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- So here's a guy, and like any guy, if we look at a little bit of his blood, let's say we look at one microliter of his blood, one microliter, and there are, of course, a million microliters in one liter. So if we look at just one microliter, what will we find? We'll find about 5,000,000 red blood cells, 5,000,000 for every microliter. And actually, by percent of volume, the blood cells make up about, you know, maybe 40 percent of his blood. So there's a lot of them, there's 5,000,000 in one microliter, red blood cells, I'm just going to write RBCs. And in that same microliter of blood there will also be platelets, now do you think there will be more or less platelets than red blood cells? So platelets are a lot smaller, so you might think that there could be more, but actually there is less. It can range quite a bit, but a number you might see is 200,000 platelets in one microliter. And that's platelets, those are the little guys that help make clots when you have problems with your blood vessels. So let's say this guy is 20 years old. Do you think that his red blood cells and his platelets are the same ones that he had five years ago? Actually, they are not because both of these things get kind of destroyed, they break down at a certain rate. And so this guy needs to be constantly making new red blood cells and platelets. And actually, it turns out that red blood cells last about 120 days on average, that's about four months. And platelets, meanwhile, only last a couple of days, so you know, a lot less than the red blood cells. So you need to be constantly making more of these things. And do you know where you do that? It turns out you do it in your bone marrow. So here I'm drawing a bone, and in the marrow inside that bone, that's where you produce these red blood cells and these platelets. Now, the way that red blood cells are made in the bone marrow, is that they start from some precursor cell. So there's some kind of cell that's not a red blood cell. And one way it's different is that it has a nucleus. So in case you didn't know, red blood cells actually don't have nuclei when they're in circulation. So some precursor cell that has a nucleus. And I'm going to kind of simplify this a little bit. But basically, this precursor with a nucleus can divide. So it undergoes the usual process of mitosis to divide. And so now we have two, and actually it will do that many, many, many, many times so you'll have a lot of them. But then some of them can actually become what we know as red blood cells, and part of that process involves, as I said, losing the nucleus. So this whole process happens in years, so I'll draw some red dots to symbolize that, and then these blood cells will go into blood vessels, and I'll just draw some blood vessels here in this bone. Bones do have blood vessels in them. So they'll go into those blood vessels and go back out to circulation and eventually up into the heart from where they'll get pumped out to the body. So this is red blood cells, and actually there's a fancy name for red blood cells which is erythrocyte. So let me write that here, erythrocyte. Might be worth knowing, might not be worth knowing. But anyway, that's another name for a red blood cell. And actually this whole process of creating red blood cells can be called erythropoiesis, erythropoiesis, and that just means creation of red blood cells. Now, platelets are made differently. Platelets are actually fragments of cells. And they come from a big cell called a mega, again more long words, megakaryocyte. The important thing about a megakaryocyte is that it's a big cell, it has lots of cytoplasm, that's the stuff that's not the nucleus. And the way that platelets are made is that they actually just kind of bud off of the megakaryocyte. So they're bits of cytoplasm that bud off and so they are obviously surrounded by membrane. Every megakaryocyte can do this many, many, many, many times and so they will release many little platelets in this manner, and then just like the red blood cells, these platelets are going to go into circulation and make their way throughout the body. So we said that red blood cells only last 120 days. And platelets only last a couple of days. So a question you might have is what happens to them after that, do they just disappear? Of course they don't just disappear. They are in your bloodstream and your body has to take them out, and so the organ that's most responsible for doing this is the spleen, and I'm drawing it here. As you can tell, it's in the left side of this guy's body. And it's sort of on the upper part of his abdomen. And the spleen is just one of the places that blood goes from the heart, so the heart pumps and it sends blood to the arms, to your muscles, to your brain, to your legs, and it also sends some to the spleen. And when some of that blood passes through the spleen, and we'll draw a cartoon of that here. When it passes through the spleen, the spleen is going to recognize the old red blood cells, the ones that are damaged, that are worn out. It's going to take those out of circulation and basically chew them up and break them down and get rid of them. And the cells that do this are called monocytes. So let's just draw a quick cartoon of that. So here we have a red blood cell which is old, so I'm going to draw it a little bit deformed. It's a little bit old and decrepit, and it's lived a good, long happy life, and it's time for it to go. So in the spleen it's going to get engulfed by a cell called a monocyte, I'll write that name here, a monocyte, we're putting a lot of new names out here. A monocyte is basically like a macrophage, if you've heard of that, it's quite similar, so this monocyte in the spleen is going to phagocytose or basically chew up this old red blood cell and break it down into its parts so that they can be reused. And I will mention that one of the things that needs to be reused and I'll actually draw it here because I think this is important, is iron. So red blood cells have iron in them because that's part of hemoglobin, and your body doesn't want to lose that iron, so the monocyte is going to recycle that iron for further use along with many of the amino acids that make up the proteins in the red blood cell. So let me just write recycle because it's important to recycle and not to waste. Now, I will mention that most of this happens in the spleen. Most of these red blood cells and platelets get broken down in the spleen but it also happens a little bit in the liver, which is an organ next to the spleen here and the process is quite similar, so this is how you get rid of old red blood cells and platelets. Now, how does your body know how many new red blood cells to make? Well, to answer that question let's go back to what is the primary purpose of red blood cells? Their primary purpose is to bring oxygen out to the different parts of the body, so if you don't have enough oxygen, so we'll say low O two, if you don't have enough oxygen, that means that you probably don't have enough red blood cells, I mean, it could also mean that your lungs aren't working or something like that. But your body most of the time will interpret it to say, okay, we don't have enough oxygen, that means we need more red blood cells, so when there's low oxygen, your body is going to produce something that's going to tell your bone marrow to produce more red blood cells. And that's something I'm going to write here. And it has a similar name to these other words up here. It's called erythropoietin, it's not erythropoiesis. It's erythropoietin, which stimulates this whole process and causes you to make a lot more erythrocytes or red blood cells. And like I said, what causes the release of erythropoietin is low oxygen, and I should mention that that happens in the kidney, so let me just write kidney here. So the kidney is going to release erythropoietin. And you might have heard of erythropoietin. It has another name which is epo, that's abbreviated name. And you might have heard of it because some athletes who feel that they need more red blood cells in order to get more oxygen to their muscles and perform better, some of those athletes will actually take epo, they'll take epo so that their bone marrow makes more red blood cells. And do you know what epo might be called? Do you know what kind of substance it might be in our vocabulary of biology? Well, epo is a hormone because it's a substance that communicates between I guess the kidney, basically really the whole body and the bone marrow. And likewise, the platelet production system also has a hormone that controls it, and that hormone is called something else, it's called thrombopoietin. And that's a much less important name than erythropoietin. You're much less likely to ever hear thrombopoietin, but all I want to do is make it clear that production of platelets also is controlled by a hormone, just like production of red blood cells.