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Life and times of RBCs and platelets

Explore the life cycle of red blood cells and platelets in the human body. Discover how these cells are produced in the bone marrow, their lifespan, and the role of the spleen in their breakdown. Learn about the hormones erythropoietin and thrombopoietin that regulate this process. Created by Patrick van Nieuwenhuizen.

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  • old spice man green style avatar for user Don Spence
    Possibly dumb question alert. The RBCs are formed in the bone marrow. The recovered iron from old and tired RBCs is in the spleen. The EPO hormone is generated in the kidneys. What is the process that brings all these factors together to make a new RBC?
    (66 votes)
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    • leaf green style avatar for user Andrew Beeler
      Interesting question. I looked it up and my understanding is that a protein called transferrin carries the iron from the spleen through the blood until it reaches the bone marrow (maybe other locations, too, I'm not sure). At that point it binds to receptors in the bone marrow which cause it to be endocytosed and the iron released. There's a bit more to it than that but I think that's the gist of it.
      (53 votes)
  • orange juice squid orange style avatar for user Yichao Alexandra Sun
    When O2 is low, the body interprets that red blood cell level is low. Does that mean individuals with lungs dysfunctions would expect to have elevated red blood cell level to compensate for the low O2?
    (21 votes)
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    • blobby green style avatar for user Billy Jabbar
      Great question. Let's consider this with an example. Heavy smokers (among other complications), are susceptible to emphysema, a disease which destroys alveoli of the lungs and greatly reduces the overall surface area of O2 diffusion through pulmonary capillaries. This causes a number of physiological responses.

      IMMEDIATE RESPONSE: The accumulation of metabolic products (CO2, H+, and wastes) are detected by special receptors of the heart (carotid and aortic nuclei). This stimuli is transmitted to the medulla oblongota of our brainstem where a special respiratory and cardiac center control both our respiratory and cardiac rate. To compensate for the low O2 (and increased CO2 and H+), we increase sympathetic input to increase our respiratory rate and also the rate of cardiac contraction to increase O2 uptake.

      LONG-TERM RESPONSE: As blood is filtered through our kidneys, specialized cells detect low O2 content and trigger the release of erythropoieten, a hormone that increases RBC production (and indirectly hemoglobin).

      We also have various metabolic and cellular changes as well, such as increasing 2,3 DPG which helps reduce the binding affinity of oxygen to hemoglobin so that it's readily expelled in tissues that desperately need it! Cells also increase mitochondria for increased energy demand.

      Of additional concern is the blood pH once these physiological changes take place. Increased respiration causes the release of excess CO2, which ultimately increases the pH of blood (remember the equilibrium relationship), so to help establish homeostasis, renal excretion of bicarbonate will increase to maintain a steady, healthy blood pH.

      What's also interesting is, because this is a chronic condition that can become progressively worse over time, the heart has to pump harder and harder to obtain more oxygen from lungs. So one notable characteristic is the right ventricle thickens). Pretty cool stuff.
      (34 votes)
  • piceratops tree style avatar for user faroukabbas.elsallak
    How come the spleen "know" which RBC needs to be removed?
    (8 votes)
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  • winston default style avatar for user David
    How many platelets can 1 megakaryocyte produce in its lifetime and how long does it live for?
    (15 votes)
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    • duskpin ultimate style avatar for user Daniel Olofsson Tassrapp
      "During its lifespan the average megakaryocyte (MK) gives rise to approximately 4,000 platelets which live an average of 9-12 days." Source: https://www.med-ed.virginia.edu/courses/path/innes/nh/platelets.cfm

      "Each megakaryocyte has been estimated to generate and release thousands of platelets (50–52). If platelet formation is restricted to a relatively limited number of proplatelet ends, platelets would have to form and release on a minute time scale. (The average megakaryocyte has approximately 5–10 original proplatelets. If 1,000 platelets are constructed, then each end would have to produce 100–200 platelets over a 4-hour time course, equivalent to 25–50 platelets per hour.) Analysis of time-lapsed video microscopy of proplatelet development from megakaryocytes grown in vitro, however, has revealed that ends are amplified in an elaborate process that repeatedly bends and bifurcates the proplatelet shaft to form new ends (27). End amplification initiates when a proplatelet shaft is bent into a sharp kink, which then folds back on itself, forming a loop in the microtubule bundle. The new loop eventually elongates, forming a new proplatelet shaft branching from the side of the original proplatelet. Loops lead the proplatelet tip and define the site where nascent platelets will assemble and where platelet-specific contents are trafficked (Figure ​(Figure3).3). In marked contrast to the microtubule-based motor that elongates proplatelets, actin-based force is used to bend the proplatelet in end amplification." Source: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1297261/

      As for it's lifespan I have not found anything.
      (7 votes)
  • male robot hal style avatar for user Wudaifu
    Does thrombopoeitin also come from the kidney like erythropoetin?
    (7 votes)
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  • primosaur sapling style avatar for user Flying Pig
    (This is a possibly stupid question). What is the use of platelets?
    (6 votes)
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    • female robot grace style avatar for user YouAreEnough
      This a good question!
      Platelets are tiny blood cells that help your body form clots to stop bleeding. If one of your blood vessels gets damaged, it sends out signals that are picked up by platelets. The platelets then rush to the site of damage and form a plug, or clot, to repair the damage. The process of spreading across the surface of a damaged blood vessel to stop bleeding is called adhesion. This is because when platelets get to the site of the injury, they grow sticky tentacles that help them adhere. They also send out chemical signals to attract more platelets to pile onto the clot in a process called aggregation. Hope you find this helpful!
      (8 votes)
  • female robot grace style avatar for user Anna
    I have read that megakaryocytes can have many nuclei(up to 60) before they produce platelets.

    Wouldn't a multinuclear cell have uncontrollable DNA replication, Protein synthesis, and Transcription?
    (3 votes)
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  • blobby green style avatar for user Sreeja Ghosh
    If Platelets don't undergo mitosis, what is the name of the process that they use to regenerate themselves? Also, can KhanAcademy make a quick video of how stem cells turn into Platelets...I'm learning this in Grade 10 Science, but am a bit confused. So I just want to see a visual representation of Stem cells differentiating into Platelets.
    (4 votes)
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    • winston default style avatar for user David
      Platelets are not directly made from stem cells, nor are they a cell at all. They are a cell fragment of a megakaryocyte (if I remember correctly) and do not have a nucleus, so they don't actually go through a process such as mitosis to replicate. They are just created through the megakaryocytes budding them off.

      Great question! Hope this helped! :)
      (6 votes)
  • piceratops tree style avatar for user Johan
    But if the one of the primary functions of the platelets is to make cloths wouldn't it be more efficient for the megakaryocyte to remain as one large unit? How is it more useful to have several smaller units than one large? Doesn't it require energy to split the platelets from the megakaryocyte? Even if only a small amount of energy is needed for the split it must be more than not splitting it at all, or have I misunderstood?
    (4 votes)
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  • female robot grace style avatar for user jeung9709
    How does body get rid of platelets, then?
    (5 votes)
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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 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.