 Platelets are one player in the process of hemostasis, which is stopping the bleeding. So I have a visual here. This is a visual from D. Silverthorn's human physiology text. And what I want you to notice about this visual, it's basically the family tree of all blood cells. And we have two locations that we're looking at. First of all, the blood cells, the grandmamas of all your blood cells, they started out in the bone marrow. And in the very beginning, you have a hematopoietic stem cell. And that stem cell is going to give rise to all your blood cells. All the formed elements that you have in blood come from this hematopoietic stem cell. This stem cell will give rise to another stem cell. Or it can give rise to somebody who is going to become a lymphocyte. A lymphocyte is a flavor of white blood cell. We're going to talk about lymphocytes in the next couple of lectures when we talk about the immune system. If you look at all of our possible white blood cells, our leukocytes, you have lots of different flavors. And when you look at them, they all came from, ultimately from this hematopoietic stem cell in the bone marrow. Even red blood cells come from this original stem cell in the bone marrow. And you can see the little process that they go through in developing into a functional red blood cell. And then you got to look at our good friend, the platelets. And that's what we're talking about right now, is they play a role in clotting. They're going to try and catch all these other guys and stop the blood flow if a blood vessel breaks. So platelets are cool because they actually, take a look at who their most close relative is. It's this giant cell called a megakaryocyte that's found in the bone marrow. And it actually tries to squeeze out of the bone marrow into the bloodstream. And when it does that, the blood is rushing by and breaks off little pieces of megakaryocyte. And literally the little pieces that broke off, those are platelets. Platelets are not cells. However, they are broken off pieces of cell so they even have endoplasmic reticulum in them. They don't have a nucleus. They're way too small for that. They can have ribosomes. They have all sorts of enzymes and other chemical producing structures so that they can do their job and they don't live very long. They last between five and eight days, which really, I mean, if you think about it, that isn't that long. If you donate blood, there's a whole process that you can do that will just like filter out your platelets since they don't last very long and they're critical to the process of clotting, blood clotting. If you don't have platelets, you are going to be sad story like a bloody mess. And in fact, like, you don't even think about it. I mean, I'm not bleeding right now. I mean, there's no like broken blood vessels in me. However, all my little tiny blood vessels, there's tiny little breaks. Like a jillion times every day and my platelets just clog the holes. And I never even know. Like, dude, I was about to bleed out. If I didn't have my platelets, yes, I would totally bleed out like right now. And thank you, megakaryocyte, for breaking into a million pieces and producing platelets. Okay, so platelets are important in the clotting process, in hemostasis. I'm going to start, if you think about it, if you're getting bloody, you broke a blood vessel. And that seems like super obvious, but it's kind of like mind-blowing to me that your blood vessels are all perfectly like contained and they make sure your blood doesn't get out. And you kind of take that for granted. Well, of course, your blood shouldn't come out. But the only way we're going to activate our platelets is if we actually have damage to a blood vessel. And then we're going to need to activate our platelets. If you activated your platelets without damage, that's going to form a blood clot that could get clogged somewhere and cause either a heart attack or a stroke, depending on what organ it gets clogged in. So we're going to imagine now we're going to have some damage to a blood vessel and we're going to look at all the things. Some of the things are mediated by platelets. Some of the responses to the damage are mediated by platelets and some of the responses to the damage are mediated by other cells, even the damaged cells. So we're going to talk about all of the processes that are involved in stopping the bleeding.