damaged blood vessel wall to help reduce blood loss. This platelet plug formation process actually occurs in 3 steps that we have numbered down below in our text as steps 2a, 2b, and 2c. Each of these steps in the text corresponds with the steps that you can see down below in the image as well. Before we continue talking about these steps of platelet plug formation, it's important to reiterate from our previous lesson videos that this process of hemostasis occurs after injury and damage to blood vessels. When blood vessels are damaged, the walls are typically going to be damaged in a way where they expose components that are not typically exposed under normal circumstances, such as collagen embedded in the walls of the blood vessel, for example. When these components, such as collagen, are exposed in the damaged blood vessel walls, it will typically serve to mark the site of damage within the blood vessel for these hemostasis components. That being said, let's dive into these steps of hemostasis so that we can get a better understanding of this. The very first step of platelet plug formation, step 2a, is adhesion of the platelets to the damaged site in the blood vessel wall. The adhesion of the platelets to the damaged site involves a plasma protein called von Willebrand factor, which is commonly abbreviated as VWF. This von Willebrand factor or VWF plasma protein is a protein that is dissolved in the plasma of the blood and circulates in the blood, and what it does is it will bind to exposed collagen at the damaged site in the blood vessel wall. When the VWF binds to this exposed collagen, it will help to serve as a bridge to anchor the platelets to the damaged site. If we take a look at this image down below of step 2a, adhesion of the platelet to the damaged site, you'll notice a damaged blood vessel, and notice that you can see the damage to the wall right here in this region. If we zoom into this damaged site, as you see here in the circle, you'll notice that within the wall of the damaged blood vessel, we are labeling the exposed collagen right here in this region. The yellow diamond right here is actually representing this Von Willebrand factor or VWF protein. We can go ahead and label it as so. This Von Willebrand factor protein is a plasma protein circulating in the blood, and it will bind to the exposed collagen on the damaged site. When it binds to this exposed collagen, it will also simultaneously serve as a bridge to anchor nearby platelets to the damaged site. Notice that the platelet here is binding to the VWF, which is bound to the exposed collagen. This allows for the adhesion of the platelet to the damaged site. Now, one thing I also want to point out is that the platelets have cytoplasmic granules that are filled with chemicals, and these chemicals are going to be important for the formation of the platelet plug. After step 2a, adhesion of the platelet to the damaged site, we have the next step of platelet plug formation, step 2b, which is the activation and degranulation of the platelet. We already know from our previous lesson videos that initially, the platelets are going to circulate in the blood in their inactive forms, and these inactive platelets need to first become activated before they can form the platelet plug. Recall that the activation of these platelets will involve physical changes to the platelets, such as them extending spiky projections, which if you take a look at the image down below for step 2b, you'll notice that this platelet is activated because its physical characteristics are changing in such a way where it extends these spiky projections, and those extended spiky projections allow the platelet to better interact with nearby platelets. Recall from our previous lesson videos that activated platelets will express some negatively charged surface proteins that are not enough to repel these aggregating platelets, but it is just enough to activate some blood clotting factors, and we'll talk more about this idea as we move forward and talk about the 3rd step of hemostasis and blood coagulation or blood clotting. Also, the activation of the platelet will also cause the degranulation of the platelet, which is really just a fancy way of saying that it is going to release the chemical-filled granules that it contains, in order to continue this process of platelet plug formation. Some of the chemicals that can be found in these granules include ADP, serotonin, and thromboxane A2, which can serve as aggregating agents to help recruit platelets to the damaged site. If we take a look at the image down below, notice that the cytoplasmic granules are degranulating, and the chemical contents, these aggregating agents, are being released, and they can go on to help recruit other nearby platelets to the damaged site. As more and more platelets aggregate to the area and are recruited to the area, they will again bind to the damaged site, and they will become activated themselves. There's this positive feedback loop between the aggregation of the platelets and the activation of the platelets. This leads us right into the 3rd and final step of platelet plug formation, which is step 2c, aggregation. In this step, we're going to see more and more platelets being recruited to this damaged site in the blood vessel wall. You can actually see some of that happening here progressively in this image. Notice that initially upon adhesion, there are very few platelets bound, but over time, as we go from left to right in our image, you'll notice that more and more platelets are slowly becoming aggregated until we get to this final step over here, 2c aggregation, where we see lots and lots of platelets aggregating into the area to create that platelet plug. Now the platelet plug has been formed. It can be effective to help reduce blood loss, and we can indicate that right here, that it can help to reduce blood loss. However, something that's very important to note is that this initial platelet plug that is formed over here is usually going to be a relatively unstable platelet plug. Although it can be effective to help reduce blood loss, because it is unstable, it's very important that this unstable platelet plug is reinforced to stabilize it further so that it can more effectively help prevent blood loss. The reinforcement of this unstable platelet plug is what happens in the 3rd and final step of hemostasis, which is blood coagulation or blood clotting, which we'll talk about in our next lesson video. The very last thing that I'll leave you with is that the formation of the platelet plug is going to be very effective at reducing blood loss, specifically when the hole in the damaged blood vessel wall is quite small, and it's even more effective at repairing, or it's even more effective at plugging holes in small blood vessels. However, if the hole is very large in the damaged blood vessel wall, then usually medical intervention is going to be required. This platelet plug formation process will only work effectively to a certain extent depending on how big the hole is in the damaged vessel wall. It's more effective the smaller the hole is. The larger the hole is, the less effective it is, and the greater the likelihood that medical intervention is required. That being said, this concludes our brief lesson on platelet plug formation, and as we move forward in our course, we'll be able to apply these concepts and learn more. I'll see you all in our next video.
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17. The Blood
Platelets: Hemostasis
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