Blood | Definition, Composition, & Functions

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class="md-content"> <div class="md-article-container template-desktop infinite-pagination"> <div class="infinite-scroll-container article last"> <article class="article-content container-lg qa-content px-0 pt-0 pb-40 py-lg-20 content md-expanded" data-topic-id="69685"> <div class="grid gx-0"> <div class="col-auto"> <div class="topic-left-rail md-article-drawer position-relative d-flex border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/blood-biochemistry">blood</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/blood-biochemistry">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref257799"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/blood-biochemistry#ref257799">Blood components</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref257800"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Plasma">Plasma</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref257801"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Plasma#ref257801">Blood cells</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref257802" class="has-children"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Red-blood-cells-erythrocytes">Red blood cells (erythrocytes)</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref257803"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Red-blood-cells-erythrocytes#ref257803">Hemoglobin</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref257804"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Production-of-red-blood-cells-erythropoiesis">Production of red blood cells (erythropoiesis)</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref257805"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Production-of-red-blood-cells-erythropoiesis#ref257805">Destruction of red blood cells</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref257806" class="has-children"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/White-blood-cells-leukocytes">White blood cells (leukocytes)</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref257807"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/White-blood-cells-leukocytes#ref257807">Granulocytes</a><ul class="list-unstyled" data-level="h5"><li data-target="#ref257808"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/White-blood-cells-leukocytes#ref257808">Neutrophils</a></li></ul><ul class="list-unstyled" data-level="h5"><li data-target="#ref257809"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/White-blood-cells-leukocytes#ref257809">Eosinophils</a></li></ul><ul class="list-unstyled" data-level="h5"><li data-target="#ref257810"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/White-blood-cells-leukocytes#ref257810">Basophils</a></li></ul></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref257811"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/White-blood-cells-leukocytes#ref257811">Monocytes</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref257812"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/White-blood-cells-leukocytes#ref257812">Lymphocytes</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref257813"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Platelets-thrombocytes">Platelets (thrombocytes)</a></li></ul></li></ul></div></li><li data-target="#ref35404"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Platelets-thrombocytes#ref35404">Functions of blood</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref35406"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Platelets-thrombocytes#ref35406">Respiration</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref35407"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Platelets-thrombocytes#ref35407">Nutrition</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref35408"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Platelets-thrombocytes#ref35408">Excretion</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref35409"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Immunity">Immunity</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref35411"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Immunity#ref35411">Temperature regulation</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref35410"><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Immunity#ref35410">Hemostasis</a></li></ul></div></li><li data-target="#ref35429"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/blood-biochemistry/Laboratory-examination-of-blood">Laboratory examination of blood</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/blood-biochemistry/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" 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Lockard Conley</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Distinguished Service Professor Emeritus of Medicine, Johns Hopkins University and Hospital, Baltimore, Maryland; Head, Hematology Division, 1947–80. Contributor to <i>Medical Physiology.</i></div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> C. Lockard Conley</span>, <div class="editor-popover popover p-0"> <a class="d-block p-20 gtm-byline font-12 byline-contributor" href="/contributor/Robert-S-Schwartz/3437" > <div class="editor-title font-16 font-weight-bold">Robert S. Schwartz</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Professor of Medicine, Tufts University, Boston; former Chief, Division of Hematology-Oncology, New England Medical Center.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Robert S. Schwartz</span><span class="text-gray-700 mx-5">•</span><a class="see-all border-gray-700 gtm-byline" rel="nofollow" href="https://www.britannica.com/science/blood-biochemistry/additional-info#contributors">All</a> </div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2024-11-21T00:00:00CST" >Nov 21, 2024</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/blood-biochemistry/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm 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href="/biography/Marcello-Malpighi" topicid="360486">Marcello Malpighi</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/science/blood-group" topicid="69795">blood group</a></dd> <dd><a href="/science/plasma-biology" topicid="463483">plasma</a></dd> <dd><a href="/science/bacteremia" topicid="48200">bacteremia</a></dd> <dd><a href="/science/human-blood" topicid="275447">human blood</a></dd> <dd><a href="/science/whole-blood" topicid="642986">whole blood</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/blood-biochemistry"> See all related content </a> </div> </div> </div> </div><div class="bg-gray-50 p-15 rounded module-spacing recent-news d-flex flex-column float-false"> <div> <h2 class="font-weight-bold font-14 m-0 d-inline"> News <span class="text-gray-600">•</span> </h2> <div class="recent-news-item first-recent-news-item d-inline"> <a class="font-14 gtm-ap-news-link" href="https://www.reuters.com/business/healthcare-pharmaceuticals/health-rounds-blood-thinners-dont-benefit-low-risk-patients-with-atrial-2024-11-20/" rel="nofollow">Health Rounds: Blood thinners don't benefit low-risk patients with atrial fibrillation</a> <span class="font-14 text-gray-600"> <span>•</span> Nov. 20, 2024, 10:52 AM ET (Reuters) <button class="btn btn-link d-inline p-0 font-12 js-toggle-recent-news"> <span class="text-gray-500">...</span><span>(Show more)</span> </button> </span> </div> </div> <div class="rest-of-recent-news-items"> <div class="recent-news-item mt-5"> <a class="font-14 gtm-ap-news-link" href="https://medicalxpress.com/news/2024-10-blood-tracks-stem-cells-decades.html" rel="nofollow">Blood study tracks what happens to stem cells decades after a transplant</a> <span class="font-14 text-gray-600"> <span>•</span> Oct. 30, 2024, 10:49 AM ET (Medical Xpress) </span> </div> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </div> </div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="68270" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="72963"><a data-id="72963" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/circuit-system-lungs-body-tissues-blood-cells/-68270"><img src="https://cdn.britannica.com/63/72963-138-BEFD12C0/circuit-system-lungs-body-tissues-blood-cells.jpg?w=800&h=450&c=crop" alt="Function of red blood cells explained" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/1m7VTPlt.m3u8" } ], "image": "https://cdn.britannica.com/63/72963-138-BEFD12C0/circuit-system-lungs-body-tissues-blood-cells.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/s3TSmsut", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/VTepqxEV.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="72963">Function of red blood cells explained</span><span>In a circuit through the cardiovascular system, red blood cells transport oxygen from the lungs to the body tissues and carry carbon dioxide from the body tissues back to the lungs.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/blood-biochemistry/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph"><strong><span id="ref62244"></span>blood</strong>, <a href="https://www.britannica.com/science/fluid-biology" class="md-crosslink autoxref " data-show-preview="true">fluid</a> that transports <a href="https://www.britannica.com/science/oxygen" class="md-crosslink " data-show-preview="true">oxygen</a> and nutrients to the <a href="https://www.britannica.com/science/cell-biology" class="md-crosslink " data-show-preview="true">cells</a> and carries away <a href="https://www.britannica.com/science/carbon-dioxide" class="md-crosslink " data-show-preview="true">carbon dioxide</a> and other waste products. Technically, blood is a transport liquid pumped by the <a href="https://www.britannica.com/science/heart" class="md-crosslink " data-show-preview="true">heart</a> (or an equivalent structure) to all parts of the body, after which it is returned to the heart to repeat the process. Blood is both a <a href="https://www.britannica.com/science/tissue" class="md-crosslink autoxref " data-show-preview="true">tissue</a> and a fluid. It is a tissue because it is a collection of similar specialized cells that serve particular functions. These cells are suspended in a liquid matrix (<a href="https://www.britannica.com/science/plasma-biology" class="md-crosslink " data-show-preview="true">plasma</a>), which makes the blood a fluid. If blood flow ceases, death will occur within minutes because of the effects of an unfavourable <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="environment" href="https://www.merriam-webster.com/dictionary/environment" data-type="MW">environment</a> on highly susceptible cells.</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="91377" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="82437"><a data-id="82437" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/circuit-system-lungs-body-tissues-blood-cells/-91377"><img src="https://cdn.britannica.com/37/82437-138-6FD73063/circuit-system-lungs-body-tissues-blood-cells.jpg?w=800&h=450&c=crop" alt="Observe how a red blood cell travels from the heart to the lungs and other body tissues to exchange oxygen and carbon dioxide" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/DiAHhSEX.m3u8" } ], "image": "https://cdn.britannica.com/37/82437-138-6FD73063/circuit-system-lungs-body-tissues-blood-cells.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/ZqaU8tut", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/ao0k7pqi.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="82437">Observe how a red blood cell travels from the heart to the lungs and other body tissues to exchange oxygen and carbon dioxide</span><span>In a circuit through the cardiovascular system, red blood cells transport oxygen from the lungs to the body tissues and transport carbon dioxide from the body tissues to the lungs.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/blood-biochemistry/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">The constancy of the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="composition" href="https://www.merriam-webster.com/dictionary/composition" data-type="MW">composition</a> of the blood is made possible by the <span id="ref62245"></span><a href="https://www.britannica.com/science/circulatory-system" class="md-crosslink " data-show-preview="true">circulation</a>, which conveys blood through the organs that regulate the concentrations of its components. In the <a href="https://www.britannica.com/science/lung" class="md-crosslink " data-show-preview="true">lungs</a>, blood acquires oxygen and releases carbon dioxide transported from the tissues. The <a href="https://www.britannica.com/science/kidney" class="md-crosslink " data-show-preview="true">kidneys</a> remove excess <a href="https://www.britannica.com/science/water" class="md-crosslink autoxref " data-show-preview="true">water</a> and dissolved waste products. Nutrient substances derived from food reach the bloodstream after absorption by the <a href="https://www.britannica.com/science/gastrointestinal-tract" class="md-crosslink " data-show-preview="true">gastrointestinal tract</a>. Glands of the <a href="https://www.britannica.com/science/endocrine-system" class="md-crosslink " data-show-preview="true">endocrine system</a> release their secretions into the blood, which transports these <a href="https://www.britannica.com/science/hormone" class="md-crosslink " data-show-preview="true">hormones</a> to the tissues in which they exert their effects. Many substances are recycled through the blood; for example, <a href="https://www.britannica.com/science/iron-chemical-element" class="md-crosslink " data-show-preview="true">iron</a> released during the destruction of old red cells is <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="conveyed" href="https://www.britannica.com/dictionary/conveyed" data-type="EB">conveyed</a> by the plasma to sites of new red <a href="https://www.britannica.com/science/cell-biology" class="md-crosslink autoxref " data-show-preview="true">cell</a> production where it is reused. Each of the numerous components of the blood is kept within appropriate concentration limits by an effective regulatory mechanism. In many instances, feedback control systems are operative; thus, a declining level of blood sugar (<a href="https://www.britannica.com/science/glucose" class="md-crosslink " data-show-preview="true">glucose</a>) leads to accelerated release of glucose into the blood so that a potentially hazardous depletion of glucose does not occur.</p><span class="marker MOD2 mod-inline"></span><span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">Unicellular organisms, primitive multicellular animals, and the early embryos of higher forms of life lack a <a href="https://www.britannica.com/science/circulatory-system" class="md-crosslink " data-show-preview="true">circulatory system</a>. Because of their small size, these organisms can absorb oxygen and nutrients and can <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="discharge" href="https://www.britannica.com/dictionary/discharge" data-type="EB">discharge</a> wastes directly into their surrounding medium by simple <span id="ref62246"></span><a href="https://www.britannica.com/science/diffusion" class="md-crosslink " data-show-preview="true">diffusion</a>. <a href="https://www.britannica.com/animal/sponge-animal" class="md-crosslink " data-show-preview="true">Sponges</a> and coelenterates (e.g., <a href="https://www.britannica.com/animal/jellyfish" class="md-crosslink " data-show-preview="true">jellyfish</a> and <a href="https://www.britannica.com/animal/Hydra-hydrozoan-genus" class="md-crosslink " data-show-preview="true">hydras</a>) also lack a blood system; the means to transport foodstuffs and oxygen to all the cells of these larger multicellular animals is provided by water, sea or fresh, pumped through spaces inside the organisms. In larger and more-complex animals, transport of adequate amounts of oxygen and other substances requires some type of blood circulation. In most such animals the blood passes through a respiratory exchange <a href="https://www.britannica.com/science/membrane-biology" class="md-crosslink autoxref " data-show-preview="true">membrane</a>, which lies in the <a href="https://www.britannica.com/science/gill-respiratory-system" class="md-crosslink " data-show-preview="true">gills</a>, lungs, or even the skin. There the blood picks up oxygen and disposes of carbon dioxide.</p><span class="marker MOD3 mod-inline"></span><span class="marker PREMOD4 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="299667" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="253594"><a data-id="253594" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/How-much-blood-is-in-the-human-body/-299667"><img src="https://cdn.britannica.com/94/253594-138-6D2D3DBC/How-much-blood-is-in-the-human-body.jpg?w=800&h=450&c=crop" alt="How much blood is in the human body?" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/KfWRKMMe.m3u8" } ], "image": "https://cdn.britannica.com/94/253594-138-6D2D3DBC/How-much-blood-is-in-the-human-body.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/LtRePXpF.vtt", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/5bE5Sntr.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="253594">How much blood is in the human body?</span><span>And how much do you really need?</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/blood-biochemistry/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">The cellular <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="composition" href="https://www.britannica.com/dictionary/composition" data-type="EB">composition</a> of blood varies from group to group in the animal kingdom. Most <span id="ref910542"></span><a href="https://www.britannica.com/animal/invertebrate" class="md-crosslink " data-show-preview="true">invertebrates</a> have various large blood cells capable of amoeboid movement. Some of these aid in transporting substances; other are capable of surrounding and digesting foreign particles or debris (<a href="https://www.britannica.com/science/phagocytosis" class="md-crosslink " data-show-preview="true">phagocytosis</a>). Compared with <a href="https://www.britannica.com/animal/vertebrate" class="md-crosslink " data-show-preview="true">vertebrate</a> blood, however, that of the invertebrates has relatively few cells. Among the vertebrates, there are several classes of amoeboid cells (white blood cells, or <a href="https://www.britannica.com/science/white-blood-cell" class="md-crosslink " data-show-preview="true">leukocytes</a>) and cells that help stop <a href="https://www.britannica.com/science/bloodletting" class="md-crosslink autoxref " data-show-preview="true">bleeding</a> (<a href="https://www.britannica.com/science/platelet" class="md-crosslink " data-show-preview="true">platelets</a>, or thrombocytes).</p><span class="marker MOD4 mod-inline"></span><span class="marker PREMOD5 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="300798" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="253595"><a data-id="253595" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/Why-is-blood-red/-300798"><img src="https://cdn.britannica.com/95/253595-138-811BABAC/Why-is-blood-red.jpg?w=800&h=450&c=crop" alt="Why is blood red?" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/TKTMPCIS.m3u8" } ], "image": "https://cdn.britannica.com/95/253595-138-811BABAC/Why-is-blood-red.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/ljhAVTXT.vtt", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/m9YOsrxq.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="253595">Why is blood red?</span><span>Are you “red-y” to find out?</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/blood-biochemistry/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">Oxygen requirements have played a major role in determining both the composition of blood and the architecture of the circulatory system. In some simple animals, including small <a href="https://www.britannica.com/animal/worm" class="md-crosslink " data-show-preview="true">worms</a> and <a href="https://www.britannica.com/animal/mollusk" class="md-crosslink " data-show-preview="true">mollusks</a>, transported oxygen is merely <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="dissolved" href="https://www.britannica.com/dictionary/dissolved" data-type="EB">dissolved</a> in the plasma. Larger and more-complex animals, which have greater oxygen needs, have pigments capable of transporting relatively large amounts of oxygen. The red pigment <span id="ref910544"></span><a href="https://www.britannica.com/science/hemoglobin" class="md-crosslink " data-show-preview="true">hemoglobin</a>, which contains iron, is found in all vertebrates and in some invertebrates. In almost all vertebrates, including humans, hemoglobin is contained exclusively within the red cells (<a href="https://www.britannica.com/science/red-blood-cell" class="md-crosslink " data-show-preview="true">erythrocytes</a>). The red cells of the lower vertebrates (e.g., birds) have a <a href="https://www.britannica.com/science/nucleus-biology" class="md-crosslink " data-show-preview="true">nucleus</a>, whereas mammalian red cells lack a nucleus. Red cells vary markedly in size among mammals; those of the <a href="https://www.britannica.com/animal/goat" class="md-crosslink " data-show-preview="true">goat</a> are much smaller than those of humans, but the goat <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="compensates" href="https://www.britannica.com/dictionary/compensates" data-type="EB">compensates</a> by having many more red cells per unit volume of blood. The concentration of hemoglobin inside the red cell varies little between species. <span id="ref62248"></span><a href="https://www.britannica.com/science/hemocyanin" class="md-crosslink ">Hemocyanin</a>, a <a href="https://www.britannica.com/science/copper" class="md-crosslink " data-show-preview="true">copper</a>-containing <a href="https://www.britannica.com/science/protein" class="md-crosslink autoxref " data-show-preview="true">protein</a> chemically unlike hemoglobin, is found in some <a href="https://www.britannica.com/animal/crustacean" class="md-crosslink " data-show-preview="true">crustaceans</a>. Hemocyanin is blue in colour when oxygenated and colourless when oxygen is removed. Some <a href="https://www.britannica.com/animal/annelid" class="md-crosslink " data-show-preview="true">annelids</a> have the iron-containing green pigment chlorocruorin, others the iron-containing red pigment hemerythrin. In many invertebrates the respiratory pigments are carried in solution in the plasma, but in higher animals, including all vertebrates, the pigments are enclosed in cells; if the pigments were freely in solution, the pigment concentrations required would cause the blood to be so viscous as to impede circulation.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/the-human-body" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/20/123920-131-0ADF15E8/arteries-veins-face-scalp.jpg" alt="Superficial arteries and veins of face and scalp, cardiovascular system, human anatomy, (Netter replacement project - SSC)" class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">The Human Body</div> </div> </a><span class="marker MOD5 mod-inline"></span><span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">This article focuses on the main components and functions of human blood. For full treatment of blood groups, <em>see</em> the article <a href="https://www.britannica.com/science/blood-group" class="md-crosslink " data-show-preview="true">blood group</a>. For information on the <a href="https://www.britannica.com/science/organ-biology" class="md-crosslink autoxref " data-show-preview="true">organ</a> system that conveys blood to all organs of the body, <em>see</em> <a href="https://www.britannica.com/science/human-cardiovascular-system" class="md-crosslink " data-show-preview="true">cardiovascular system</a>. For additional information on blood in general and comparison of the blood and <a href="https://www.britannica.com/science/lymph" class="md-crosslink autoxref " data-show-preview="true">lymph</a> of <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="diverse" href="https://www.merriam-webster.com/dictionary/diverse" data-type="MW">diverse</a> organisms, <em>see</em> <a href="https://www.britannica.com/science/circulatory-system" class="md-crosslink " data-show-preview="true">circulation</a>.</p><span class="marker MOD6 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref257799"><h2 class="h1">Blood components</h2> <span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">In humans, blood is an <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="opaque" href="https://www.merriam-webster.com/dictionary/opaque" data-type="MW">opaque</a> red fluid, freely flowing but denser and more viscous than water. The characteristic colour is imparted by <span id="ref663134"></span><a href="https://www.britannica.com/science/hemoglobin" class="md-crosslink " data-show-preview="true">hemoglobin</a>, a unique iron-containing protein. Hemoglobin brightens in colour when saturated with oxygen (oxyhemoglobin) and darkens when oxygen is removed (deoxyhemoglobin). For this reason, the partially deoxygenated blood from a <a href="https://www.britannica.com/science/vein-blood-vessel" class="md-crosslink " data-show-preview="true">vein</a> is darker than oxygenated blood from an <a href="https://www.britannica.com/science/artery" class="md-crosslink " data-show-preview="true">artery</a>. The red blood cells (<span id="ref910564"></span><a href="https://www.britannica.com/science/red-blood-cell" class="md-crosslink " data-show-preview="true">erythrocytes</a>) <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="constitute" href="https://www.merriam-webster.com/dictionary/constitute" data-type="MW">constitute</a> about 45 percent of the volume of the blood, and the remaining cells (white blood cells, or <a href="https://www.britannica.com/science/white-blood-cell" class="md-crosslink " data-show-preview="true">leukocytes</a>, and <a href="https://www.britannica.com/science/platelet" class="md-crosslink " data-show-preview="true">platelets</a>, or thrombocytes) less than 1 percent. The fluid portion, <a href="https://www.britannica.com/science/plasma-biology" class="md-crosslink " data-show-preview="true">plasma</a>, is a clear, slightly sticky, yellowish liquid. After a fatty meal, plasma transiently appears turbid. Within the body the blood is permanently fluid, and <a href="https://www.britannica.com/science/turbulent-flow" class="md-crosslink autoxref " data-show-preview="true">turbulent flow</a> assures that cells and plasma are fairly homogeneously mixed.</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=black-friday-2024">Subscribe</a> </div> </div> </div> </DIV></div><div class="one-good-fact-module"> </div><span class="marker MOD7 mod-inline"></span> <span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph">The total amount of blood in humans varies with age, sex, weight, body type, and other factors, but a rough average figure for adults is about 60 millilitres per kilogram of body weight. An average young male has a plasma volume of about 35 millilitres and a red cell volume of about 30 millilitres per kilogram of body weight. There is little variation in the <span id="ref249249"></span><a href="https://www.britannica.com/science/blood-volume" class="md-crosslink ">blood volume</a> of a healthy person over long periods, although each component of the blood is in a <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="continuous" href="https://www.britannica.com/dictionary/continuous" data-type="EB">continuous</a> state of flux. In particular, water rapidly moves in and out of the bloodstream, achieving a balance with the extravascular fluids (those outside the blood vessels) within minutes. The normal volume of blood provides such an adequate reserve that appreciable blood loss is well tolerated. Withdrawal of 500 millilitres (about a pint) of blood from normal blood donors is a harmless procedure. Blood volume is rapidly replaced after blood loss; within hours, plasma volume is restored by movement of extravascular fluid into the circulation. Replacement of red cells is completed within several weeks. The vast area of <span id="ref249253"></span><a href="https://www.britannica.com/science/capillary" class="md-crosslink " data-show-preview="true">capillary</a> membrane, through which water passes freely, would permit instantaneous loss of the plasma from the <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="circulation" href="https://www.britannica.com/dictionary/circulation" data-type="EB">circulation</a> were it not for the plasma proteins—in particular, <span id="ref249254"></span><a href="https://www.britannica.com/science/serum-albumin" class="md-crosslink " data-show-preview="true">serum albumin</a>. Capillary membranes are impermeable to serum albumin, the smallest in weight and highest in concentration of the plasma proteins. The osmotic effect of serum albumin retains fluid within the circulation, opposing the hydrostatic forces that tend to drive the fluid outward into the tissues.</p><span class="marker MOD8 mod-inline"></span> </section><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article></div> </div></div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":8,"pageId":69685,"pageLength":1215,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/topic-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"C","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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Blood | Definition, Composition, & Functions | Britannica