If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:35

Video transcript

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 five million red blood cells five million for every micro liter and actually by percent volume the blood cells make up about you know maybe forty percent of his blood there's a lot of them there's five million in one microliter red blood cells are just going to write RBC's and in that same microliter of blood there 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 think you might think that there could be more but actually there is less usually 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 helped 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're 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 turns out that red blood cells last about 120 days on average that's about four months and platelets meanwhile only lasts 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 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 we'll 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 here so I'll draw some some red dots to simplify that to symbolize that rather 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 we'll go into those blood vessels and go back out to circulation and eventually up into the heart where 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 a wreath ro site so let me write that here a wreath ro site might be worth knowing might not be worth knowing but anyway that's another name for red blood cell and actually this whole process of creating red blood cells can be called a wreath ropo 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 mega cario sites 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 the platelets are made is that they actually just kind of bud off of the megakaryocyte so they're bits of cytoplasm that butt off and so they're obviously surrounded by membrane and every megakaryocytes 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 they just disappear and of course they don't just disappear they're 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 drawing it here as you can tell it's on 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 and 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 base like a macrophage if you've heard of that it's quite similar so this monocyte in the spleen is going to phagocyte toes 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 lo oh - 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 the time will interpret it to say okay we don't have Anna Fox engine that means we need more red blood cells so when there's low oxygen you're going 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 a wreath row poet in that's not a wreath rope Oasis it's a wreath rope wheat in 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 a wreath ropo eaten and you might have heard of a wreath ropo eaten it has another name which is EEP oh that's a 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 Ipoh he'll take a po so that their bone marrow makes more red blood cells and you know what EPO might be called do you know what kind of substance it might be in our vocabulary of biology well Ipoh 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 thrombo poet in' and that's a much less important name than then we throw poitain 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