In this video, we're going to begin our overview of hemostasis. Now although the terms are related, the term hemostasis is not to be confused with the term homeostasis, which, recall, applies to any process that maintains stable conditions. Whereas the term hemostasis refers specifically to the fast, local, and controlled process to prevent and control bleeding after an injury. And so the end result of hemostasis is the formation of a blood clot, which can effectively help to prevent blood loss after an injury. Now, it's very important to note that under normal circumstances, when there is no injury to the blood vessel, the blood does not clot. And the reason for this is because the clotting factors that are involved in forming the blood clot are going to circulate in the blood in their inactive forms. And these inactive clotting factors must first become activated before the blood clotting process can begin. And the activation of these clotting factors happens after an injury to the blood vessel. And so upon blood vessel injury, hemostasis is actually going to consist of three steps that we will overview below in this video. But moving forward in separate videos, we're going to dive into more detail for each of these steps. And so again, this process of hemostasis occurs after injury to the blood vessels. And so notice that we're highlighting an injury here to this person's hand when they cut themselves while cutting some carrots. And so notice that the very first step of hemostasis is vascular spasm, and vascular spasm refers to when the blood vessels will constrict or contract immediately after blood vessel injury. And so you can see that the contraction of this blood vessel is being highlighted here by these arrows that you see here. And the contraction is going to decrease the diameter of the blood vessel, and that is ultimately going to decrease the blood flow. And so notice that we're indicating decreased blood flow with a down arrow, and the decreased blood flow is going to help to prevent blood loss and to help buy some time for the rest of the hemostasis steps to occur. And so the second step of hemostasis is platelet plug formation, and as its name implies, this is when the platelets are going to become activated and form a platelet plug. And so notice here in this image that the platelets are aggregating and plugging the hole in the damaged blood vessel wall. And so, the platelets are going to aggregate to plug that hole, and this is going to be an unstable fix. So it is going to help prevent blood loss, but this platelet plug needs to be reinforced to make it more effective at preventing blood loss. And so that's exactly what this third and final step is all about. That is coagulation or blood clotting. And so in this third and final step, coagulation, or blood clotting, that platelet plug that was formed in step number two is going to be reinforced with a protein called fibrin. And that fibrin protein is going to create a stable and more effective blood clot that helps to prevent blood loss. And once step three here is complete, then the hemostasis process is complete. However, it is important to note that after hemostasis is complete, there are two other steps that help complete the healing process, and those two steps are going to be clot retraction and fibrinolysis. And so we'll talk about these two steps after we talk about the three steps of hemostasis in more detail. And so this here concludes our brief overview of hemostasis, and we'll be able to apply these concepts and learn more as we move forward. So I'll see you all in our next video.
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17. The Blood
Platelets: Hemostasis
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