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{{short description|Organic fluid which transports nutrients throughout the organism}}
{{Infobox anatomy
|Name = Blood
|Image = Venous and arterial blood.jpg
|Caption = Venous (darker) and arterial (brighter) blood
}}

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'''Blood''' is a [[body fluid]] in the [[circulatory system]] of [[humans]] and other [[vertebrate]]s that delivers necessary substances such as [[nutrient]]s and [[oxygen]] to the [[Cell (biology)|cells]], and transports [[Metabolic waste|metabolic waste products]] away from those same cells.

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Blood is composed of blood cells suspended in [[blood plasma]]. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume), and contains [[protein]]s, [[glucose]], mineral [[ion]]s, and [[hormone]]s. The blood cells are mainly [[red blood cell]]s (erythrocytes), [[white blood cell]]s (leukocytes), and (in mammals) [[platelet]]s (thrombocytes). The most abundant cells are red blood cells. These contain [[hemoglobin]], which facilitates oxygen transport by reversibly binding to it, increasing its solubility. [[Gnathostomata|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 [[Coagulation|clotting]] of blood.

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Blood is circulated around the body through [[blood vessel]]s by the pumping action of the [[heart]]. In animals with [[lung]]s, [[artery|arterial]] blood carries oxygen from inhaled air to the tissues of the body, and [[vein|venous]] blood carries carbon dioxide, a waste product of [[metabolism]] produced by cells, from the tissues to the lungs to be exhaled. Blood is bright red when its hemoglobin is oxygenated and dark red when it is [[Deoxygenated hemoglobin|deoxygenated]].

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Medical terms related to blood often begin with ''hemo-'', ''hemato-'', ''haemo-'' or ''haemato-'' from the Greek word {{lang|grc|αἷμα}} (''{{Lang|grc-latn|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== <!--T:6-->
[[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 (bound to [[hemoglobin]], which is carried in red cells)
* Supply of nutrients such as [[glucose]], [[amino acid]]s, and [[fatty acid]]s (dissolved in the blood or bound to [[Blood proteins|plasma proteins]] (e.g., [[blood lipid]]s))
* Removal of waste such as [[carbon dioxide]], [[urea]], and [[lactic acid]]
* Immunological functions, including circulation of [[white blood cell]]s, and detection of foreign material by [[Antibody|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 [[hormone]]s and the signaling of [[tissue (biology)|tissue]] damage
* Regulation of core [[body temperature]]
* [[Hydraulics|Hydraulic]] functions

==Constituents== <!--T:7-->
===In mammals===
{{See also|Reference ranges for common blood tests}}

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Blood accounts for 7% of the human body weight, with an average density around 1060&nbsp;kg/m<sup>3</sup>, very close to pure water's density of 1000&nbsp;kg/m<sup>3</sup>. The average adult has a [[blood volume]] of roughly {{convert|5|L|USpt|lk=in}} or 1.3 gallons, which is composed of plasma and ''formed elements''. The formed elements are the two types of blood cell or ''corpuscle'' – the [[red blood cell]]s, (erythrocytes) and [[white blood cell]]s (leukocytes), and the cell fragments called [[platelet]]s that are involved in clotting. By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, and white cells about 0.7%.

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Whole blood (plasma and cells) exhibits [[non-Newtonian fluid|non-Newtonian]] [[Blood viscosity|fluid dynamics]].

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<gallery>
File:Krew Frakcjonowana.jpg|Human blood fractioned by centrifugation: Plasma (upper, yellow layer), buffy coat (middle, thin white layer) and erythrocyte layer (bottom, red layer) can be seen.
File:Blutkreislauf.png|Blood circulation: Red = oxygenated, blue = deoxygenated
File:Blausen 0425 Formed Elements.png|Illustration depicting formed elements of blood
File:Blut-EDTA.jpg|Two tubes of [[EDTA]]-anticoagulated blood.<br /> Left tube: after standing, the RBCs have settled at the bottom of the tube.<br /> Right tube: Freshly drawn blood
</gallery>

====Cells==== <!--T:11-->
{{Further|Complete blood count}}

<!--T:12-->
[[Image:Red White Blood cells.jpg|thumb|right|A [[scanning electron microscope]] (SEM) image of a normal [[red blood cell]] (left), a [[platelet]] (middle), and a [[white blood cell]] (right)]]
One microliter of blood contains:
* '''4.7 to 6.1&nbsp;million (male), 4.2 to 5.4&nbsp;million (female) [[erythrocyte]]s:''' Red blood cells contain the blood's [[hemoglobin]] and distribute oxygen. Mature red blood cells lack a [[Cell nucleus|nucleus]] and [[organelle]]s in mammals. The red blood cells (together with [[endothelial]] vessel cells and other cells) are also marked by [[glycoprotein]]s that define the different [[Human blood group systems|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.
* '''4,000–11,000 [[leukocyte]]s:''' 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 ([[pathogen]]s) and foreign substances. The cancer of leukocytes is called [[leukemia]].
* '''200,000–500,000 [[thrombocytes]]:''' Also called [[platelet]]s, they take part in blood clotting ([[coagulation]]). Fibrin from the coagulation cascade creates a mesh over the [[platelet plug]].

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{| class="wikitable" style="float:right; margin-left:15px"
|+ Constitution of normal blood
|-
!Parameter
!Value
!Refs.
|-
| [[Hematocrit]] ||
45 ± 7 (38–52%) for males<br />
42 ± 5 (37–47%) for females
|
|-
| [[pH]] || 7.35–7.45
|
|-
| [[base excess]] || −3 to +3
|
|-
| P[[oxygen|O<sub>2</sub>]] || 10–13 kPa (80–100&nbsp;mm Hg)
|
|-
| P[[carbon dioxide|CO<sub>2</sub>]] || 4.8–5.8 kPa (35–45&nbsp;mm Hg)
|
|-
| [[Carbonic acid|HCO<sub>3</sub><sup>−</sup>]] || 21–27 mM
|
|-
| [[Oxygen saturation]] ||
Oxygenated: 98–99%<br />
Deoxygenated: 75%
|
|}

====Plasma==== <!--T:14-->
{{Main|Blood 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 [[water|aqueous]] solution containing 92% water, 8% blood plasma [[protein]]s, and trace amounts of other materials. Plasma circulates dissolved nutrients, such as [[glucose]], [[amino acid]]s, and [[fatty acid]]s (dissolved in the blood or bound to plasma proteins), and removes waste products, such as [[carbon dioxide]], [[urea]], and [[lactic acid]].

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Other important [[List of human blood components|components]] include:
* [[Serum albumin]]
* Blood-clotting factors (to facilitate [[coagulation]])
* Immunoglobulins ([[Antibody|antibodies]])
* [[lipoprotein]] particles
* Various other [[protein]]s
* Various [[electrolyte]]s (mainly [[Sodium in biology|sodium]] and [[chloride]])

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

====Acidity==== <!--T:17-->
{{See also|Acid–base homeostasis}}

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[[pH#Living systems|Blood pH]] is regulated to stay within the narrow range of 7.35 to 7.45, making it slightly [[Base (chemistry)|basic]] (compensation). Extra-cellular fluid in blood that has a pH below 7.35 is too [[acid]]ic, whereas blood pH above 7.45 is too basic. A pH below 6.9 or above 7.8 is usually lethal. Blood pH, [[Oxygen#partial pressure|partial pressure of oxygen (pO<sub>2</sub>)]], [[PCO2|partial pressure of carbon dioxide (pCO<sub>2</sub>)]], and [[bicarbonate]] (HCO<sub>3</sub><sup>−</sup>) are carefully regulated by a number of [[homeostasis|homeostatic mechanisms]], which exert their influence principally through the [[respiratory system]] and the [[urinary system]] to control the [[acid–base homeostasis|acid–base balance]] and respiration, which is called compensation. An [[arterial blood gas test]] measures these. Plasma also circulates [[hormone]]s transmitting their messages to various tissues. The list of normal [[Reference ranges for blood tests|reference ranges]] for various blood electrolytes is extensive.

===In non-mammals=== <!--T:19-->
[[File:Newbloodm.jpg|thumbnail|Vertebrate red blood cell types, measurements in micrometers]]
[[Image:Frogrbc1000x.jpg|thumb|Frog red blood cells magnified 1000 times|left]]
[[Image:Turtlerbc1000x.jpg|thumb|right|Turtle red blood cells magnified 1000 times]]
[[Image:Chickenrbc1000x.jpg|thumb|right|Chicken red blood cells magnified 1000 times]]
[[Image:Humanrbc1000x.jpg|thumb|Human red blood cells magnified 1000 times]]

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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:
* 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== <!--T:21-->
===Circulatory system===
[[File:Diagram of the human heart (cropped).svg|thumb|Circulation of blood through the human heart]]

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{{main|Circulatory system}}
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.

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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]].

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

===Cell production and degradation=== <!--T:25-->
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]].
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]].

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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]].

==={{anchor|Oxygen transport}}Oxygen transport=== <!--T:27-->
{{Further|Oxygen saturation (medicine)}}
[[File:Oxyhaemoglobin dissociation curve.png|thumb|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)|left]]
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 [[hemoglobin]]. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in [[mammal]]s and many other species. Hemoglobin has an oxygen binding capacity between 1.36 and 1.40 ml O<sub>2</sub> per gram hemoglobin, which increases the total [[blood oxygen capacity]] seventyfold, compared to if oxygen solely were carried by its solubility of 0.03&nbsp;ml O<sub>2</sub> per liter blood per mm&nbsp;Hg partial pressure of oxygen (about 100&nbsp;mm&nbsp;Hg in arteries).

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With the exception of pulmonary and [[Umbilical artery|umbilical arteries]] and their corresponding veins, arteries carry '''oxygenated blood''' away from the heart and deliver it to the body via [[arteriole]]s and [[capillary|capillaries]], where the oxygen is consumed; afterwards, [[venule]]s and veins carry '''deoxygenated blood''' back to the heart.

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Under normal conditions in adult humans at rest, hemoglobin in blood leaving the lungs is about 98–99% [[Oxygen saturation (medicine)|saturated with oxygen]], achieving an oxygen delivery between 950 and 1150 ml/min to the body. In a healthy adult at rest, oxygen consumption is approximately 200–250 ml/min, (70 to 78%) 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. Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia). Sustained hypoxia (oxygenation less than 90%), is dangerous to health, and severe hypoxia (saturations less than 30%) may be rapidly fatal.

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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), so fetuses produce another form of hemoglobin with a much higher affinity for oxygen ([[Fetal hemoglobin|hemoglobin F]]) to function under these conditions.

===Carbon dioxide transport=== <!--T:31-->
CO<sub>2</sub> 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 {{chem2|HCO3−}} by the enzyme [[carbonic anhydrase]] in the red blood cells by the reaction {{chem2|CO2 + H2O -> H2CO3 -> H+ + HCO3−}}; about 7% is dissolved in the plasma; and about 23% is bound to hemoglobin as [[carbamino]] compounds.

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Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO<sub>2</sub> 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 regulation|allosteric]] effects on the hemoglobin molecule, the binding of CO<sub>2</sub> 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 CO<sub>2</sub> or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the [[Bohr effect]].

===Transport of hydrogen ions=== <!--T:33-->
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=== <!--T:34-->
{{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.

===Thermoregulation=== <!--T:35-->
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.

=== Rate of flow === <!--T:36-->
Rate of blood flow varies greatly between different organs. Liver has the most abundant blood supply with an approximate flow of 1350 ml/min. Kidney and brain are the second and the third most supplied organs, with 1100 ml/min and ~700 ml/min, respectively.

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Relative rates of blood flow per 100 g of tissue are different, with kidney, adrenal gland and thyroid being the first, second and third most supplied tissues, respectively.

===Hydraulic functions=== <!--T:38-->
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]].

<!--T:39-->
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.

==Color== <!--T:40-->
{{Multiple image
| image1            =
| image2            =
| caption1          = Blood from a [[vein]]
| caption2          = Blood from an [[artery]]
| alt1              = Black blood
| alt2              = Normal red blood
}}

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{{main|Hemoglobin}}
[[File:Bleeding finger.jpg|thumb|Capillary blood from a bleeding finger]]
Hemoglobin is the principal determinant of the color of blood ('''hemochrome'''). Each molecule has four heme groups, and their interaction with various molecules alters the exact color. 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.

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Blood in [[carbon monoxide poisoning]] is bright red, because [[carbon monoxide]] causes the formation of [[carboxyhemoglobin]]. In [[cyanide]] poisoning, the body cannot use 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&nbsp;– a symptom called [[cyanosis]]. If the heme is oxidized, [[methemoglobin]], 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.

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Veins close to the surface of the skin appear blue for a variety of reasons. However, the factors that contribute to this alteration of [[visual perception|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.

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[[Skink]]s in the genus ''[[Prasinohaema]]'' have green blood due to a buildup of the waste product [[biliverdin]].

==Disorders== <!--T:45-->
===General medical ===
* Disorders of volume
** Injury can cause blood loss through bleeding. A healthy adult can lose almost 20% of blood volume (1&nbsp;L) before the first symptom, restlessness, begins, and 40% of volume (2&nbsp;L) before [[shock (circulatory)|shock]] sets in. [[Thrombocyte]]s 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 (circulatory)|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, [[Hypertension|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 === <!--T:46-->
{{see also|Hematology}}
* Anemia
** Insufficient red cell mass ([[anemia]]) can be the result of bleeding, blood disorders like [[thalassemia]], or [[Illnesses related to poor nutrition|nutritional deficiencies]], and may require one or more [[blood transfusion]]s. Anemia can also be due to a [[genetic disorder]] in which the red blood cells do not function effectively. Anemia can be confirmed by a [[blood test]] if the hemoglobin value is less than 13.5&nbsp;gm/dl in men or less than 12.0&nbsp;gm/dl in women. Several countries have [[blood bank]]s 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 [[cancer (medicine)|cancers]] of the blood-forming tissues and cells.
** Non-cancerous overproduction of red cells ([[polycythemia vera]]) or platelets ([[essential thrombocytosis]]) may be [[Premalignant condition|premalignant]].
** [[Myelodysplastic syndrome]]s involve ineffective production of one or more cell lines.
* Disorders of coagulation
** [[Hemophilia]] is a [[Genetic disorder|genetic illness]] that causes dysfunction in one of the blood's [[coagulation|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 [[Hepatitis C|C]] are transmitted primarily through blood contact. Owing to [[blood-borne infection]]s, bloodstained objects are treated as a [[Biological hazard|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=== <!--T:47-->
{{main|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.

==Treatments== <!--T:48-->
===Transfusion===
{{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.

<!--T:49-->
Other blood products administered [[intravenous]]ly are platelets, blood plasma, cryoprecipitate, and specific coagulation factor concentrates.

===Intravenous administration=== <!--T:50-->
Many forms of medication (from [[antibiotic]]s to [[chemotherapy]]) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.

<!--T:51-->
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.

===Letting=== <!--T:52-->
{{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.

==Etymology== <!--T:53-->
[[Image:Jan Janský, 1902.jpg|thumb|upright|[[Jan Janský]] is credited with the first classification of blood into four types (A, B, AB, and O)]]
English ''blood'' ([[Old English]] ''blod'') derives from [[Germanic languages|Germanic]] and has cognates with a similar range of meanings in all other Germanic languages (e.g. German ''Blut'', Swedish ''blod'', Gothic ''blōþ''). There is no accepted [[Proto-Indo-European language|Indo-European]] etymology.

==History== <!--T:54-->
===Classical Greek medicine===
[[Robin 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").

===Types=== <!--T:55-->
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, and O) in 1907, which remains in use today. In 1907 the first [[blood transfusion]] was performed that used the ABO system to predict compatibility. The first non-direct transfusion was performed on 27 March 1914. The Rhesus factor was discovered in 1937.

==Culture and religion== <!--T:56-->
{{See also|Blood libel}}

<!--T:57-->
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]]".

<!--T:58-->
Blood is given particular emphasis in the [[Islam|Islamic]], [[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.

<!--T:59-->
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=== <!--T:60-->
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.}} 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=== <!--T:61-->
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.

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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=== <!--T:63-->
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 Church]].

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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.

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At the [[Council of Jerusalem]], the [[Apostles in the New Testament|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.

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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).

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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}})''.

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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=== <!--T:69-->
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. Eggs must also be checked and any blood spots removed before consumption. Although blood from fish is biblically kosher, it is rabbinically forbidden to consume fish blood to avoid the appearance of breaking the Biblical prohibition.

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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.

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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.

===Islam=== <!--T:72-->
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."

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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=== <!--T:74-->
{{main|Jehovah's Witnesses and blood transfusions}}

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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.

===Vampirism=== <!--T:76-->
{{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.

==Invertebrates== <!--T:77-->
{{Main|Hemolymph}}
In invertebrates, a body fluid analogous to blood called hemolymph is found, the main difference being that hemolymph is not contained in a closed circulatory system. Hemolymph may function to carry oxygen, although hemoglobin is not necessarily used. [[Crustacean]]s and [[mollusk]]s use [[hemocyanin]] instead of hemoglobin. In most insects, their hemolymph does not contain oxygen-carrying molecules because their bodies are small enough for their [[invertebrate trachea|tracheal system]] to suffice for supplying oxygen.

==Other uses== <!--T:78-->
===Forensic and archaeological===
[[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.

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Blood residue analysis is also a technique used in [[archeology]].

===Artistic=== <!--T:80-->
Blood is one of the body fluids that has been used in art. In particular, the performances of [[Viennese Actionism|Viennese Actionist]] [[Hermann Nitsch]], [[Istvan Kantor]], [[Franko B]], [[Lennie Lee]], [[Ron Athey]], [[Yang Zhichao]], [[Justice Yeldham|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.

===Genealogical=== <!--T:81-->
The term ''blood'' is used in [[Genealogy|genealogical circles]] to refer to one's [[Ancestor|ancestry]], [[Human evolution|origins]], and [[Ethnic group|ethnic background]] as in the word ''[[Heredity|bloodline]]''. Other terms where blood is used in a family history sense are ''[[Nobility#"Blue" blood|blue-blood]]'', ''[[Royal descent|royal blood]]'', ''[[mixed-blood]]'' and ''[[Kinship terminology|blood relative]]''.

== See also == <!--T:82-->
{{Div col|colwidth=20em}}
* [[Autotransfusion]]
* [[Blood as food]]
* [[Blood pressure]]
* [[Blood substitutes]] ("artificial blood")
* [[Blood test]]
* [[Hematology]]
* [[Blood phobia|Hemophobia]]
* [[Luminol]], a visual test for blood left at crime scenes.
* [[Oct-1-en-3-one]] ("Smell" of blood)
* [[Taboo food and drink#Blood|Taboo food and drink: Blood]]
{{div col end}}

== External links == <!--T:83-->
{{wikiquote}}
{{wiktionary}}
{{commons category|Blood}}
* [https://www.ncbi.nlm.nih.gov/books/NBK2261 Blood Groups and Red Cell Antigens.] Free online book at [[National Center for Biotechnology Information|NCBI]] Bookshelf ID: NBK2261
* {{In Our Time|Blood|p00548ym|Blood}}
* [https://web.archive.org/web/20161016200738/http://histology-world.com/photoalbum/thumbnails.php?album=7 Blood Photomicrographs]

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{{blood}}
{{Lymphocytes}}
{{transfusion medicine}}

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{{二次利用|date=13 February 2024}}
[[Category:Blood| ]]
[[Category:Hematology]]
[[Category:Tissues (biology)]]
[[Category:Articles containing video clips]]
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