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{{short description|Organic fluid which transports nutrients throughout the organism}}

'''Blood''' is a body fluid in humans and other animals that delivers necessary nutrients to cells and transports their waste products away.<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/blood |title=Definition of BLOOD |access-date=4 March 2017 |url-status=live |archive-url=https://web.archive.org/web/20170323030751/https://www.merriam-webster.com/dictionary/blood |archive-date=23 March 2017 |df=dmy-all }}</ref>

Vertebrate blood is composed of a variety of blood cells suspended in [[w:blood plasma|plasma]]. Plasma, which constitutes 55% of blood fluid, is mostly [[water ]] (92% by volume)<ref>
{{cite web |url=http://www.fi.edu/learn/heart/blood/blood.html |title=Blood – The Human Heart |access-date=19 March 2009 |author=The Franklin Institute Inc. |url-status=dead |archive-url=https://web.archive.org/web/20090305043654/http://www.fi.edu/learn/heart/blood/blood.html |archive-date=5 March 2009 |df=dmy-all}}</ref> and colored [[red]] because of its [[w:hemoglobin|hemoglobin]], an [[iron]]-containing protein. 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 [[w:hemocyanin|hemocyanin]] to carry oxygen instead of hemoglobin. Insects and some mollusks use a fluid called [[w:hemolymph|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. Blood originates from a vertebrate's bones and acts as a connective tissue across the entire body.

==Functions==
[[File:1GZX Haemoglobin.png|right|thumb|Hemoglobin, a globular protein<br />green = haem (or heme) groups<br />red & blue = protein subunits]]
Blood performs many important functions within the body, including:
* Supply of [[oxygen]] to tissues
* Supply of nutrients
* Removal of waste
* Immunological functions, including circulation of [[white blood cell]]s, and detection of foreign material by antibodies
* Coagulation, the response to a broken blood vessel, the conversion of blood from a liquid to a semisolid gel to stop bleeding
* Messenger functions, including the transport of hormones and the signaling of tissue damage
* Hydraulic functions (erectile tissue)
* Thermoregulation by changing blood flow to the skin

==Physiology==
===Cardiovascular system===
[[File:Diagram of the human heart (cropped).svg|thumb|Circulation of blood through the human heart]]

Blood is circulated around the body through [[blood vessel]]s by the pumping action of the [[heart]]. In humans, blood is pumped from the strong [[left ventricle]] of the heart through [[artery|arteries]] to peripheral [[Tissue (biology)|tissues]] and returns to the right [[Atrium (heart)|atrium of the heart]] through [[vein]]s. It then enters the right [[Ventricle (heart)|ventricle]] and is pumped through the [[pulmonary artery]] to the [[lung]]s and returns to the left atrium through the [[pulmonary vein]]s. 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 [[Cell (biology)|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 muscle]]s, 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.<ref>{{cite web |title=Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus |url=http://www.rarebookroom.org/Control/hvyexc/index.html |first=William |last=Harvey | name-list-format = vanc |authorlink=William Harvey |language=Latin |year=1628 |url-status=live |archive-url=https://web.archive.org/web/20101127002138/http://rarebookroom.org/Control/hvyexc/index.html |archive-date=27 November 2010 |df=dmy-all }}</ref>

===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]].<ref>{{cite book | last1 = Williams | first1 = Peter W. | last2 = Gray | first2 = Henry David | name-list-format = vanc |title=Gray's anatomy |publisher=C. Livingstone |location=New York |year=1989 |isbn=978-0-443-02588-4 |edition= 37th}}</ref>
The proteinaceous component of blood (including clotting proteins) is produced predominantly by the [[liver]], while hormones are produced by the [[endocrine gland]]s 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 cell]]s in the liver. The liver also clears some proteins, lipids, and amino acids. The kidney actively secretes waste products into the [[urine]].

===Lymphatic system===
{{main|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.

==Treatments==
===Blood products===
{{further|Blood transfusion}}
[[File:Bloodbags.jpg|thumb|Venous blood collected during blood donation]]
Blood for transfusion is obtained from human donors by blood donation and stored in a [[blood bank]]. There are many different [[blood type]]s 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 [[intravenous]]ly are platelets, blood plasma, cryoprecipitate, and specific coagulation factor concentrates.

===Intravenous administration===
Many forms of medication (from [[antibiotic]]s 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, CaCl<sub>2</sub> 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===
{{main|bloodletting}}
In modern [[evidence-based medicine]], bloodletting is used in management of a few rare diseases, including [[hemochromatosis]] and [[polycythemia]]. However, [[bloodletting]] and [[Hirudotherapy|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 [[Hippocrates|Hippocratic]] medicine.

==Magic and religion==
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 descendence, rather than marriage. This bears closely to [[bloodline]]s, and sayings such as "blood is thicker than water" and "[[wikt:bad blood|bad blood]]", as well as "[[Blood brother]]".

Blood is given particular emphasis in the [[Judaism|Jewish]] and [[Christianity|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.

===Classical Greek medicine===
{{ill|Robin Fåhræus|pl||sv}} (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").<ref>{{cite journal | vauthors = Hart GD | title = Descriptions of blood and blood disorders before the advent of laboratory studies | journal = British Journal of Haematology | volume = 115 | issue = 4 | pages = 719–28 | date = December 2001 | pmid = 11843802 | doi = 10.1046/j.1365-2141.2001.03130.x | url = http://www.bloodmed.com/home/hannpdf/bjh3130.pdf | archive-url = https://web.archive.org/web/20110708073035/http://www.bloodmed.com/home/hannpdf/bjh3130.pdf | df = dmy | url-status = dead | archive-date = 8 July 2011 }}{{failed verification|date=January 2015}}</ref>

===Indigenous Australians===
In many [[Indigenous Australians|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:<blockquote>
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.<ref>{{cite book | last = Lawlor | first = Robert | name-list-format = vanc |title=Voices of the first day: awakening in the Aboriginal dreamtime |publisher=Inner Traditions International |location=Rochester, VT |year=1991 |pages=102–103 |isbn=978-0-89281-355-1}}</ref></blockquote> 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 field]]s, because iron is [[magnetism|magnetic]].

===European paganism===
{{more citations needed|date=October 2012}}
Among the [[Germanic peoples|Germanic tribes]], blood was used during their sacrifices; the ''[[Blót]]s''. 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 language|Old English]], and the terminology was borrowed by the [[Roman Catholic Church]] becoming ''to bless'' and ''blessing''. The [[Hittite language|Hittite]] word for blood, ''ishar'' was a cognate to words for "oath" and "bond", see [[Ishara]].
The [[Ancient Greece|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 [[Book of Genesis|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&nbsp;– 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 in Christianity|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 Orthodox Church|Eastern Orthodoxy]], [[Oriental Orthodoxy]], and the [[Assyrian Church of the East]] teach that, when consecrated, the [[Eucharist]]ic wine [[transubstantiation|actually becomes]] the [[Blood of Christ|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 [[Twelve Apostles|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." ({{sourcetext|source=Bible|version=King James|book=Luke |chapter=22|verse=20}})''.

Most forms of Protestantism, especially those of a [[Methodist]] 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 [[consubstantiation|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 [[diet (nutrition)|dietary]] laws ([[Kashrut]]). Blood is purged from meat by rinsing and soaking in water (to loosen clots), [[salting (food)|salting]] and then rinsing with water again several times.<ref>[http://www.chabad.org/library/article_cdo/aid/82678/jewish/Koshering-Meat.htm Koshering Meat.] {{webarchive|url=https://web.archive.org/web/20131216084120/http://www.chabad.org/library/article_cdo/aid/82678/jewish/Koshering-Meat.htm |date=16 December 2013 }} Chabad.org.</ref> Eggs must also be checked and any blood spots removed before consumption.<ref>[http://www.chabad.org/library/howto/wizard_cdo/aid/113470/jewish/Removing-the-Blood.htm Removing the Blood.] {{webarchive|url=https://web.archive.org/web/20131216084034/http://www.chabad.org/library/howto/wizard_cdo/aid/113470/jewish/Removing-the-Blood.htm |date=16 December 2013 }} Chabad.org.</ref> Although blood from fish is biblically kosher, it is rabbinically forbidden to consume fish blood to avoid the appearance of breaking the Biblical prohibition.<ref>Citron, R. Aryeh. [http://www.chabad.org/library/article_cdo/aid/1182104/jewish/All-About-Kosher-Fish.htm All About Kosher Fish.] {{webarchive|url=https://web.archive.org/web/20131216084101/http://www.chabad.org/library/article_cdo/aid/1182104/jewish/All-About-Kosher-Fish.htm |date=16 December 2013 }} Chabad.org.</ref>

Another ritual involving blood involves the covering of the blood of [[fowl]] and [[Game (food)|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.<ref>[[Menachem M. Schneerson|Schneerson, R. Menachem M.]] ''Igrot Kodesh,'' vol. vii, p. 270.</ref>

===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. When an animal has been slaughtered, the animal's neck is cut in a way to ensure that the spine is not severed, hence the brain may send commands to the heart to pump blood to it for oxygen. In this way, blood is removed from the body, and the meat is generally now safe to cook and eat. In modern times, blood transfusions are generally not considered against the rules.

===Jehovah's Witnesses===
{{main|Jehovah's Witnesses and blood transfusions}}

Based on their interpretation of scriptures such as Acts 15:28, 29 ("Keep abstaining...from blood."), many [[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.<ref>''The Watchtower'' 15 June 2004, p. 22, "Be Guided by the Living God"</ref>

===Chinese medicine===
In Chinese medicine, blood is one of the five body constituents (blood, [[moisture]], [[Qi]], [[essence]], [[shen]]).

===Vampire legends===
{{main|Vampire}}
[[Vampire]]s 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]], [[leech]]es, female [[mosquito]]es, [[vampire bat]]s, 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.

== References ==
{{Reflist}}

[[Category:Blood]]