Tachyon

From Wikipedia, the free encyclopedia

Because a tachyon would always move faster than light, it would not be possible to see it approaching. After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions. The black line is the shock wave of Cherenkov radiation, shown only in one moment of time. This double image effect is most prominent for an observer located directly in the path of a superluminal object (in this example a sphere, shown in grey). The right hand bluish shape is the image formed by the blue-doppler shifted light arriving at the observer—who is located at the apex of the black Cherenkov lines—from the sphere as it approaches. The left-hand reddish image is formed from red-shifted light that leaves the sphere after it passes the observer. Because the object arrives before the light, the observer sees nothing until the sphere starts to pass the observer, after which the image-as-seen-by-the-observer splits into two—one of the arriving sphere (to the right) and one of the departing sphere (to the left).

A tachyon /ˈtæki.ɒn/ or tachyonic particle is a hypothetical particle that always moves faster than light. The word comes from the Greek: ταχύς or tachys, meaning "swift, quick, fast, rapid", and was coined in 1967 by Gerald Feinberg.^[1] The complementary particle types are called luxon (always moving at the speed of light) and bradyon (always moving slower than light), which both exist. The possibility of particles moving faster than light was first proposed by Bilaniuk, Deshpande, and George Sudarshan in 1962, although the term they used for it was "meta-particle".^[2]

Most physicists think that faster-than-light particles cannot exist because they are not consistent with the known laws of physics.^[3][4] If such particles did exist, they could be used to build a tachyonic antitelephone and send signals faster than light, which (according to special relativity) would lead to violations of causality.^[4] Potentially consistent theories that allow faster-than-light particles include those that break Lorentz invariance, the symmetry underlying special relativity, so that the speed of light is not a barrier.

In the 1967 paper that coined the term,^[1] Feinberg proposed that tachyonic particles could be quanta of a quantum field with negative squared mass. However, it was soon realized that excitations of such imaginary mass fields do not in fact propagate faster than light,^[5] and instead represent an instability known as tachyon condensation.^[3] Nevertheless, negative squared mass fields are commonly referred to as "tachyons",^[6] and in fact have come to play an important role in modern physics.

Despite theoretical arguments against the existence of faster-than-light particles, experiments have been conducted to search for them. No compelling evidence for their existence has been found. In September 2011, it was reported that a tau neutrino had travelled faster than the speed of light in a major release by CERN; however, later updates from CERN on the OPERA project indicate that the faster-than-light readings were resultant from "a faulty element of the experiment's fibre optic timing system".^[7]

In paper accepted for publication in Astroparticle Physics by professor Robert Ehrlich it is stated that six observations based on data and fits to data from a variety of areas are consistent with the hypothesis that the electron neutrino is a tachyon with imaginary mass.^[8]

Tachyons in relativistic theory

In special relativity, a faster-than-light particle would have space-like four-momentum,^[1] in contrast to ordinary particles that have time-like four-momentum. It would also have imaginary mass and proper time.^{[citation needed]} Being constrained to the spacelike portion of the energy–momentum graph, it could not slow down to subluminal speeds.^[1]

Mass

In a Lorentz invariant theory, the same formulas that apply to ordinary slower-than-light particles (sometimes called "bradyons" in discussions of tachyons) must also apply to tachyons. In particular the energy–momentum relation:

(where p is the relativistic momentum of the bradyon and m is its rest mass) should still apply, along with the formula for the total energy of a particle:

This equation shows that the total energy of a particle (bradyon or tachyon) contains a contribution from its rest mass (the "rest mass–energy") and a contribution from its motion, the kinetic energy.
When v is larger than c, the denominator in the equation for the energy is "imaginary", as the value under the radical is negative. Because the total energy must be real, the numerator must also be imaginary: i.e. the rest mass m must be imaginary, as a pure imaginary number divided by another pure imaginary number is a real number.

Speed

One curious effect is that, unlike ordinary particles, the speed of a tachyon increases as its energy decreases. In particular, approaches zero when approaches infinity. (For ordinary bradyonic matter, E increases with increasing speed, becoming arbitrarily large as v approaches c, the speed of light). Therefore, just as bradyons are forbidden to break the light-speed barrier, so too are tachyons forbidden from slowing down to below c, because infinite energy is required to reach the barrier from either above or below.

As noted by Einstein, Tolman, and others, special relativity implies that faster-than-light particles, if they existed, could be used to communicate backwards in time.^[9]

Neutrinos

In 1985 Chodos et al. proposed that neutrinos can have a tachyonic nature.^[10] The possibility of standard model particles moving at superluminal speeds can be modeled using Lorentz invariance violating terms, for example in the Standard-Model Extension.^[11][12][13] In this framework, neutrinos experience Lorentz-violating oscillations and can travel faster than light at high energies. This proposal was strongly criticized.^[14]

Cherenkov radiation

A tachyon with an electric charge would lose energy as Cherenkov radiation^[15]—just as ordinary charged particles do when they exceed the local speed of light in a medium. A charged tachyon traveling in a vacuum therefore undergoes a constant proper time acceleration and, by necessity, its worldline forms a hyperbola in space-time. However reducing a tachyon's energy increases its speed, so that the single hyperbola formed is of two oppositely charged tachyons with opposite momenta (same magnitude, opposite sign) which annihilate each other when they simultaneously reach infinite speed at the same place in space. (At infinite speed, the two tachyons have no energy each and finite momentum of opposite direction, so no conservation laws are violated in their mutual annihilation. The time of annihilation is frame dependent.)

Even an electrically neutral tachyon would be expected to lose energy via gravitational Cherenkov radiation, because it has a gravitational mass, and therefore increase in speed as it travels, as described above. If the tachyon interacts with any other particles, it can also radiate Cherenkov energy into those particles. Neutrinos interact with the other particles of the Standard Model, and Andrew Cohen and Sheldon Glashow recently used this to argue that the faster-than-light neutrino anomaly cannot be explained by making neutrinos propagate faster than light, and must instead be due to an error in the experiment.^[16]

Causality

Causality is a fundamental principle of physics. If tachyons can transmit information faster than light, then according to relativity they violate causality, leading to logical paradoxes of the "kill your own grandfather" type. This is often illustrated with thought experiments such as the "tachyon telephone paradox"^[9] or "logically pernicious self-inhibitor."^[17]

The problem can be understood in terms of the relativity of simultaneity in special relativity, which says that different inertial reference frames will disagree on whether two events at different locations happened "at the same time" or not, and they can also disagree on the order of the two events (technically, these disagreements occur when spacetime interval between the events is 'space-like', meaning that neither event lies in the future light cone of the other).^[18]

If one of the two events represents the sending of a signal from one location and the second event represents the reception of the same signal at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity ensures that all reference frames agree that the transmission-event happened before the reception-event.^[18] However, in the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received before it was sent, so that the signal could be said to have moved backwards in time. Because one of the two fundamental postulates of special relativity says that the laws of physics should work the same way in every inertial frame, if it is possible for signals to move backwards in time in any one frame, it must be possible in all frames. This means that if observer A sends a signal to observer B which moves faster than light in A's frame but backwards in time in B's frame, and then B sends a reply which moves faster than light in B's frame but backwards in time in A's frame, it could work out that A receives the reply before sending the original signal, challenging causality in every frame and opening the door to severe logical paradoxes.^[19] Mathematical details can be found in the tachyonic antitelephone article, and an illustration of such a scenario using spacetime diagrams can be found in Baker, R. (2003)^[20]

Reinterpretation principle

The reinterpretation principle^[1][2][19] asserts that a tachyon sent back in time can always be reinterpreted as a tachyon traveling forward in time, because observers cannot distinguish between the emission and absorption of tachyons. The attempt to detect a tachyon from the future (and violate causality) would actually create the same tachyon and send it forward in time (which is causal).

However, this principle is not widely accepted as resolving the paradoxes.^[9][19][21] Instead, what would be required to avoid paradoxes is that unlike any known particle, tachyons do not interact in any way and can never be detected or observed, because otherwise a tachyon beam could be modulated and used to create an anti-telephone^[9] or a "logically pernicious self-inhibitor".^[17] All forms of energy are believed to interact at least gravitationally, and many authors state that superluminal propagation in Lorentz invariant theories always leads to causal paradoxes.^[22][23]

Fundamental models

In modern physics, all fundamental particles are regarded as excitations of quantum fields. There are several distinct ways in which tachyonic particles could be embedded into a field theory.

Fields with imaginary mass

In the paper that coined the term "tachyon", Gerald Feinberg studied Lorentz invariant quantum fields with imaginary mass.^[1] Because the group velocity for such a field is superluminal, naively it appears that its excitations propagate faster than light. However, it was quickly understood that the superluminal group velocity does not correspond to the speed of propagation of any localized excitation (like a particle). Instead, the negative mass represents an instability to tachyon condensation, and all excitations of the field propagate subluminally and are consistent with causality.^[5] Despite having no faster-than-light propagation, such fields are referred to simply as "tachyons" in many sources.^{[3][6][24][25][26][27]}
Tachyonic fields play an important role in modern physics. Perhaps the most famous is the Higgs boson of the Standard Model of particle physics, which—in its uncondensed phase—has an imaginary mass. In general, the phenomenon of spontaneous symmetry breaking, which is closely related to tachyon condensation, plays a very important role in many aspects of theoretical physics, including the Ginzburg–Landau and BCS theories of superconductivity. Another example of a tachyonic field is the tachyon of bosonic string theory.^[24][26][28]

Tachyons are predicted by bosonic string theory and also the NS (which is the open bosonic sector) and NS-NS (which is the closed bosonic sector) sectors of RNS Superstring theory before GSO projection. However, due to the Sen conjecture—also known as tachyon condensation—this is not possible. This resulted in the necessity for the GSO projection.

Lorentz-violating theories

In theories that do not respect Lorentz invariance the speed of light is not (necessarily) a barrier, and particles can travel faster than the speed of light without infinite energy or causal paradoxes.^[22] A class of field theories of that type are the so-called Standard Model extensions. However, the experimental evidence for Lorentz invariance is extremely good, so such theories are very tightly constrained.^[29][30]

Fields with non-canonical kinetic term

By modifying the kinetic energy of the field, it is possible to produce Lorentz invariant field theories with excitations that propagate superluminally.^[5][23] However, such theories in general do not have a well-defined Cauchy problem (for reasons related to the issues of causality discussed above), and are probably inconsistent quantum mechanically.

History

As mentioned above, the term "tachyon" was coined by Gerald Feinberg in a 1967 paper titled "Possibility of Faster-Than-Light Particles".^[1] He had been inspired by the science-fiction story "Beep" by James Blish.^[31] Feinberg studied the kinematics of such particles according to special relativity. In his paper he also introduced fields with imaginary mass (now also referred to as "tachyons") in an attempt to understand the microphysical origin such particles might have.

The first hypothesis regarding faster-than-light particles is sometimes attributed to German physicist Arnold Sommerfeld in 1904,^[32] and more recent discussions happened in 1962^[2] and 1969.^[33]

In fiction

Tachyons have appeared in many works of fiction. They have been used as a standby mechanism upon which many science fiction authors rely to establish faster-than-light communication, with or without reference to causality issues. The word tachyon has become widely recognized to such an extent that it can impart a science-fictional connotation even if the subject in question has no particular relation to superluminal travel (a form of technobabble, akin to positronic brain).
Also referenced in the movie, "K-PAX". Kevin Spacey's character claims to have traveled to Earth at Tachyon speeds. In the Watchmen comic book universe, the use of tachyons can disrupt Dr. Manhattan's ability to perceive time. Tachyons also figure prominently in the Star Trek universe, and are often associated with time travel scenarios in the Star Trek universe. Also recently mentioned in The Flash episode "the Man in the Yellow Suit".^[34]

Bill Nye

From Wikipedia, the free encyclopedia

Bill Nye
Nye at Bridgewater State College on April 10, 2007.
Born	William Sanford Nye (1955-11-27) November 27, 1955 (age 59) Washington, D.C., U.S.
Residence	Los Angeles, California, U.S.
Nationality	American
Fields	Mechanical engineering
Institutions	Boeing Cornell University Planetary Society
Alma mater	Cornell University (B.S.)
Known for	Bill Nye the Science Guy
Signature

William Sanford "Bill" Nye (born November 27, 1955), popularly known as Bill Nye the Science Guy, is an American science educator, comedian, television host, actor, writer, scientist, and former mechanical engineer, best known as the host of the Disney/PBS children's science show Bill Nye the Science Guy (1993–98) and for his many subsequent appearances in popular media as a science educator.

Early life

Nye was born on November 27, 1955,^[1][2][3] in Washington, D.C., to Jacqueline (née Jenkins; 1921–2000), a codebreaker during World War II, and Edwin Darby "Ned" Nye (1917–97), also a World War II veteran, whose experience without electricity in a Japanese prisoner of war camp led him to become a sundial enthusiast.^[4][5][6] His maternal grandmother was French, from Dancevoir.^[7]
After attending Lafayette Elementary and Alice Deal Junior High in the city, he was accepted to the private Sidwell Friends School on a partial scholarship and graduated in 1973.^[8][9] He studied mechanical engineering at Cornell University (where he took an astronomy class taught by Carl Sagan)^[10] and graduated with a bachelor of science degree in mechanical engineering in 1977.^[11] Nye occasionally returns to Cornell as a professor to guest-lecture introductory-level astronomy and human ecology classes.^[12]

Career

Nye began his career in Seattle at Boeing, where (among other things) he starred in training films and developed a hydraulic pressure resonance suppressor for the 747. Later, he worked as a consultant in the aeronautics industry. In 1999 he told the St. Petersburg Times that he applied to be a NASA astronaut every few years, but was always rejected.^[13]

The Science Guy

Nye began his professional entertainment career as a writer/actor on a local sketch comedy television show in Seattle, Washington, called Almost Live!. The host of the show, Ross Shafer, suggested he do some scientific demonstrations in a six-minute segment, and take on the nickname "The Science Guy".^[14] His other main recurring role on Almost Live! was as Speedwalker, a speedwalking Seattle superhero.
From 1991 to 1993, he appeared in the live-action educational segments of Back to the Future: The Animated Series in the nonspeaking role of assistant to Dr. Emmett Brown (played by Christopher Lloyd), in which he would demonstrate science while Lloyd explained. The segments' national popularity led to Nye's hosting an educational television program, Bill Nye the Science Guy, from 1993 to 1998. Each of the 100 episodes aimed to teach a specific topic in science to a preteen audience, yet it garnered a wide adult audience as well.^[15] With its comedic overtones, the show became popular as a teaching aid in schools. When portraying "The Science Guy", Nye wears a light blue lab coat and bow tie.

Nye has written several books as The Science Guy. In addition to hosting, he was a writer and producer for the show, which was filmed entirely in Seattle.

Nye's Science Guy persona appears alongside Ellen DeGeneres and Alex Trebek in a video at Ellen's Energy Adventure, an attraction that has played since 1996 at the Universe of Energy pavilion inside Epcot at Walt Disney World. His voice is heard in the Dinosaur attraction in Disney's Animal Kingdom park, teaching guests about the dinosaurs while they queue for the ride. He appears in video form in the "Design Lab" of CyberSpace Mountain, inside DisneyQuest at Walt Disney World, where he refers to himself as "Bill Nye the Coaster Guy."

Entertainment/edutainment

Nye and Executive Director of The Planetary Society received the Committee for Skeptical Inquiry's "In Praise of Reason" Award at CSICON 2011 in New Orleans

Nye remained interested in science education through entertainment. He played a science teacher in Disney's 1998 TV movie The Principal Takes a Holiday; he made a hovercraft to demonstrate science in an unusual classroom manner. From 2000 to 2002, Nye was the technical expert in BattleBots. In 2004 and 2005, Nye hosted 100 Greatest Discoveries, an award-winning series produced by THINKFilm for The Science Channel and in high definition on the Discovery HD Theater. He was also host of an eight-part Discovery Channel series called Greatest Inventions with Bill Nye. He created a 13-episode PBS KCTS-TV series about science, called The Eyes of Nye, aimed at an older audience than his previous show had been. Airing in 2005, it often featured episodes based on politically relevant themes such as genetically modified food, global warming, and race. Nye has guest-starred in several episodes of the crime drama Numb3rs as an engineering faculty member. A lecture Nye gave several years ago on exciting children about math was an inspiration for creating Numb3rs.^[16] He has also made guest appearances on the VH1 reality show America's Most Smartest Model.^[17]

Nye has appeared numerous times on the talk show Larry King Live, speaking about topics such as global warming and UFOs. He argued that global warming is an issue that should be addressed by governments of the world in part because it could be implicated in the record-setting 2005 Atlantic hurricane season.^[18] On UFOs, he has been skeptical of extraterrestrial explanations for sightings such as those at Roswell and Malmstrom Air Force Base in 1967.^[19]

Nye appears in segments of The Climate Code on The Weather Channel, telling his personal ways of saving energy. He still makes regular appearances on the show, often asking quiz questions. In the fall of 2008, Nye also appeared periodically on the daytime game show Who Wants to Be a Millionaire as part of the show's reintroduced "Ask the Expert" lifeline. In 2008, he also hosted Stuff Happens, a show on the then new Planet Green network. In November 2008, Nye appeared in an acting role as himself in the fifth-season episode "Brain Storm" of Stargate Atlantis alongside fellow television personality and astrophysicist Neil deGrasse Tyson.^[20]

In 2009, portions of Bill Nye's shows were used as lyrics and portions of the second Symphony of Science music education video by composer John Boswell. Nye recorded a short YouTube video (as himself, not his TV persona) advocating clean-energy climate-change legislation on behalf of Al Gore's Repower America campaign in October 2009.^[21] Bill joined the American Optometric Association in a multimedia advertising campaign to persuade parents to get their children comprehensive eye examinations.^[22] Nye made an appearance in Palmdale's 2010 video "Here Comes the Summer";^[23] the band's lead singer Kay Hanley is his neighbor. Nye (as his TV persona) also made a guest appearance on The Dr. Oz Show.

In September 2012, Nye claimed that creationist views threaten science education and innovation in the United States.^[24][25][26]

In February 2014, Nye debated creationist Ken Ham at the Creation Museum on the topic of whether creation is a viable model of origins in today's modern, scientific era.^[27][28][29]

On February 28, 2014, Nye was a celebrity guest and interviewer at the White House Student Film Festival.^[30]

Scientific work

Nye orating in October 2010.

In the early 2000s, Nye assisted in the development of a small sundial that was included in the Mars Exploration Rover missions.^[2] Known as MarsDial, it included small colored panels to provide a basis for color calibration in addition to helping keep track of time.^[31] From 2005 to 2010, Nye was the vice president of The Planetary Society, an organization that advocates space science research and the exploration of other planets, particularly Mars.^[32] He became the organization's second Executive Director in September 2010 when Louis Friedman stepped down.^[33][34]

In November 2010, Nye became the face of a new permanent exhibition at the Chabot Space & Science Center in Oakland, California. Bill Nye’s Climate Lab features Nye as commander of the Clean Energy Space Station, and invites visitors on an urgent mission to thwart climate change. Beginning with a view of Planet Earth from space, visitors explore air, water, and land galleries to discover how climate change affects Earth’s connected systems, and how to use the Sun, wind, land, and water to generate clean energy. In an interview about the exhibit, Nye said, “Everything in the exhibit is geared to showing you that the size of the problem of climate change is big. Showing you a lot about energy use ... It’s a huge opportunity ... We need young people, entrepreneurs, young inventors, young innovators to change the world.”^[35]

Nye with the Chief of Naval Research Rear. Adm. Nevin Carr following the presentation of a "Powered by Naval Research" pocket protector during the Navy Office of General Counsel Spring 2011 Conference.

Nye gave a solar noon clock atop Rhodes Hall to Cornell on Aug 27 following a public lecture that filled the 715-seat Statler Auditorium. Nye talked about his father's passion for sundials and timekeeping, his time at Cornell, his work on the sundials mounted on the Mars rovers and the story behind the Bill Nye Solar Noon Clock.^[36] Bill Nye conducted a Q&A session after the 2012 Mars Rover Landing.^[37]

Nye holds several United States patents,^[38] including one for ballet pointe shoes^[32] and another for an educational magnifying glass created by filling a clear plastic bag with water.^[39][40] From 2001 to 2006, Nye served as Frank H.T. Rhodes Class of '56 University Professor at Cornell University.^[11][41] Nye supported the 2006 reclassification of Pluto from planet to dwarf planet by the International Astronomical Union.^[42]

Nye is a fellow of the Committee for Skeptical Inquiry, a U.S. non-profit scientific and educational organization whose aim is to promote scientific inquiry, critical investigation, and the use of reason in examining controversial and extraordinary claims.^[43] Interviewed by John Rael for the Independent Investigation Group IIG, Nye stated that his "concern right now... scientific illiteracy... you [the public] don't have enough rudimentary knowledge of the universe to evaluate claims."^[44] In November 2012, Nye launched a Kickstarter project for an educational Aerodynamics game, AERO 3D. The project was not funded.^[45] His book, Undeniable: Evolution and the Science of Creation, was released on November 4, 2014.^[46]

Dancing with the Stars

Nye was a contestant in the 17th season of Dancing with the Stars in 2013, partnering newcomer Tyne Stecklein. They were eliminated early in the season after Nye sustained an injury to his quadriceps tendon on Week 3.^[47]

Dance	Score	Music	Result
Cha-cha-cha	14(5-4-5)	"Weird Science"—Oingo Boingo	No Elimination
Paso Doble	17(6-5-6)	"Symphony No. 5"—Ludwig van Beethoven	Safe
Jazz	16(6-5-5)	"Get Lucky"—Daft Punk feat. Pharrell Williams	Eliminated

Personal life

Since 2006, Nye has lived in Los Angeles, though he has also owned a house on Mercer Island.^[48] As of July 2007, Nye and environmental activist Ed Begley, Jr. are engaging in a friendly competition "to see who could have the lowest carbon footprint," according to Begley.^[49] In a 2008 interview, Nye joked that he wants to "crush Ed Begley" in their environmental competition.^[50] Nye and Begley are neighbors in Los Angeles, and sometimes dine together at a local vegetarian restaurant.^[50] Nye often appears on Begley's Planet Green reality show Living with Ed. Nye enjoys baseball and occasionally does experiments involving the physics of the game. As a longtime Seattle resident before becoming an entertainer, he is said to have been a fan of the Seattle Mariners, although recently he has voiced his preference (as a D.C. native) for the Washington Nationals.^[8] He also played Ultimate while in college and for a period of time while living in Seattle.^[51] In July 2012, Nye endorsed President Barack Obama's reelection bid.^[52]

Nye announced his engagement during an appearance on The Late Late Show with Craig Ferguson and was married to his fiancée of five months, musician Blair Tindall, on February 3, 2006. The ceremony was performed by Rick Warren at The Entertainment Gathering at the Skirball Cultural Center in Los Angeles. Yo-Yo Ma provided the music.^[53] Nye left the relationship seven weeks later when the marriage license was declared invalid.^[54] In 2007, Nye received a protective order against Tindall after an incident in which she came onto his property and used herbicide to damage his garden. Tindall admitted this, but denied being a threat to him.^[55] In 2012, Nye sued Tindall for unpaid attorney's fees he incurred while he went to court in 2009 to enforce the protective order against Tindall after she allegedly violated it. According to Nye's court filings, she was ordered to pay these fees; to date, she has not paid any of it.^[56]

Nye is an avid swing dancer.^[57] He has been spotted at local dances in the Los Angeles area as well as at nationwide events such as entompology (Rochester, NY).^[58] Nye describes himself as agnostic.^[59]

Awards and honors

In May 1999, Nye was the commencement speaker at Rensselaer Polytechnic Institute where he was awarded an honorary doctor of science degree.^[60] He was awarded an honorary doctorate by Johns Hopkins University in May 2008.^[61] In May 2011, he received an honorary doctor of science degree from Willamette University, where he was the keynote speaker for that year's commencement exercises.^[62] In addition, Bill Nye also received an honorary doctor of pedagogy degree from Lehigh University on May 20, 2013, at the commencement ceremony.^[63] Nye received the 2010 Humanist of the Year Award from the American Humanist Association.^[64]

Blood

From Wikipedia, the free encyclopedia

Blood
Human blood smear: a – erythrocytes; b – neutrophil; c – eosinophil; d – lymphocyte.
A scanning electron microscope (SEM) image of a normal red blood cell, a platelet, and a white blood cell.
Details
Latin	haema
Identifiers
TA	A12.0.00.009
FMA	9670
Anatomical terminology

Human blood fractioned by centrifugation. Plasma (upper, yellow layer), buffy coat (middle, thin white layer) and erythrocyte layer (bottom, red layer) can be seen.^{[citation needed]}

Blood circulation:
Red = oxygenated
Blue = deoxygenated

Blood is a bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. When it reaches the lungs, gas exchange occurs when carbon dioxide is diffused out of the blood into the alveoli and oxygen is diffused into the blood. This oxygenated blood is pumped to the left hand side of the heart in the pulmonary vein and enters the left atrium. From here it passes through the bicuspid valve, through the ventricle and taken all around the body by the aorta. Blood contains antibodies, nutrients, oxygen and much more to help the body work.

In vertebrates, it is composed of blood cells suspended in blood plasma. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume),^[1] and contains dissipated proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), and blood cells themselves. Albumin is the main protein in plasma, and it functions to regulate the colloidal osmotic pressure of blood. The blood cells are mainly red blood cells (also called RBCs or erythrocytes) and white blood cells, including leukocytes and platelets. The most abundant cells in vertebrate blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates oxygen transport by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is almost entirely transported extracellularly dissolved in plasma as bicarbonate ion.

Vertebrate blood is bright red when its hemoglobin is oxygenated and dark red when it is deoxygenated. Some animals, such as crustaceans and mollusks, use hemocyanin to carry oxygen, instead of hemoglobin. Insects and some mollusks use a fluid called hemolymph instead of blood, the difference being that hemolymph is not contained in a closed circulatory system. In most insects, this "blood" does not contain oxygen-carrying molecules such as hemoglobin because their bodies are small enough for their tracheal system to suffice for supplying oxygen.

Jawed vertebrates have an adaptive immune system, based largely on white blood cells. White blood cells help to resist infections and parasites. Platelets are important in the clotting of blood. Arthropods, using hemolymph, have hemocytes as part of their immune system.

Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals with lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.

Medical terms related to blood often begin with hemo- or hemato- (also spelled haemo- and haemato-) from the Greek word αἷμα (haima) for "blood". In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its origin in the bones and the presence of potential molecular fibers in the form of fibrinogen.

Functions

Haemoglobin, a globular protein
green = haem groups
red & blue = protein subunits

Heme

Blood performs many important functions within the body including:

• Supply of oxygen to tissues (bound to hemoglobin, which is carried in red cells)
• Supply of nutrients such as glucose, amino acids, and fatty acids (dissolved in the blood or bound to plasma proteins (e.g., blood lipids))
• Removal of waste such as carbon dioxide, urea, and lactic acid
• Immunological functions, including circulation of white blood cells, and detection of foreign material by antibodies
• Coagulation, the response to a broken blood vessel, the conversion of blood from a liquid to a semi-solid gel to stop bleeding.
• Messenger functions, including the transport of hormones and the signaling of tissue damage
• Regulation of body pH
• Regulation of core body temperature
• Hydraulic functions

Constituents of human blood

Illustration depicting formed elements of blood.

Two tubes of EDTA-anticoagulated blood.
Left tube: after standing, the RBCs have settled at the bottom of the tube.
Right tube: contains freshly drawn blood.

Blood accounts for 7% of the human body weight,^[2][3] with an average density of approximately 1060 kg/m³, very close to pure water's density of 1000 kg/m³.^[4] The average adult has a blood volume of roughly 5 litres (11 US pt),^[3] which is composed of plasma and several kinds of cells. These blood cells (which are also called corpuscles or "formed elements") consist of erythrocytes (red blood cells, RBCs), leukocytes (white blood cells), and thrombocytes (platelets). By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, and white cells about 0.7%.

Whole blood (plasma and cells) exhibits non-Newtonian fluid dynamics; its flow properties are adapted to flow effectively through tiny capillary blood vessels with less resistance than plasma by itself. In addition, if all human hemoglobin were free in the plasma rather than being contained in RBCs, the circulatory fluid would be too viscous for the cardiovascular system to function effectively.

Cells

One microliter of blood contains:

• 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes:^[5] Red blood cells contain the blood's hemoglobin and distribute oxygen. Mature red blood cells lack a nucleus and organelles in mammals. The red blood cells (together with endothelial vessel cells and other cells) are also marked by glycoproteins that define the different blood types. The proportion of blood occupied by red blood cells is referred to as the hematocrit, and is normally about 45%. The combined surface area of all red blood cells of the human body would be roughly 2,000 times as great as the body's exterior surface.^[6]
• 4,000–11,000 leukocytes:^[7] White blood cells are part of the body's immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances. The cancer of leukocytes is called leukemia.
• 200,000–500,000 thrombocytes:^[7] Also called platelets, they take part in blood clotting (coagulation). Fibrin from the coagulation cascade creates a mesh over the platelet plug.

Constitution of normal blood

Parameter	Value
Hematocrit	45 ± 7 (38–52%) for males 42 ± 5 (37–47%) for females
pH	7.35–7.45
base excess	−3 to +3
PO2	10–13 kPa (80–100 mm Hg)
PCO2	4.8–5.8 kPa (35–45 mm Hg)
HCO3−	21–27 mM
Oxygen saturation	Oxygenated: 98–99% Deoxygenated: 75%

Plasma

About 55% of blood is blood plasma, a fluid that is the blood's liquid medium, which by itself is straw-yellow in color. The blood plasma volume totals of 2.7–3.0 liters (2.8–3.2 quarts) in an average human. 

It is essentially an aqueous solution containing 92% water, 8% blood plasma proteins, and trace amounts of other materials. Plasma circulates dissolved nutrients, such as glucose, amino acids, and fatty acids (dissolved in the blood or bound to plasma proteins), and removes waste products, such as carbon dioxide, urea, and lactic acid.Other important components include:

• Serum albumin
• Blood-clotting factors (to facilitate coagulation)
• Immunoglobulins (antibodies)
• lipoprotein particles
• Various other proteins
• Various electrolytes (mainly sodium and chloride)

The term serum refers to plasma from which the clotting proteins have been removed. Most of the proteins remaining are albumin and immunoglobulins.

Narrow range of pH values

Blood pH is regulated to stay within the narrow range of 7.35 to 7.45, making it slightly basic.^[8][9] Blood that has a pH below 7.35 is too acidic, whereas blood pH above 7.45 is too basic. Blood pH, partial pressure of oxygen (pO₂), partial pressure of carbon dioxide (pCO₂), and HCO₃⁻ are carefully regulated by a number of homeostatic mechanisms, which exert their influence principally through the respiratory system and the urinary system in order to control the acid-base balance and respiration. An arterial blood gas test will measure these. Plasma also circulates hormones transmitting their messages to various tissues. The list of normal reference ranges for various blood electrolytes is extensive.

Blood in non-mammalian vertebrates

Vertebrate red blood cell types, measurements in micrometers

Frog red blood cells magnified 1000 times

Turtle red blood cells magnified 1000 times

Chicken red blood cells magnified 1000 times

Human red blood cells magnified 1000 times

Human blood is typical of that of mammals, although the precise details concerning cell numbers, size, protein structure, and so on, vary somewhat between species. In non-mammalian vertebrates, however, there are some key differences:^[10]

• Red blood cells of non-mammalian vertebrates are flattened and ovoid in form, and retain their cell nuclei
• There is considerable variation in the types and proportions of white blood cells; for example, acidophils are generally more common than in humans
• Platelets are unique to mammals; in other vertebrates, small nucleated, spindle cells called thrombocytes are responsible for blood clotting instead

Physiology

Cardiovascular system

The circulation of blood through the human heart

Blood is circulated around the body through blood vessels by the pumping action of the heart. In humans, blood is pumped from the strong left ventricle of the heart through arteries to peripheral tissues and returns to the right atrium of the heart through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air to all of the cells of the body, and venous blood carries carbon dioxide, a waste product of metabolism by cells, to the lungs to be exhaled. However, one exception includes pulmonary arteries, which contain the most deoxygenated blood in the body, while the pulmonary veins contain oxygenated blood.

Additional return flow may be generated by the movement of skeletal muscles, which can compress veins and push blood through the valves in veins toward the right atrium.

The blood circulation was famously described by William Harvey in 1628.^[11]

Production and degradation of blood cells

In vertebrates, the various cells of blood are made in the bone marrow in a process called hematopoiesis, which includes erythropoiesis, the production of red blood cells; and myelopoiesis, the production of white blood cells and platelets. During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs. In addition, during childhood, the thymus gland, found in the mediastinum, is an important source of T lymphocytes.^[12] The proteinaceous component of blood (including clotting proteins) is produced predominantly by the liver, while hormones are produced by the endocrine glands and the watery fraction is regulated by the hypothalamus and maintained by the kidney.

Healthy erythrocytes have a plasma life of about 120 days before they are degraded by the spleen, and the Kupffer cells in the liver. The liver also clears some proteins, lipids, and amino acids. The kidney actively secretes waste products into the urine.

Oxygen transport

Basic hemoglobin saturation curve. It is moved to the right in higher acidity (more dissolved carbon dioxide) and to the left in lower acidity (less dissolved carbon dioxide)

About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with the Hgb. About 1.5% is physically dissolved in the other blood liquids and not connected to Hgb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below). Hemoglobin has an oxygen binding capacity of between 1.36 and 1.37 ml O₂ per gram hemoglobin,^[13] which increases the total blood oxygen capacity seventyfold,^[14] compared to if oxygen solely were carried by its solubility of 0.03 ml O₂ per liter blood per mm Hg partial pressure of oxygen (approximately 100 mm Hg in arteries).^[14]

With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules, and veins carry deoxygenated blood back to the heart.

Under normal conditions in adult humans at rest; hemoglobin in blood leaving the lungs is about 98–99% saturated with oxygen, achieving an oxygen delivery of between 950 and 1150 ml/min^[15] to the body. In a healthy adult at rest, oxygen consumption is approximately 200 - 250 ml/min,^[15] and deoxygenated blood returning to the lungs is still approximately 75%^[16][17] (70 to 78%)^[15] saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions.^[18] Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia). Sustained hypoxia (oxygenation of less than 90%), is dangerous to health, and severe hypoxia (saturations of less than 30%) may be rapidly fatal.^[19]

A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 21% of the level found in an adult's lungs), and, so, fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to function under these conditions.^[20]

Carbon dioxide transport

CO₂ is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood). Most of it (about 70%) is converted to bicarbonate ions HCO−
3 by the enzyme carbonic anhydrase in the red blood cells by the reaction CO₂ + H₂O → H₂CO₃ → H⁺ + HCO−
3; about 7% is dissolved in the plasma; and about 23% is bound to hemoglobin as carbamino compounds.^[21] Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO₂ bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO₂ decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane effect, and is important in the transport of carbon dioxide from the tissues to the lungs. A rise in the partial pressure of CO₂ or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr effect.

Transport of hydrogen ions

Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin binds most of the hydrogen ions as it has a much greater affinity for more hydrogen than does oxyhemoglobin.

Lymphatic system

In mammals, blood is in equilibrium with lymph, which is continuously formed in tissues from blood by capillary ultrafiltration. Lymph is collected by a system of small lymphatic vessels and directed to the thoracic duct, which drains into the left subclavian vein where lymph rejoins the systemic blood circulation.

Thermoregulation

Blood circulation transports heat throughout the body, and adjustments to this flow are an important part of thermoregulation. Increasing blood flow to the surface (e.g., during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss. In contrast, when the external temperature is low, blood flow to the extremities and surface of the skin is reduced and to prevent heat loss and is circulated to the important organs of the body, preferentially.

Hydraulic functions

The restriction of blood flow can also be used in specialized tissues to cause engorgement, resulting in an erection of that tissue; examples are the erectile tissue in the penis and clitoris.

Another example of a hydraulic function is the jumping spider, in which blood forced into the legs under pressure causes them to straighten for a powerful jump, without the need for bulky muscular legs.^[22]

Invertebrates

In insects, the blood (more properly called hemolymph) is not involved in the transport of oxygen. (Openings called tracheae allow oxygen from the air to diffuse directly to the tissues). Insect blood moves nutrients to the tissues and removes waste products in an open system.

Other invertebrates use respiratory proteins to increase the oxygen-carrying capacity. Hemoglobin is the most common respiratory protein found in nature. Hemocyanin (blue) contains copper and is found in crustaceans and mollusks. It is thought that tunicates (sea squirts) might use vanabins (proteins containing vanadium) for respiratory pigment (bright-green, blue, or orange).

In many invertebrates, these oxygen-carrying proteins are freely soluble in the blood; in vertebrates they are contained in specialized red blood cells, allowing for a higher concentration of respiratory pigments without increasing viscosity or damaging blood filtering organs like the kidneys.

Giant tube worms have unusual hemoglobins that allow them to live in extraordinary environments. These hemoglobins also carry sulfides normally fatal in other animals.

Color

The coloring matter of blood (hemochrome) is largely due to the protein in the blood responsible for oxygen transport. Different groups of organisms use different proteins.

Hemoglobin

Capillary blood from a bleeding finger

Venous blood collected during blood donation

Hemoglobin is the principal determinant of the color of blood in vertebrates. Each molecule has four heme groups, and their interaction with various molecules alters the exact color. In vertebrates and other hemoglobin-using creatures, arterial blood and capillary blood are bright red, as oxygen imparts a strong red color to the heme group. Deoxygenated blood is a darker shade of red; this is present in veins, and can be seen during blood donation and when venous blood samples are taken. This is because the spectrum of light absorbed by hemoglobin differs between the oxygenated and deoxygenated states.^[23]

Blood in carbon monoxide poisoning is bright red, because carbon monoxide causes the formation of carboxyhemoglobin. In cyanide poisoning, the body cannot utilize oxygen, so the venous blood remains oxygenated, increasing the redness. There are some conditions affecting the heme groups present in hemoglobin that can make the skin appear blue—a symptom called cyanosis. If the heme is oxidized, methaemoglobin, which is more brownish and cannot transport oxygen, is formed. In the rare condition sulfhemoglobinemia, arterial hemoglobin is partially oxygenated, and appears dark red with a bluish hue.

Veins close to the surface of the skin appear blue for a variety of reasons. However, the factors that contribute to this alteration of color perception are related to the light-scattering properties of the skin and the processing of visual input by the visual cortex, rather than the actual color of the venous blood.^[24]

Skinks in the genus Prasinohaema have green blood due to a buildup of the waste product biliverdin.^[25]

Hemocyanin

The blood of most mollusks – including cephalopods and gastropods – as well as some arthropods, such as horseshoe crabs, is blue, as it contains the copper-containing protein hemocyanin at concentrations of about 50 grams per liter.^[26] Hemocyanin is colorless when deoxygenated and dark blue when oxygenated. The blood in the circulation of these creatures, which generally live in cold environments with low oxygen tensions, is grey-white to pale yellow,^[26] and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed.^[26] This is due to change in color of hemocyanin when it is oxidized.^[26] Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in mammals by hemoglobin in RBCs.^[26]

Chlorocruorin

The blood of most annelid worms and some marine polychaetes use chlorocruorin to transport oxygen. It is green in color in dilute solutions.^[27]

Hemerythrin

Hemerythrin is used for oxygen transport in the marine invertebrates sipunculids, priapulids, brachiopods, and the annelid worm, magelona. Hemerythrin is violet-pink when oxygenated.^[27]

Hemovanadin

The blood of some species of ascidians and tunicates, also known as sea squirts, contains proteins called vanabins. These proteins are based on vanadium, and give the creatures a concentration of vanadium in their bodies 100 times higher than the surrounding sea water. Unlike hemocyanin and hemoglobin, hemovanadin is not an oxygen carrier. When exposed to oxygen, however, vanabins turn a mustard yellow.

Pathology

General medical disorders

• Disorders of volume
  • Injury can cause blood loss through bleeding.^[28] A healthy adult can lose almost 20% of blood volume (1 L) before the first symptom, restlessness, begins, and 40% of volume (2 L) before shock sets in. Thrombocytes are important for blood coagulation and the formation of blood clots, which can stop bleeding. Trauma to the internal organs or bones can cause internal bleeding, which can sometimes be severe.
  • Dehydration can reduce the blood volume by reducing the water content of the blood. This would rarely result in shock (apart from the very severe cases) but may result in orthostatic hypotension and fainting.
• Disorders of circulation
  • Shock is the ineffective perfusion of tissues, and can be caused by a variety of conditions including blood loss, infection, poor cardiac output.
  • Atherosclerosis reduces the flow of blood through arteries, because atheroma lines arteries and narrows them. Atheroma tends to increase with age, and its progression can be compounded by many causes including smoking, high blood pressure, excess circulating lipids (hyperlipidemia), and diabetes mellitus.
  • Coagulation can form a thrombosis, which can obstruct vessels.
  • Problems with blood composition, the pumping action of the heart, or narrowing of blood vessels can have many consequences including hypoxia (lack of oxygen) of the tissues supplied. The term ischemia refers to tissue that is inadequately perfused with blood, and infarction refers to tissue death (necrosis), which can occur when the blood supply has been blocked (or is very inadequate)

Hematological disorders

• Anemia
  • Insufficient red cell mass (anemia) can be the result of bleeding, blood disorders like thalassemia, or nutritional deficiencies; and may require blood transfusion. Several countries have blood banks to fill the demand for transfusable blood. A person receiving a blood transfusion must have a blood type compatible with that of the donor.
  • Sickle-cell anemia
• Disorders of cell proliferation
  • Leukemia is a group of cancers of the blood-forming tissues and cells.
  • Non-cancerous overproduction of red cells (polycythemia vera) or platelets (essential thrombocytosis) may be premalignant.
  • Myelodysplastic syndromes involve ineffective production of one or more cell lines.
• Disorders of coagulation
  • Hemophilia is a genetic illness that causes dysfunction in one of the blood's clotting mechanisms. This can allow otherwise inconsequential wounds to be life-threatening, but more commonly results in hemarthrosis, or bleeding into joint spaces, which can be crippling.
  • Ineffective or insufficient platelets can also result in coagulopathy (bleeding disorders).
  • Hypercoagulable state (thrombophilia) results from defects in regulation of platelet or clotting factor function, and can cause thrombosis.
• Infectious disorders of blood
  • Blood is an important vector of infection. HIV, the virus that causes AIDS, is transmitted through contact with blood, semen or other body secretions of an infected person. Hepatitis B and C are transmitted primarily through blood contact. Owing to blood-borne infections, bloodstained objects are treated as a biohazard.
  • Bacterial infection of the blood is bacteremia or sepsis. Viral Infection is viremia. Malaria and trypanosomiasis are blood-borne parasitic infections.

Carbon monoxide poisoning

Substances other than oxygen can bind to hemoglobin; in some cases this can cause irreversible damage to the body. Carbon monoxide, for example, is extremely dangerous when carried to the blood via the lungs by inhalation, because carbon monoxide irreversibly binds to hemoglobin to form carboxyhemoglobin, so that less hemoglobin is free to bind oxygen, and fewer oxygen molecules can be transported throughout the blood. This can cause suffocation insidiously. A fire burning in an enclosed room with poor ventilation presents a very dangerous hazard, since it can create a build-up of carbon monoxide in the air. Some carbon monoxide binds to hemoglobin when smoking tobacco.^{[citation needed]}

Medical treatments

Blood products

Blood for transfusion is obtained from human donors by blood donation and stored in a blood bank. There are many different blood types in humans, the ABO blood group system, and the Rhesus blood group system being the most important. Transfusion of blood of an incompatible blood group may cause severe, often fatal, complications, so crossmatching is done to ensure that a compatible blood product is transfused.
Other blood products administered intravenously are platelets, blood plasma, cryoprecipitate, and specific coagulation factor concentrates.

Intravenous administration

Many forms of medication (from antibiotics to chemotherapy) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.

After severe acute blood loss, liquid preparations, generically known as plasma expanders, can be given intravenously, either solutions of salts (NaCl, KCl, CaCl₂ etc.) at physiological concentrations, or colloidal solutions, such as dextrans, human serum albumin, or fresh frozen plasma. In these emergency situations, a plasma expander is a more effective life-saving procedure than a blood transfusion, because the metabolism of transfused red blood cells does not restart immediately after a transfusion.

Bloodletting

In modern evidence-based medicine, bloodletting is used in management of a few rare diseases, including hemochromatosis and polycythemia. However, bloodletting and leeching were common unvalidated interventions used until the 19th century, as many diseases were incorrectly thought to be due to an excess of blood, according to Hippocratic medicine.

History

Jan Janský is credited with the first classification of blood into the four types (A, B, AB, O)

According to the Oxford English Dictionary, the word "blood" dates to the oldest English, circa 1000 AD. The word is derived from Middle English, which is derived from the Old English word blôd, which is akin to the Old High German word bluot, meaning blood. The modern German word is (das) Blut.

Classical Greek medicine

Fåhræus (a Swedish physician who devised the erythrocyte sedimentation rate) suggested that the Ancient Greek system of humorism, wherein the body was thought to contain four distinct bodily fluids (associated with different temperaments), were based upon the observation of blood clotting in a transparent container. When blood is drawn in a glass container and left undisturbed for about an hour, four different layers can be seen. A dark clot forms at the bottom (the "black bile"). Above the clot is a layer of red blood cells (the "blood"). Above this is a whitish layer of white blood cells (the "phlegm"). The top layer is clear yellow serum (the "yellow bile").^{[29][not in citation given]}

Human blood

The ABO blood group system was discovered in the year 1900 by Karl Landsteiner. Jan Janský is credited with the first classification of blood into the four types (A, B, AB, O) in 1907, which remains in use today. In 1907 the first successful blood transfusion was performed. The first non-direct transfusion was performed on March 27, 1914. The Rhesus factor was discovered in 1937.

Cultural and religious beliefs

Due to its importance to life, blood is associated with a large number of beliefs. One of the most basic is the use of blood as a symbol for family relationships through birth/parentage; to be "related by blood" is to be related by ancestry or descendance, rather than marriage. This bears closely to bloodlines, and sayings such as "blood is thicker than water" and "bad blood", as well as "Blood brother".

Blood is given particular emphasis in the Jewish and Christian religions, because Leviticus 17:11 says "the life of a creature is in the blood." This phrase is part of the Levitical law forbidding the drinking of blood or eating meat with the blood still intact instead of being poured off.

Mythic references to blood can sometimes be connected to the life-giving nature of blood, seen in such events as childbirth, as contrasted with the blood of injury or death.

Indigenous Australians

In many indigenous Australian Aboriginal peoples' traditions, ochre (particularly red) and blood, both high in iron content and considered Maban, are applied to the bodies of dancers for ritual. As Lawlor states:

In many Aboriginal rituals and ceremonies, red ochre is rubbed all over the naked bodies of the dancers. In secret, sacred male ceremonies, blood extracted from the veins of the participant's arms is exchanged and rubbed on their bodies. Red ochre is used in similar ways in less-secret ceremonies. Blood is also used to fasten the feathers of birds onto people's bodies. Bird feathers contain a protein that is highly magnetically sensitive.^[30]

Lawlor comments that blood employed in this fashion is held by these peoples to attune the dancers to the invisible energetic realm of the Dreamtime. Lawlor then connects these invisible energetic realms and magnetic fields, because iron is magnetic.

European paganism

Among the Germanic tribes, blood was used during their sacrifices; the Blóts. The blood was considered to have the power of its originator, and, after the butchering, the blood was sprinkled on the walls, on the statues of the gods, and on the participants themselves. This act of sprinkling blood was called blóedsian in Old English, and the terminology was borrowed by the Roman Catholic Church becoming to bless and blessing. The Hittite word for blood, ishar was a cognate to words for "oath" and "bond", see Ishara. The Ancient Greeks believed that the blood of the gods, ichor, was a substance that was poisonous to mortals.

As a relic of Germanic Law the cruentation, an ordeal where the corpse of the victim was supposed to start bleeding in the presence of the murderer was used until the early 17th century.

Christianity

In Genesis 9:4, God prohibited Noah and his sons from eating blood (see Noahide Law). This command continued to be observed by the Eastern Orthodox.

It is also found in the Bible that when the Angel of Death came around to the Hebrew house that the first-born child would not die if the angel saw lamb's blood wiped across the doorway.

At the Council of Jerusalem, the apostles prohibited certain Christians from consuming blood—this is documented in Acts 15:20 and 29. This chapter specifies a reason (especially in verses 19-21): It was to avoid offending Jews who had become Christians, because the Mosaic Law Code prohibited the practice.

Christ's blood is the means for the atonement of sins. Also, ″… the blood of Jesus Christ his [God] Son cleanseth us from all sin." (1 John 1:7), “… Unto him [God] that loved us, and washed us from our sins in his own blood." (Revelation 1:5), and "And they overcame him (Satan) by the blood of the Lamb [Jesus the Christ], and by the word of their testimony …” (Revelation 12:11).

Some Christian churches, including Roman Catholicism, Eastern Orthodoxy, Oriental Orthodoxy, and the Assyrian Church of the East teach that, when consecrated, the Eucharistic wine actually becomes the blood of Jesus for worshippers to drink. Thus in the consecrated wine, Jesus becomes spiritually and physically present. This teaching is rooted in the Last Supper, as written in the four gospels of the Bible, in which Jesus stated to his disciples that the bread that they ate was his body, and the wine was his blood. "This cup is the new testament in my blood, which is shed for you." (Luke 22:20).

Most forms of Protestantism, especially those of a Wesleyan or Presbyterian lineage, teach that the wine is no more than a symbol of the blood of Christ, who is spiritually but not physically present. Lutheran theology teaches that the body and blood is present together "in, with, and under" the bread and wine of the Eucharistic feast.

Judaism

In Judaism, animal blood may not be consumed even in the smallest quantity (Leviticus 3:17 and elsewhere); this is reflected in Jewish dietary laws (Kashrut). Blood is purged from meat by rinsing and soaking in water (to loosen clots), salting and then rinsing with water again several times.^[31]
Eggs must also be checked and any blood spots removed before consumption.^[32] Although blood from fish is Biblically kosher, it is rabbinically forbidden to consume fish blood to avoid the appearance of breaking the Biblical prohibition.^[33]

Another ritual involving blood involves the covering of the blood of fowl and game after slaughtering (Leviticus 17:13); the reason given by the Torah is: "Because the life of the animal is [in] its blood" (ibid 17:14). In relation to human beings, Kabbalah expounds on this verse that the animal soul of a person is in the blood, and that physical desires stem from it.

Likewise, the mystical reason for salting temple sacrifices and slaughtered meat is to remove the blood of animal-like passions from the person. By removing the animal's blood, the animal energies and life-force contained in the blood are removed, making the meat fit for human consumption.^[34]

Islam

Consumption of food containing blood is forbidden by Islamic dietary laws. This is derived from the statement in the Qur'an, sura Al-Ma'ida (5:3): "Forbidden to you (for food) are: dead meat, blood, the flesh of swine, and that on which has been invoked the name of other than Allah."

Blood is considered unclean, hence there are specific methods to obtain physical and ritual status of cleanliness once bleeding has occurred. Specific rules and prohibitions apply to menstruation, postnatal bleeding and irregular vaginal bleeding. In modern times, however, blood transfusions are generally not considered against the rules.

Jehovah's Witnesses

Based on their interpretation of scriptures such as Acts 15:28, 29 ("Keep abstaining...from blood."), Jehovah's Witnesses neither consume blood nor accept transfusions of whole blood or its major components: red blood cells, white blood cells, platelets (thrombocytes), and plasma. Members may personally decide whether they will accept medical procedures that involve their own blood or substances that are further fractionated from the four major components.^[35]

East Asian culture

In south East Asian popular culture, it is often said that if a man's nose produces a small flow of blood, he is experiencing sexual desire. This often appears in Chinese-language and Hong Kong films as well as in Japanese and Korean culture parodied in anime, manga, and drama. Characters, mostly males, will often be shown with a nosebleed if they have just seen someone nude or in little clothing, or if they have had an erotic thought or fantasy; this is based on the idea that a male's blood pressure will spike dramatically when aroused.^{[36][unreliable source?]}

Vampire legends

Vampires are mythical creatures that drink blood directly for sustenance, usually with a preference for human blood. Cultures all over the world have myths of this kind; for example the 'Nosferatu' legend, a human who achieves damnation and immortality by drinking the blood of others, originates from Eastern European folklore. Ticks, leeches, female mosquitoes, vampire bats, and an assortment of other natural creatures do consume the blood of other animals, but only bats are associated with vampires. This has no relation to vampire bats, which are new world creatures discovered well after the origins of the European myths.

Applications

In the applied sciences

Blood residue can help forensic investigators identify weapons, reconstruct a criminal action, and link suspects to the crime. Through bloodstain pattern analysis, forensic information can also be gained from the spatial distribution of bloodstains.

Blood residue analysis is also a technique used in archeology.

In art

Blood is one of the body fluids that has been used in art.^[37] In particular, the performances of Viennese Actionist Hermann Nitsch, Istvan Kantor, Franko B, Lennie Lee, Ron Athey, Yang Zhichao, Lucas Abela and Kira O' Reilly, along with the photography of Andres Serrano, have incorporated blood as a prominent visual element. Marc Quinn has made sculptures using frozen blood, including a cast of his own head made using his own blood.

In genealogy and family history

The term blood is used in genealogical circles to refer to one's ancestry, origins, and ethnic background as in the word bloodline. Other terms where blood is used in a family history sense are blue-blood, royal blood, mixed-blood and blood relative.