tag:theconversation.com,2011:/us/topics/platelets-40452/articlesPlatelets – The Conversation2022-01-31T13:00:06Ztag:theconversation.com,2011:article/1745812022-01-31T13:00:06Z2022-01-31T13:00:06ZWhy do we bleed? A hematologist explains how the body prevents blood loss after injury<figure><img src="https://images.theconversation.com/files/441609/original/file-20220119-25-1kfjasu.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2039%2C1471&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The body starts plugging up wounds as quickly as it can to prevent blood loss and infection.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/finger-with-a-bead-of-blood-royalty-free-image/545247995"> Jonathan Knowles/Stone via Getty Images</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
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<p><strong>Why do we bleed? – Michael, age 9, Seattle, Washington</strong></p>
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<p>“Ouch!” I cried out, looking down at where my family’s puppy, Hercules, had just bitten me. Two tiny dots of red started to bloom where he had sunk his canine teeth into my finger.</p>
<p>“Bad dog,” I said to him. He looked up at me sheepishly. “Well, I guess I’m a bad human for putting my finger near your mouth,” I admitted. He wagged his tail and licked my arm.</p>
<p>I put pressure on my finger until the bleeding stopped. Three weeks later the area had healed, and we had taught Hercules to heel, too!</p>
<p><a href="https://scholar.google.com/citations?user=n2R5Ow4AAAAJ&hl=en">I am a doctor</a> who specializes in <a href="https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/hematology">hematology</a> – the study of blood. Here, I’ll share with you how blood travels through the body, how we bleed and, more importantly, how we stop bleeding.</p>
<h2>What’s inside blood?</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/441790/original/file-20220120-9299-1mqm0vp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Diagram showing normal blood composition" src="https://images.theconversation.com/files/441790/original/file-20220120-9299-1mqm0vp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/441790/original/file-20220120-9299-1mqm0vp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=731&fit=crop&dpr=1 600w, https://images.theconversation.com/files/441790/original/file-20220120-9299-1mqm0vp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=731&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/441790/original/file-20220120-9299-1mqm0vp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=731&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/441790/original/file-20220120-9299-1mqm0vp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=919&fit=crop&dpr=1 754w, https://images.theconversation.com/files/441790/original/file-20220120-9299-1mqm0vp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=919&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/441790/original/file-20220120-9299-1mqm0vp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=919&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Blood is a mix of red and white blood cells, platelets and plasma.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:1901_Composition_of_Blood.jpg">OpenStax College/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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<p>The blood in our bodies is a <a href="https://training.seer.cancer.gov/leukemia/anatomy/composition.html">complex fluid</a>. It contains red blood cells, which bring oxygen to our body’s tissues; white blood cells, our body’s infection-fighting army; and platelets, which clump together to help stop bleeding. A special protein called <a href="https://kidshealth.org/en/kids/blood.html">hemoglobin</a> in red blood cells is what gives our blood its beautiful red color.</p>
<p>The yellowy, liquid portion of blood is called <a href="https://www.redcrossblood.org/donate-blood/dlp/plasma-information.html">plasma</a>. Most of it – <a href="https://www.britannica.com/science/plasma-biology">over 90% of it</a> – is water, but the remaining portion contains molecules that help our blood clot and stop bleeding.</p>
<h2>Where does blood live and where does it travel?</h2>
<p>The <a href="https://emedicine.medscape.com/article/1968326-overview">bone marrow</a>, located in the center of our bones, is the factory that makes the cells circulating in our bloodstream.</p>
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<figcaption><span class="caption">Bone marrow transplants are given to patients with blood cancers like leukemia and lymphoma.</span></figcaption>
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<p>Once blood cells are made, they move into <a href="https://www.ncbi.nlm.nih.gov/books/NBK279250/">blood vessels</a> – rubbery tubes that form roads traveling through our entire body. Veins are the vessels that carry blood to the heart, while arteries carry blood to the rest of the body. The biggest vessels can be the width of a garden hose, while the smallest can be tinier than a human hair. Blood <a href="https://www.pbs.org/wgbh/nova/heart/heartfacts.html">travels 12,000 miles</a> through the vessels of an adult every single day!</p>
<p>The heart is the powerful engine that pushes blood through those tubes to deliver oxygen and nutrients to our organs and skin. First, the right side of the heart pumps the blood into the lungs, which contains oxygen from the air we breathe. Here, the <a href="https://doi.org/10.1002/cphy.c080113">hemoglobin</a> in red blood cells grabs oxygen and take it back to the heart. </p>
<p>The left side of the heart then pumps that oxygen-rich blood to the rest of the body. After red blood cells drop off their oxygen cargo, they are pumped back to the right side of the heart to get more oxygen and repeat their tour of the body again. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/441612/original/file-20220119-15-h5wzp6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Diagram of human circulatory system" src="https://images.theconversation.com/files/441612/original/file-20220119-15-h5wzp6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/441612/original/file-20220119-15-h5wzp6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/441612/original/file-20220119-15-h5wzp6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/441612/original/file-20220119-15-h5wzp6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/441612/original/file-20220119-15-h5wzp6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/441612/original/file-20220119-15-h5wzp6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/441612/original/file-20220119-15-h5wzp6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Blood delivers oxygen to the body by flowing through the circulatory system using a system of vessels.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/flat-design-circulatory-system-infographic-royalty-free-illustration/1286314341">Nadezhda Ivanova/iStock via Getty Images Plus</a></span>
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<h2>Why do we bleed?</h2>
<p>The skin forms a protective layer over our bodies. When that layer is damaged, the blood vessels immediately underneath it are torn, allowing blood to leak out. </p>
<p>A small cut tears just the small vessels near the skin, and only a tiny amount of blood drips out. A deep cut might tear larger vessels and cause more bleeding. When we bang an arm or a leg, tearing the smaller vessels without damaging our skin, blood collects under the unbroken skin to form a <a href="https://medlineplus.gov/bruises.html">bruise</a>.</p>
<p>Because the heart is still pumping blood throughout the body, more blood will continue to leak out until the damage is patched up. To stop the leaking, a torn blood vessel releases chemicals that <a href="https://doi.org/10.1161/01.ATV.0000130465.23430.74">activate proteins</a> that clump together and form a plug to <a href="https://medlineplus.gov/ency/imagepages/19462.htm">fill the hole</a>. This seals up the blood vessel and prevents more blood from getting out. You’ve seen the results of this on your skin before: the scab that forms on top of a scrape or cut.</p>
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<a href="https://images.theconversation.com/files/441613/original/file-20220119-23-1hvgipb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Diagram showing process of platelet and fibrin clumping to close a wound to a blood vessel." src="https://images.theconversation.com/files/441613/original/file-20220119-23-1hvgipb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/441613/original/file-20220119-23-1hvgipb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=652&fit=crop&dpr=1 600w, https://images.theconversation.com/files/441613/original/file-20220119-23-1hvgipb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=652&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/441613/original/file-20220119-23-1hvgipb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=652&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/441613/original/file-20220119-23-1hvgipb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=819&fit=crop&dpr=1 754w, https://images.theconversation.com/files/441613/original/file-20220119-23-1hvgipb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=819&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/441613/original/file-20220119-23-1hvgipb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=819&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Platelets and proteins called fibrin plug the hole in a blood vessel to stop bleeding.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/hemostasis-basic-steps-of-wound-healing-royalty-free-illustration/1179785857">ttsz/iStock via Getty Images Plus</a></span>
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<p>Applying pressure to a wound helps stop bleeding because it pushes the walls of the vessel together to slow the flow of blood. This helps form a clot to plug the tear, starting the healing process.</p>
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<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/174581/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Mikkael A. Sekeres does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Blood plays a vital role in keeping us alive, from delivering oxygen to the body’s organs to fighting off infections.Mikkael A. Sekeres, Professor of Medicine and Chief, Division of Hematology, Sylvester Cancer Center, University of MiamiLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1585222021-04-08T07:08:00Z2021-04-08T07:08:00ZWhat is thrombocytopenia, the rare blood condition possibly linked to the AstraZeneca vaccine?<figure><img src="https://images.theconversation.com/files/393933/original/file-20210408-22-ibd3zq.jpg?ixlib=rb-1.1.0&rect=30%2C0%2C3340%2C2225&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>The federal government has asked Australia’s medical and vaccine regulators to <a href="https://www.abc.net.au/news/2021-04-08/australia-government-urgently-look-astrazeneca-covid-vaccine/100054896">urgently consider</a> the European Medicines Agency’s finding of a possible link between the Oxford/AstraZeneca COVID vaccine and rare blood clots.</p>
<p>This follows reports over recent weeks of blood clots in a small number of people around the world who had received the AstraZeneca vaccine, including <a href="https://www.smh.com.au/national/astrazeneca-vaccine-rollout-to-continue-after-clotting-case-in-melbourne-20210401-p57ftc.html">one man</a> who was hospitalised in Melbourne.</p>
<p>Scientists have termed the condition “vaccine induced prothrombotic immune thrombocytopenia” (<a href="https://www.afr.com/policy/health-and-education/scientists-name-clotting-syndrome-associated-with-astrazeneca-vaccine-20210406-p57guv">VIPIT</a>). But what does this actually mean, how significant is the risk, and what are the implications for Australia’s vaccine rollout — which is currently relying predominantly on the AstraZeneca jab?</p>
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<h2>A paucity of platelets</h2>
<p>As indicated by its name, VIPIT is a form of something called thrombocytopenia.</p>
<p>Thrombocytopenia is a condition whereby the numbers of thrombocytes (very small blood particles, or platelets) <a href="https://www.healthline.com/health/thrombocytopenia">are markedly reduced</a>. Platelets form clots to stop bleeding, so when you don’t have enough platelets in your blood, your body can’t form clots. This can lead to excessive bleeding.</p>
<p>The condition has a <a href="https://pubmed.ncbi.nlm.nih.gov/31275945/">genetic component</a>, but can also arise from more than <a href="https://pubmed.ncbi.nlm.nih.gov/20008194/">300 common medicines</a>, including <a href="https://pubmed.ncbi.nlm.nih.gov/16872245/">penicillin</a> and <a href="https://pubmed.ncbi.nlm.nih.gov/15588119/">certain pain killers</a>. <a href="https://pubmed.ncbi.nlm.nih.gov/24092609/">Quinine</a>, which is added to tonic water for flavour, can also very rarely cause thrombocytopenia.</p>
<p>The <a href="https://www.thanz.org.au/documents/item/577">symptoms of VIPIT can include</a> severe headaches, abdominal pain, seizures and visual changes. These are similar to the symptoms of thrombocytopenia unrelated to the vaccine.</p>
<p>In <a href="https://www.ema.europa.eu/en/news/covid-19-vaccine-astrazeneca-benefits-still-outweigh-risks-despite-possible-link-rare-blood-clots">rare cases</a> of thrombocytopenia, clots can develop in the vessels draining blood from the brain. The European Medicines Agency said it had received reports of <a href="https://www.abc.net.au/news/2021-04-08/astrazeneca-vaccine-linked-to-rare-blood-clots-ema/100054774">169 cases</a> of brain blood clots in people who had been vaccinated with the AstraZeneca shot. </p>
<p>In severe cases, thrombocytopenia can be fatal. There have been deaths from blood clots reportedly associated with the AstraZeneca vaccine, <a href="https://www.bbc.com/news/world-europe-56665150">including 19</a> in the United Kingdom. </p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/what-can-go-wrong-in-the-blood-a-brief-overview-of-bleeding-clotting-and-cancer-76400">What can go wrong in the blood? A brief overview of bleeding, clotting and cancer</a>
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<p>VIPIT appears to present <a href="https://www.thanz.org.au/documents/item/577">4-20 days</a> after vaccination, and so far, the issue has been largely associated with <a href="https://www.sciencemag.org/news/2021/03/rare-clotting-disorder-may-cloud-worlds-hopes-astrazenecas-covid-19-vaccine">women under the age of 65</a>.</p>
<p>So how could this vaccine potentially cause thrombocytopenia? The “prothrombotic immune” part of the name denotes it’s caused by an over-activation of the immune system, which gives us a clue. </p>
<h2>Platelets and COVID-19</h2>
<p>The AstraZeneca vaccine prompts cells to make a specific part of SARS-CoV-2 (the virus that causes COVID-19), called the spike protein, which the virus uses to attach to cells when infecting us.</p>
<p>The vaccine stimulates our immune system to generate antibodies against the spike protein, which then primes the body to mount an immune response against SARS-CoV-2, if it encounters the virus in the future. </p>
<p>But <a href="https://www.reuters.com/article/us-health-coronavirus-science-idUSKBN2BL2R3">in some people</a>, the AstraZeneca vaccine seems to produce antibodies that react with platelets, making them stick together, leading the blood to clot. This in turn reduces circulating platelet numbers, and hence the thrombocytopenia.</p>
<p>These antibodies are similar to those found in some people on a blood thinning drug called heparin. The immune response to heparin generates antibodies that bind to platelets. This can lead to blood clots in some people, called <a href="https://www.bmj.com/content/350/bmj.g7566#:%7E:text=Heparin%20induced%20thrombocytopenia%20(HIT)%20is,and%20produce%20a%20hypercoagulable%20state.">heparin induced thrombocytopenia</a>. As many as one in 20 patients <a href="https://pubmed.ncbi.nlm.nih.gov/17400685/">receiving heparin</a> develop thrombocytopenia. </p>
<p>Keeping in mind we’re yet to establish cause and effect, it’s a possibility that the biological mechanism by which we believe heparin leads to thrombocytopenia could be the same biological mechanism by which the AstraZeneca vaccine might.</p>
<h2>How common is it?</h2>
<p>Naturally occurring thrombocytopenia affects about <a href="https://rarediseases.org/rare-diseases/immune-thrombocytopenia/">one in 30,000 adults</a> a year in the United States. </p>
<p>As for the suspected vaccine-induced kind, according to data collated by the Thrombosis and Haemostasis Society of Australia and New Zealand, VIPIT is as rare as <a href="https://www.thanz.org.au/documents/item/577">one in 500,000 people</a>. But the society notes the data are incomplete.</p>
<p>Different countries have reported different rates. Norway, for example, has so far reported <a href="https://www.sciencemag.org/news/2021/03/rare-clotting-disorder-may-cloud-worlds-hopes-astrazenecas-covid-19-vaccine">one in 25,000</a> vaccinated adults under the age of 65 have experienced low platelet counts, bleeding, and widespread thromboses (blood clots).</p>
<p>Of course, the possibility that some of these cases of thrombocytopenia may have occurred regardless of the vaccine makes understanding vaccine-induced cases more complicated. But taken together, thrombocytopenia appears to be more common in the general population than among those who have been vaccinated. </p>
<p>As we continue to vaccinate the world, it’s likely small subsets of people will continue to experience this complication. Whether we can establish a causal link between the AstraZeneca vaccine and thrombocytopenia is subject to continued investigation.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/politics-with-michelle-grattan-stephen-duckett-on-whats-gone-wrong-with-the-rollout-158526">Politics with Michelle Grattan: Stephen Duckett on what's gone wrong with the rollout</a>
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<h2>Be aware, but not alarmed</h2>
<p>Amid this ongoing investigation, some countries, <a href="https://www.aljazeera.com/news/2021/3/26/norway-extends-astrazeneca-vaccine-suspension-for-3-weeks">such as Norway</a>, have paused their rollouts of the AstraZeneca vaccine. Others have restricted use of the vaccine in certain groups, like <a href="https://www.aljazeera.com/news/2021/3/29/canada-to-pause-use-of-astrazeneca-vaccine-for-those-under-55">Canada</a>, which is using it only for adults older than 55, who may have higher risks from COVID and lower risk of blood clots. Meanwhile, the UK has pledged to make <a href="https://www.theguardian.com/society/2021/apr/07/under-30s-in-uk-should-be-offered-alternative-covid-vaccine-to-astrazeneca-jab-says-regulator">other vaccine options available</a> for younger people. </p>
<p>We will wait to see how the Australian experts respond. But for the general adult population, we agree with the <a href="https://www.abc.net.au/news/2021-04-08/astrazeneca-vaccine-linked-to-rare-blood-clots-ema/100054774">current guidance</a> from bodies including the European Medicines Agency and the World Health Organization that the benefits of the AstraZeneca vaccine outweigh the risks. </p>
<p>That said, it’s not unreasonable to be cautious. You should <a href="https://www.ema.europa.eu/en/news/astrazenecas-covid-19-vaccine-ema-finds-possible-link-very-rare-cases-unusual-blood-clots-low-blood">monitor for these symptoms</a> up to 28 days after receiving the jab: </p>
<ul>
<li><p>breathlessness</p></li>
<li><p>pain in the chest or stomach</p></li>
<li><p>swelling or coldness in the leg</p></li>
<li><p>severe or worsening headache</p></li>
<li><p>blurred vision</p></li>
<li><p>persistent bleeding</p></li>
<li><p>multiple small bruises, reddish or purplish spots, or blood blisters under the skin.</p></li>
</ul>
<p>If you’re experiencing any of these symptoms and you’re concerned, seek medical advice.</p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/data-suggest-no-increased-risk-of-blood-clots-from-the-astrazeneca-vaccine-australia-shouldnt-pause-its-rollout-157137">Data suggest no increased risk of blood clots from the AstraZeneca vaccine. Australia shouldn't pause its rollout</a>
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<img src="https://counter.theconversation.com/content/158522/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Anthony Zulli received funding from NHMRC and NHF in the past.</span></em></p><p class="fine-print"><em><span>Maximilian de Courten has received funding from the NHMRC in the past. He is affiliated with the Mitchell Institute for Education and Health Policy. </span></em></p><p class="fine-print"><em><span>Vasso Apostolopoulos has received funding from NHMRC in the past. </span></em></p><p class="fine-print"><em><span>Maja Husaric does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Scientists have called it “vaccine-induced prothrombotic immune thrombocytopenia”, or VIPIT. The condition is characterised by a shortage of platelets in the blood.Anthony Zulli, Associate professor, Victoria UniversityMaja Husaric, Senior Lecturer; MD, Victoria UniversityMaximilian de Courten, Professor in Global Public Health and Director of the Mitchell Institute, Victoria UniversityVasso Apostolopoulos, Professor of Immunology and Associate Provost, Research Partnerships, Victoria UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1144952019-05-03T10:43:42Z2019-05-03T10:43:42ZPlatelets: The chameleons of cancer biology<figure><img src="https://images.theconversation.com/files/272278/original/file-20190502-103082-1li7vsn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Activated platelets (purple) on their way to heal a wound.
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/3d-rendered-medically-accurate-illustration-active-1343108396?src=1hKNrj9qhV9M8ikZLW1h-A-1-20">Sebastian Kaulitzki/Shutterstock.com</a></span></figcaption></figure><p>Have you ever been in a classroom and wondered to yourself whether the information being presented could be wrong? </p>
<p>During graduate school, I audited a medical school class in which the professor remarked that at some point in the students’ medical training, a fact would be presented that later may be shown to be incorrect. </p>
<p>New discoveries are continuously upending our most fundamentally held dogmas. One area currently being reexamined is the complicated interactions between the components of blood with your body. Don’t worry; just like your high school textbook says, red blood cells still bring oxygen from the lungs to rest of the body, and white blood cells are still protecting your body from foreign invaders. However, the more subtle roles of blood, especially in diseases like cancer, are not fully understood.</p>
<p>I am a cell and molecular biologist, and my background in research began with studying changes within cells that <a href="https://doi.org/10.1038/oncsis.2016.36">cause</a> <a href="https://doi.org/10.21873/anticanres.11074">cancer</a>. My interest in cancer eventually led me to explore how tumors are supplied with adequate nutrition. One blood component continued to arise in literature, displaying an interesting and dynamic role after a person develops cancer: platelets. </p>
<h2>Platelets and cancer</h2>
<p>Platelets are the component of blood that springs into action when you get a paper cut or scrape your knee. The signals governing the recognition of a wound, and how to address healing, are quite complicated. However, under normal physiological conditions, these signals mediate a process called platelet activation, which results in healing the wound by closing broken blood vessels. </p>
<p>In addition to this well-known role, scientists have uncovered many other functions of platelets. In fact, an association between higher numbers of platelets and the progression of cancer has been recognized for nearly five <a href="https://doi.org/10.1002/ijc.2910110322">decades</a>. Platelet numbers are elevated in cancer patients, and studies have shown that platelet-mediated blood clots are associated with tumor cell <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3665369/">metastasis</a>.</p>
<p>That suggests that platelets may be linked to the spread of cancer to other parts of the body. But that may not be the only thing the platelets do for cancer.</p>
<p>Platelets, along with other cell types, release microparticles which respond to a variety of stimuli, including those which mediate platelet <a href="https://doi.org/10.1111/j.1365-2141.1967.tb08741.x">activation</a> for wound healing. </p>
<p>While the mechanisms of microparticles are not completely understood, several <a href="http://doi.org/10.1371/journal.pone.0050746">studies</a> have demonstrated they contain molecules that modulate which genes are turned on and off. Altering gene activity ultimately affects the production or turnover of proteins, which can cause a multitude of changes that affect health. Interestingly, microparticles contain molecules that have been shown to alter gene expression, and are associated with cancer progression.</p>
<p>That brings up an interesting question: Is it possible that platelets and the microparticles they release upon activation could also modulate cancer progression?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/272032/original/file-20190501-113830-9e06ae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/272032/original/file-20190501-113830-9e06ae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/272032/original/file-20190501-113830-9e06ae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=524&fit=crop&dpr=1 600w, https://images.theconversation.com/files/272032/original/file-20190501-113830-9e06ae.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=524&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/272032/original/file-20190501-113830-9e06ae.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=524&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/272032/original/file-20190501-113830-9e06ae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=659&fit=crop&dpr=1 754w, https://images.theconversation.com/files/272032/original/file-20190501-113830-9e06ae.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=659&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/272032/original/file-20190501-113830-9e06ae.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=659&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Platelets heal broken blood vessels.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-vector/platelets-thrombocyte-activation-fibrin-blood-vessel-1014485002?src=CiRk4tbLuy1nvQ4IZstkMw-1-15">VectorMine/Shutterstock.com</a></span>
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<h2>Do platelets drive cancer?</h2>
<p>My colleagues and I sought to address this, beginning with perhaps the most important question: whether platelet-derived microparticles were detectable in patient cancer tissue. Indeed, using a high-resolution microscope we observed microparticles in <a href="https://doi.org/10.1182/blood-2016-11-751099">cancers from many tissue types</a>. </p>
<p>Importantly, we did not see microparticles in normal tissue from the same patient, indicating they were specifically targeting cancer cells. </p>
<p>This was a particularly interesting observation, because until then, these vesicle-like structures were hypothesized to affect only other cells within the blood vessel, and not target any other surrounding cells. Our team demonstrated that platelet-derived microparticles specifically target tumors. </p>
<p>Moreover, those targeted cells absorbed the gene-regulating molecules within the microparticles. However, contrary to our original hypothesis, we observed that microparticles inhibited solid tumor growth. This finding showed that platelets may not be so easy to pick on as only a bad guy in cancer progression. </p>
<h2>Rethinking the role of platelets</h2>
<p>New information has enabled researchers like myself to reevaluate our understanding of the role of platelets in cancer. Until recently, relatively few studies have observed a protective role of <a href="https://doi.org/10.1002/ijc.29847">platelets in cancer</a>. </p>
<p>While there is substantial evidence linking intact platelets to promoting cancer, the dual pro- and anti-cancerous roles of platelets raise some fascinating questions about both intact platelets and the microparticles that they release. </p>
<p>In this way, it is tempting to consider platelets as so-called chameleons within the realm of cancer biology. The intact platelets appear to promote cancer progression, but the microparticles do the opposite, creating a natural check-and-balance system.</p>
<p>Further, the so-called leaky nature of tumor blood vessels has been posited as a likely reason for microparticles to have access to cancerous, but not normal, <a href="https://doi.org/10.3389/fcvm.2018.00013">tissue</a>. It is enticing to consider that someday, with a bit more characterization regarding the cargo being transferred, scientists could utilize the body’s natural delivery of extracellular vesicles, like these microparticles, as a therapeutic resource.</p><img src="https://counter.theconversation.com/content/114495/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>James Michael received funding from NHLBI, grant ID F32 HL139035. </span></em></p>Platelets heal wounds. But they also seem to play a paradoxical role in both promoting and inhibiting the growth of solid tumors.James Michael, Lecturer of Biochemistry, Thomas Jefferson UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/750662017-07-24T20:12:06Z2017-07-24T20:12:06ZExplainer: what’s actually in our blood?<figure><img src="https://images.theconversation.com/files/170731/original/file-20170524-5786-oxlpch.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Our blood has more functions than we probably realise - all vital for life. </span> <span class="attribution"><span class="source">from www.shutterstock.com.au</span></span></figcaption></figure><p><em>This week we’re running a series in collaboration with the Australian Red Cross Blood Service looking at blood: what it actually does, why we need it, and what happens when something goes wrong with the fluid that gives us life. Read other articles in the series <a href="https://theconversation.com/au/topics/blood-series-39533">here</a>.</em></p>
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<p>Blood is vitally important for our body. As it’s pumped around our body through veins and arteries, it transports oxygen from our lungs to all of the other organs, tissues and cells that need it. Blood also removes waste products from our organs and tissues, taking them to the liver and kidneys, where they’re removed from the body.</p>
<p>About 45% of our blood consists of different types of cells and the other 55% is plasma, a pale yellow fluid. Blood transports nutrients, hormones, proteins, vitamins and minerals around our body, suspended in the plasma. They provide energy to our cells and also signal for growth and tissue repair. The average adult has about five litres of blood.</p>
<p>The different types of blood cells include red blood cells, platelets, and white blood cells, and these are produced in the bone marrow, in the centre of our bones.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/177639/original/file-20170710-587-zksmvf.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/177639/original/file-20170710-587-zksmvf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/177639/original/file-20170710-587-zksmvf.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=306&fit=crop&dpr=1 600w, https://images.theconversation.com/files/177639/original/file-20170710-587-zksmvf.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=306&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/177639/original/file-20170710-587-zksmvf.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=306&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/177639/original/file-20170710-587-zksmvf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=384&fit=crop&dpr=1 754w, https://images.theconversation.com/files/177639/original/file-20170710-587-zksmvf.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=384&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/177639/original/file-20170710-587-zksmvf.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=384&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="attribution"><span class="source">The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<h2>Red blood cells</h2>
<p>Red blood cells are essential for transporting oxygen around the body. Red cells are very small, donut-shaped cells with <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3678251/">an average lifespan of 120 days</a> within the body. They contain a protein called haemoglobin, which contains iron and binds very strongly to oxygen, giving blood its red colour. </p>
<p>Red cells are flexible and able to squeeze through even the tiniest of our blood vessels, called capillaries, to deliver oxygen to all of the cells in our body. When the red cells reach our organs and tissues, haemoglobin releases the oxygen.</p>
<h2>Platelets</h2>
<p>Platelets are even smaller than red blood cells. In fact, they are tiny fragments of another much larger type of cell, called a megakaryocyte, which is located in the bone marrow. Platelets are formed by budding off from the megakaryocyte. Platelets have an average lifespan of eight to 10 days within the body, so they are constantly being produced. When body tissue is damaged, <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4925965/">chemicals are released that attract</a> platelets. </p>
<p>Platelets clump together and stick to the damaged tissue, which starts to form a clot to stop bleeding. Many of the proteins that help the clot to form are contained in plasma. Platelets also release growth factors that help with tissue healing.</p>
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<p><a href="https://theconversation.com/from-animal-experiments-to-saving-lives-a-history-of-blood-transfusions-80391"><em>Infographic - From animal experiments to saving lives: a history of blood transfusions</em></a></p>
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<h2>White blood cells</h2>
<p>Blood also carries white blood cells, which are an essential part of our immune system. Some white cells are able to kill micro-organisms by engulfing and ingesting them. Other types of white cells, called lymphocytes, release antibodies that help to fight infection.</p>
<p>Blood cells don’t act alone; they work together for normal body function. For example, when we cut our skin, platelets help plug the cut to stop it bleeding, plasma delivers nutrients and clotting proteins, white cells help to prevent the cut from becoming infected, and red cells deliver oxygen to help keep the skin tissue healthy.</p>
<h2>Blood transfusions</h2>
<p>Sometimes patients who are having surgery, cancer treatment or when they are seriously injured need a blood transfusion. This is usually because they have lost a lot of platelets, red cells or plasma, or because their cancer treatment has killed many of their blood cells.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/5ZMuPciBVy8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The journey of blood.</span></figcaption>
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<p>In Australia, blood is donated by voluntary blood donors at the <a href="http://www.donateblood.com.au/">Australian Red Cross Blood Service</a>. A typical whole blood donation is just over 450 mL, and it takes around ten minutes to collect. Every time a donation is made, the donor is screened for infectious diseases such as hepatitis and HIV, so these aren’t transferred to the patient receiving the blood. </p>
<p>After donation, the blood is separated into its different parts: platelets, red cells and plasma, which are known as blood components. White cells are removed because they can cause problems in patients who receive them. Once the blood has been separated, it’s stored until it’s needed by hospitals. The red blood cells are stored in a refrigerator and the plasma is frozen. The red cells can be stored for six weeks, and the plasma can be stored for up to a year. Platelets can only be stored for five days. When a hospital needs blood it’s packed into special blood shippers, and transported to the hospital blood bank to be transfused.</p>
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<p><em><strong>Read other articles in the series:</strong></em></p>
<p><em><a href="https://theconversation.com/essays-on-blood-why-do-we-actually-have-it-75064">Essays on blood: why do we actually have it?</a></em></p>
<p><em><a href="http://theconversation.com/from-animal-experiments-to-saving-lives-a-history-of-blood-transfusions-80391">From animal experiments to saving lives: a history of blood transfusions</a></em></p><img src="https://counter.theconversation.com/content/75066/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Denese Marks receives funding from the Australian and New Zealand Society of Blood Transfusion, Defence Health Foundation and MacoPharma Pty Ltd. Australian governments fund the Australian Red Cross Blood Service for the provision of blood, blood products and services to the Australian community.</span></em></p>Blood transports nutrients, hormones, proteins, vitamins and minerals around our body.Denese Marks, Adjunct Associate Professor, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/750642017-07-23T20:10:04Z2017-07-23T20:10:04ZEssays on blood: why do we actually have it?<figure><img src="https://images.theconversation.com/files/178776/original/file-20170719-13561-1ibysp7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">We know blood is vital for life, do we know why?</span> <span class="attribution"><span class="source">Illuminations: Blood Equality by Jordan Eagles (USA) Image credit: David Meanix and courtesy of artist as part of Science Gallery Melbourne’s BLOOD exhibition</span></span></figcaption></figure><p><em>This week we’re running a series in collaboration with the Australian Red Cross Blood Service looking at blood: what it actually does, why we need it, and what happens when something goes wrong with the fluid that gives us life. Read other articles in the series <a href="https://theconversation.com/au/topics/blood-series-39533">here</a>.</em></p>
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<p>Just as a village can’t grow into a city without some form of transport (road, rail or river) that provides necessary interconnections for it to flourish, living things are limited in the size they can reach unless they have some form of circulatory system to transport nutrients and remove waste.</p>
<p>Single celled organisms such as bacteria and fungi, and some multicellular creatures such as sponges, corals and flatworms, simply absorb the nutrients they need and get rid of their waste using a passive process known as diffusion (which is much like soaking in and draining out).</p>
<p>More complex animals have developed some kind of circulatory system. A variety of different systems and pumps (hearts) have developed, but they all have a few things in common. These include something to carry oxygen around their bodies, a fluid of some sort, and some “plumbing” – in humans (and a number of other species) the fluid is called blood and the plumbing is our arteries, veins and capillaries. The oxygen carrier is haemoglobin.</p>
<p>Depending on the organism and where it has adapted to live, its oxygen carrier can come in different forms, often giving its “blood” different colours. Spiders, crustaceans, octopuses and squid use haemocyanin, which is based on copper and gives them blue blood. This carrier works well in low oxygen environments and in the cold. </p>
<p>Segmented worms and some leeches use an iron based carrier called chlorocruorin, which can appear either green or red, depending on its chemical environment. Vertebrates, including humans, use haemoglobin, which makes their blood red.</p>
<p>A truly special case is the <a href="https://blogs.scientificamerican.com/brainwaves/how-the-antarctic-icefish-lost-its-red-blood-cells-but-survived-anyway/">Antarctic icefish</a>, which lost its haemoglobin long ago as a result of a presumably random mutation. It has adapted though, and now survives by transporting oxygen that is simply dissolved in its blood. This is possible thanks to the cold conditions it lives in.</p>
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<a href="https://images.theconversation.com/files/176694/original/file-20170704-7743-1u6unbb.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/176694/original/file-20170704-7743-1u6unbb.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/176694/original/file-20170704-7743-1u6unbb.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=419&fit=crop&dpr=1 600w, https://images.theconversation.com/files/176694/original/file-20170704-7743-1u6unbb.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=419&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/176694/original/file-20170704-7743-1u6unbb.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=419&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/176694/original/file-20170704-7743-1u6unbb.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=526&fit=crop&dpr=1 754w, https://images.theconversation.com/files/176694/original/file-20170704-7743-1u6unbb.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=526&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/176694/original/file-20170704-7743-1u6unbb.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=526&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<h2>What is our blood made of?</h2>
<p>Human blood, and that of all creatures with backbones (Antarctic ice fish excepted), is red. The colour comes from a chemical known as haem, which contains iron. It’s the iron that is the crucial ingredient for carrying oxygen. Oxygen is needed for our cells to burn sugars, fats and proteins in a controlled way. This provides us with the energy we need to live.</p>
<p>Outside our bodies, we know that when iron is exposed to oxygen, it rusts. And it doesn’t easily “unrust”. But to work as an oxygen carrier in our bodies, iron needs to “rust” and “unrust” on demand - picking up oxygen where it is in plentiful supply (our lungs), and releasing it where it is required (the cells in our organs).</p>
<p>This on/off oxygen switch is made possible with help from complex larger molecules. The first is haem, a flat ring structure that holds an iron atom at its centre. Haem is held closely by proteins known as globin, and this combination forms haemoglobin, which is itself packaged up in red blood cells to be transported around the body.</p>
<hr>
<p><a href="https://theconversation.com/from-animal-experiments-to-saving-lives-a-history-of-blood-transfusions-80391"><em>Infographic - From animal experiments to saving lives: a history of blood transfusions</em></a></p>
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<p>The molecular structure of haemoglobin is delicately tuned to allow it to bind oxygen in the lungs and drop it off in areas where there is less oxygen available.</p>
<p>Red cells are specialised parcels, lacking DNA, that are able to squeeze through the tiniest capillaries, down to four millionths of a meter (equivalent to roughly half their diameter). Their donut shape maximises their surface area to make sure they can efficiently deliver oxygen, while keeping them small enough to fit through the smallest blood vessels.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/176861/original/file-20170705-4592-1q4j491.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/176861/original/file-20170705-4592-1q4j491.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/176861/original/file-20170705-4592-1q4j491.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=286&fit=crop&dpr=1 600w, https://images.theconversation.com/files/176861/original/file-20170705-4592-1q4j491.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=286&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/176861/original/file-20170705-4592-1q4j491.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=286&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/176861/original/file-20170705-4592-1q4j491.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=360&fit=crop&dpr=1 754w, https://images.theconversation.com/files/176861/original/file-20170705-4592-1q4j491.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=360&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/176861/original/file-20170705-4592-1q4j491.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=360&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Antarctic ice fish managed to evolve past needing red blood cells and instead absorbs oxygen.</span>
<span class="attribution"><span class="source">Wikimedia Commons</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
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<h2>More than just the red stuff</h2>
<p>As well as red cells, our blood contains other cells and chemicals that repair and maintain the transport system and send signals around the body.</p>
<p>White blood cells, also known as leukocytes, repel or destroy invaders. Some white blood cells (lymphocytes) manufacture molecules known as antibodies that tag viruses and bacteria for destruction, while others called neutrophils and macrophages (literally “big eaters”) engulf bacteria, fungi and parasites to keep our circulation clean. When neutrophils have done their job you sometimes might see them as the main component of pus.</p>
<p>Platelets are very small fragments of larger cells called megakaryocytes. They react to any breaches to the walls of blood vessels, gathering together and triggering reactions that form a plug (or a clot) for the damaged section. If a person doesn’t have enough platelets, they can suffer from uncontrollable bleeding.</p>
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<h2>Where does it come from?</h2>
<p>All blood cells (red cells, white cells and platelets) develop from haematopoietic (literally meaning “blood-making”) stem cells, located in the bone marrow. It has recently been found that many <a href="https://www.ncbi.nlm.nih.gov/pubmed/28329764">platelets are made in the lungs</a>, from megakaryocytes that have migrated there from the bone marrow.</p>
<p>As stem cells develop, they progressively specialise into the many different types of blood cells, making developmental choices along the way. The specialisation of cells during development is tightly controlled by a symphony of growth factors. In some types of blood cancers and serious diseases, stem cell or bone marrow transplants can be used to “reboot” the blood making system.</p>
<p>As our knowledge of the control of blood cell development grows, we’re making progress towards being able to <a href="https://www.newscientist.com/article/2131517-human-blood-stem-cells-grown-in-the-lab-for-the-first-time/">reproduce this process in cells grown in the laboratory</a>. This is still some time away from being a broadly available process, but an exciting area to watch as it develops.</p>
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<p><em>Update: the sentence outlining the shape of red blood cells was incorrect and has been reworded.</em></p><img src="https://counter.theconversation.com/content/75064/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Irving is employed by the Australian Red Cross Blood Service and has research collaborations with others receiving NHMRC and ARC research grants.
Australian governments fund the Australian Red Cross Blood Service for the provision of blood, blood products and services to the Australian community.</span></em></p>Everything you never knew about the red stuff in your veins.David Irving, Adjunct Professor, University of Technology SydneyLicensed as Creative Commons – attribution, no derivatives.