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BloodThe earliest evidence in the fossil record of a blood-like substance, found on the remains of an armored arthropod, dates back at least 520 million years.
Blood has evolved several times across the animal kingdom, giving rise to a rainbow of color, composition, and purpose. Some creatures can skip having blood entirely, a nearly heretical approach to humans who can't live without it.
In humans, there are two main blood groups. Blood group ABO refers to the presence of A and/or B antigens. The Rh blood group describes Rh antigens' presence (positive) or absence (negative). Groups describe the type of antigens that determines blood type, leading to four main types (A, B, AB, and O) with positive and negative variations. The combination of these types determines how the immune system responds to foreign blood, such as in the case of transfusions.Explore Blood
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Genetic blood test offers an estimated clock on when Alzheimer's symptoms may appearThe 2026 research finding from researchers at Washington University School of Medicine in St. Louis—if validated in broader populations—suggests that an already available blood test could estimate when Alzheimer's symptoms may begin within a margin of several years. The test detects a protein called p-tau217 found in blood plasma and the researchers incorporated the findings from that blood test into a statistical model to come up with the symptom timeline predictions. Medical News TodayUntreated malaria can lead to blocked blood vessels in the brain, coma, and organ failureMalaria is treatable with medications, but sometimes even after treatment, infection can come back because the disease-causing parasite wasn't completely eliminated. Patients may also have symptoms that emerge years after the initial infection and treatment. Cleveland ClinicBlood tests for Lyme disease tend to come up as false negatives in the week after a biteIt can take several weeks for Lyme antibodies to develop, which is what blood tests would be detecting. That's why many clinicians will prescribe antibiotic treatment immediately after a bite if the individual develops the characteristic bullseye rash of Lyme or gets bitten in an area of the country known to have high rates of Lyme. Columbia University Irving Medical CenterRed blood, skin, hair, and nail cells destroy their DNA upon maturingTo maximize the amount of oxygen they carry and more easily alter their shape to fit into narrow blood capillaries, red blood cells destroy their own nuclei. Cornified cells found in hair, nails, and the outermost layer of skin destroy their nuclei and other cellular contents to make room for keratin, which provides durability and structural support. West Texas A&M UniversityEnglishmen in the 17th century mixed their blood into wine to show devotionIn order to demonstrate their love for a woman, some young Englishmen would stab themselves in the arm and mix the blood into their wine. They'd then toast the object of their affection and drink to their honor. NPRCold-blooded reptiles seek heat, and that behavior may help combat fungal infectionsThis Radiolab podcast episode takes on the idea that though our human bodies are too warm for the majority of fungal species to effectively replicate inside our bodies, that's not the case for reptiles. Yet when those species warm their bodies by basking in the sun on rocks, they can soak up heat and may also help keep fungal infections in check. RadiolabHow much your blood really costsIn the US, donating plasma twice a week can earn you $400 a month. But this system only works by targeting people who need the money most. Plasma clinics are placed in low-income zip codes, and frequent donation may increase health risks. 1440Watch a Yale professor’s take on O’Connor’s ‘Wise Blood’O’Connor’s first novel is about Hazel Motes, a World War II veteran who returns home and sets out to form an anti-religious congregation. It’s a striking debut and one that Yale professor Amy Hungerford taught in her course “The American Novel Since 1945.” YaleCoursesNanoplastics can bypass the blood-brain barrier and may harm the nervous systemNanoplastics are formed from the breakdown of microplastics and can breach cells. Their large surface area relative to size allows them to carry higher concentrations of forever chemicals than the surrounding environment into sensitive organs. It's ComplicatedThe rarest blood type in the worldRh-null, also known as “golden blood,” is a blood type so rare, fewer than 50 people are known to have it. It lacks all 61 Rh antigens, making it a universal donor for rare Rh cases, but a nightmare in emergencies, since only Rh-null can be given back. 1440 Trials ongoing for a 99% accurate, AI-powered blood testThe UK's National Health Service has launched a trial for a blood test that can detect a dozen types of cancers with just ten drops of blood. An AI tool looks for genetic material known to be released by tumors, allowing for cancer detection before tumors are large enough to be imaged. Sky NewsFentanyl is better at crossing the blood-brain barrier than other opioidsAmong opioids, fentanyl is uniquely adept at crossing the blood-brain barrier—the brain's security fence against harmful substances. This enables a lightning-fast high. It ends just as quickly, sometimes within minutes (where a heroin high lasts hours), and still results in the same withdrawal symptoms. KurzgesagtBears have evolved natural mechanisms to avoid blood clots while hibernatingMovement is essential to keeping blood from clotting, which raises the question of why animals such as hibernating bears don't suffer from clots despite going months lying still. New research suggesting that the animals may have evolved to have lower concentrations of a specific protein may explain this phenomenon. ScienceBlood clotting is critical to maintaining life, but dysfunctional clots can be fatalClotting is how our body stops bleeding in the event of an injury. Typically, platelets adhere to exposed tissue, becoming sticky and forming a plug. Chemical signals trigger the coagulation cascade, converting fibrinogen into fibrin threads that stabilize the plug. This prevents bleeding but can obstruct vessels if overactive or misplaced. MedlinePlusExercise improves brain blood flow, stimulates neural growth, and moreExercise has dramatic effects on human biology, including reducing some of the long-term harms associated with chronic stress. Its effects are also pronounced in the brain, including improved memory retention, sleep, cognitive functioning, and more. TED TalksBlood transports oxygen and vital nutrients throughout the bodyBlood is the most vital substance in a body, ferrying oxygen, nutrients, and hormones to cells in need while regulating body temperature and collecting cellular waste and byproducts, such as carbon dioxide, for removal. Every adult human has over a gallon of the red liquid coursing through their veins. 1440What is the blood brain barrier?Every blood vessel is lined with endothelial cells that act as filters between blood and the rest of the body. In the brain, these cells are tightly packed together to keep a close watch on what substances can access the vital organ. This protective measure poses an obstacle to drugmakers looking to deliver drugs to the well-defended brain for conditions like Alzheimer's. Watch a quick overview here. STATTruman Capote's 'In Cold Blood,' a pioneer of the true crime genre, isn't wholly accurateThe nonfiction novel "In Cold Blood" influenced true crime's narrative and investigative bend and was supposedly only 92% accurate. Journalist Ben Yagoda reflects on its accuracy after finding the fact-checking file for the four-part series published before the story was converted into hardback. Yagoda says the inaccuracies are "especially striking" because the story was originally published in one of the magazines credited with establishing fact-checking as a practice. (Some users may experience a paywall.) SlateThe strange endurance of consuming blood for healthBlood's importance to humanity is best summarized this way: without it, we die. This importance naturally begets assumptions about its medicinal properties, many of which appear accessible through drinking or eating the fluid connective tissue. This article details the history of blood drinking, from ancient Roman gladiators to modern-day sanguinarians. Atlas ObscuraHow did segregation in the US affect blood donations?When the US established its first nationwide network of blood banks in 1941 with the hopes of restoring the dwindling reserves of military members, it was quick to emphasize blood donated by Black Americans wouldn't coat the veins of white servicemen. Historians say this policy was meant to appease white Southerners who believed Black blood was inferior to their own.This policy was leaked to the press that same year. Undark MagazineThe economics of horseshoe crab blood Horseshoe crabs are older than dinosaurs and have survived five mass extinctions that wiped most life from the planet. But it's only now that their future as a species is in danger. This blood, worth about $60,000 per gallon, is hyper-sensitized to certain bacteria that are fatal to humans. This 17-minute podcast dives into humanity's pursuit of horseshoe crab blood, a "somewhat magical" mixture heavily relied on by the pharmaceutical industry. The Economics of Everyday ThingsWhy don't we synthesize blood?We're trying. Scientists are currently working on creating viable synthetic blood, an alternative to the presently relied-upon donations that manage to fall short of demand. More than half of the world's nations struggle to meet their population's need for blood. This article details current and historical efforts to develop synthetic blood, a venture that started more than 400 years ago. VoxWhen was the first blood transfusion?In the 1600s, an English doctor transferred the blood from one dog to another via quills. The procedure ended when one of the dogs died. This article details the early days of blood transfusions, which turned toward humans shortly after the 1666 dog experiment and involved putting the blood of a sheep into a human recipient (who reportedly survived the encounter). JSTORThe ultrarare 'Er' blood type has only been documented around 40 timesIn 2022, the tragic loss of a pregnancy led researchers to the discovery that the rare 'Er' blood type could potentially cause a mismatch in blood types between mother and fetus during pregnancy, with devastating consequences. WIREDA brief history of human blood groupsThe blood group that determines whether you have type A, B, AB, or O blood was discovered near the turn of the 20th century. This short academic article dives into three main theories about the evolution and emergence of blood groups, the exact nature of which remains unclear. National Institutes of HealthHow the brain protects itself from blood-borne threatsIf the brain were a computer, the skull would be the hardware that blocks outside interference, and the blood-brain barrier would be the software that stops rogue or unwanted programs from running. This semi-permeable membrane regulates molecule and ion movement between the blood and brain, an exchange necessary for blood to bring nutrients to the brain and sweep away cell waste. This 13-minute Quanta Science Podcast episode explains how the blood-brain barrier works (and what happens when it doesn't). Quanta Science PodcastThe history of blood, as told by a hematologist Since ancient times, blood has been seen as a semi-magical substance. Alchemists theorized it was a mixture of the four elements. Others proposed it was one of four bodily fluids whose balance dictated health. This 2019 essay by a hematologist reviews blood's reputation across cultures and centuries. The New YorkerBlood leaves the heart at speeds around 12-16 miles per hourBlood is pumped around the body through the circulatory system, aka the cardiovascular system. This system comprises the heart, arteries, veins, and capillaries. This video is the first in a multi-part series on the circulatory system, focusing on the role of the heart, the pump that powers the entire system. Professor Dave ExplainsAn interactive explanation of blood transfusionThe antigens and Rh factors of blood cells (which determines your blood type) must be compatible with your immune system—which is why blood transfusions between patients must have matching types. In a mismatch, the immune system will attack and destroy donor blood cells, triggering a potentially fatal reaction. Nobel PrizeSeven things you might not know about bloodThe red stuff currently pumping throughout your body is a type of connective tissue that is both a solid and a liquid. The liquid portion—plasma—acts as a carrier for the solid portion, which includes blood cells and their biological brethren. Your body is constantly refreshing its supply of blood, creating new cells to replace those that have expired or died. This article overviews some lesser-known facts about blood (and why it's not always red). The ConversationBone marrow generates roughly 500 billion new blood cells each daySome of our bones are stuffed to the brim with a fatty, jam-like substance called marrow. Bone marrow comes in yellow and red, with the latter responsible for producing new blood cells, and has become critical in treating diseases like leukemia. Institute of Human AnatomyNew blood cells are created by hematopoietic stem cellsBlood is made up of several cells with various functions and short lifespans, so the body must have a constant supply of replacements. These replacements are created by hematopoietic stem cells, rare cells that live in bone marrow and can regenerate themselves and blood cells. Stem Cell NetworkWhat does blood do?Blood's primary purpose is to ferry oxygen and nutrients to cells in need while collecting cell waste and byproducts for removal. It also enables the body's repair and immune systems to traverse our internal landscape to address cell damage or external invaders, such as viruses and parasites. Zooming out from the exact roles of blood's various cells, the fluid connective tissue also assists with maintaining homeostasis via temperature regulation and pH balance. OpenStax/Oregon State UniversityBlood evolved many times over hundreds of millions of yearsThe earliest evidence of a blood-like substance comes from ancient arthropods, which appear to have had copper-based blood. In the 500 million years since then, some creatures have grown to rely on other variants, such as humanity's iron-based blood. PBS EonsThe proteins and sugars on the surface of your blood cells determine your blood typeThe antigens dotting the outside of a red blood cell determine blood type. That's why the four main blood types are A, B, AB, and O, each with positive and negative variations. Secondly, the presence or absence of the Rh factor protein determines whether the type is positive or negative. Interactive Biology'Buffy coats': The 1% of blood that helps fight off diseaseWhen blood is centrifuged, white blood cells and platelets form a layer called the buffy coat between the plasma and red blood cells. This layer makes up less than 1% of blood, but its heightened leukocyte concentration makes it useful for toxicology research. Akadeum Life SciencesInsulin is a hormone that controls blood sugar by helping absorb it into cellsBlood sugar levels increase after a meal when the body starts turning food into usable fuel—sugars, proteins, and more—which are absorbed into the bloodstream through the intestines. Blood sugar levels rise, releasing insulin, and letting sugar in the blood be absorbed into hungry cells, thereby dropping blood sugar levels. NatureThe pancreas releases digestive enzymes and regulates blood sugarThe pancreas weighs about the same as a deck of cards and is smaller than the average adult human hand. This lopsided glandular organ controls blood sugar levels through insulin production and helps the body digest food by creating a neutralizing tonic filled with digestive enzymes. TED-EdBlood of young mice extends life span of oldA recent study confirms that transfusing the blood of younger mice with older mice successfully extends the life span of older mice by over 10 percent. The technique, called heterochronic parabiosis, surgically connects the circulatory system of two mice for an extended period of time. Though blood transfusion has been historically believed to fight aging, this was the first study of its kind to demonstrate that the length of the transfusion correlates to the length of the benefits. The benefits occur because younger blood has more components than older blood to assist with the maintenance and regeneration of organs and tissues. Though the effects on humans remain unknown, scientists estimate the additional time added to the mice's life would translate to about eight years in the life of a person. National Institutes of HealthThe bloody history of the true crime genreThe true crime genre appears to have crawled out of an unlocked closet between 1550 and 1700. Rising literacy rates and printing technologies during this period led to leaflets and ballads-turned-broadsides that graphically regurgitated the findings. These products often included woodcuts of the crime's "unsavory acts.". Such content was snapped up by the "artisan class and above" as poorer classes lacked the funds to fuel the hobby. JSTORWhy hibernating bears don't get blood clotsFor mammals, long periods of inactivity can lead to health problems—blood flow slows and pools, veins kink and distort, and clots potentially form. Hibernation takes this risk to the extreme, yet for decades why animals like bears did not exhibit... The ScientistWhy do we harvest horseshoe crab blood?Each year, thousands of horseshoe crabs are captured from the mid-Atlantic coast, taken to a lab, and partially drained of their blood. The goal? Amebocytes. This type of immune cell is exceptionally sensitive to the presence of bacterial endotoxins. Endotoxin molecules are released from the cell walls of certain bacteria—like E. coli—upon the bacteria's death and are toxic to humans once they enter the bloodstream. The cells have become vital in checking for contamination in vaccines. TED-EdBlood pressure readings correspond to the peaks and valleys of heart beatsPushing 2,000 gallons of blood through the body every day exerts a force known as blood pressure. Blood pressure is highest during the heart's contractile phase, known as systole, which forces blood through the circulatory system, and lowest during the heart's resting phase between beats, known as diastole. TED TalksSpectacular shots of 2022's Blood Moon eclipseA total lunar eclipse took place early Tuesday on Election Day. This occurs when the moon and the sun are on exact opposite sides of Earth, NASA says. See photos of the event here. USA TODAYExploring the microscopic composition of bloodVisually, blood looks like a single, red fluid. Zooming in, it is one of the richest substances known, with hemoglobin, plasma, proteins, red and white blood cells, platelets, and more. Visual CapitalistBlood Pressure and DiabetesNew meta-analysis shows blood pressure reductions with ACE inhibitors and ARBs are associated with a lower diabetes risk, but other types of antihypertensive agents have the opposite effect. MedscapeAdams had a deep contempt for Alexander HamiltonAdams regarded him as dangerously ambitious and untrustworthy. Her letters are among the most vivid contemporary assessments of Hamilton, providing a portrait of one of the founding era's most contentious figures. 'Dracula' was inspired by a seafood-induced nightmareBram Stoker, the author of the classic horror novel, said that a dinner of "dressed crab" led to unsettling dreams about "a vampire king rising from the tomb." He'd revisit that vision, eventually creating the iconic titular vampire. Mental FlossExplore the abundance of different types of cells in the human bodyLacking a nucleus and other organelles to maximize space for oxygen-carrying hemoglobin and enhance maneuverability through narrow capillaries, there are about four times as many red blood cells and platelets as all other cell types in the body combined. However, about half of the body's cell biomass consists of skeletal myocytes, or muscle fibers. Human Cell Tree MapAmerican archaeology has a complicated legacyMany national parks were discovered through American archaeological expeditions and, consequently, included the looting of Native American cultural sites and artifacts. Women In Archaeology
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