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MCAT
Course: MCAT > Unit 7
Lesson 8: Hematologic systemLife and times of RBCs and platelets
Created by Patrick van Nieuwenhuizen.
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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?(65 votes)
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.(52 votes)
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)
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)
How come the spleen "know" which RBC needs to be removed?(7 votes)
It doesn't. The structure of the spleen bends and stresses the RBCs as they pass through it. The old and damaged ones break under the stress, and then macrophages in the spleen clean up after the broken cells.(26 votes)
How many platelets can 1 megakaryocyte produce in its lifetime and how long does it live for?(15 votes)
"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)
Does thrombopoeitin also come from the kidney like erythropoetin?(7 votes)
Some of it does but it is also made by the liver, bone marrow, spleen, and lung.(6 votes)
(This is a possibly stupid question). What is the use of platelets?(6 votes)
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)
- 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)
No actually there are many cells that are multi-nucleated, including skeletal muscle cells. The processes behind controlling replication, transcription, and translation are unaffected by the amount of nuclei in a given cell(7 votes)
- 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)
- 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)
- 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)
- Megakaryocytes are extremely large cells, average size is 50-100 μm. to have cells that large circulating in the blood can be pretty dangerous. So while it would be more efficient, your body doesn't want to take the risk(6 votes)
- How does body get rid of platelets, then?(5 votes)
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.