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[[Image:Blood smear.jpg|350px|thumb|left|Human [[blood smear]]: <br>a - [[erythrocytes]]; b - [[neutrophil]]; c - [[eosinophil]]; d - [[lymphocyte]].]]


==Overview==
'''Blood''' is a specialized biological fluid consisting of [[red blood cell]]s (also called RBCs or erythrocytes), [[white blood cell]]s (also called leukocytes) and [[platelet]]s (also called thrombocytes) suspended in a complex fluid medium known as [[blood plasma]].  

By far the most abundant cells in blood are red blood cells. These contain [[hemoglobin]] which gives blood its red color.  The [[iron]]-containing heme portion of Hemoglobin facilitates hemoglobin-bound transportation of [[oxygen]] and [[carbon dioxide]] by selectively binding to these [[Breath|respiratory]] gasses and greatly increasing their solubility in blood.  White blood cells help to resist infections, and platelets are important in the [[Coagulation|clotting]] of blood. 

Blood is circulated around the body through [[blood vessel]]s by the pumping action of the [[heart]].  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 of the heart through [[vein]]s, blood then enters the right 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 in the lungs to all of the cells of the body, and venous blood carries carbon dioxide, produced as a waste product of [[metabolism]] by [[Cell (biology)|cells]], to the lungs to be exhaled.

Medical terms related to blood often begin with '''''hemo-''''' or '''''hemato-''''' (BE: ''haemo-'' and ''haemato-'') from the [[Greek language|Greek]] word "''haima''" for "blood."  Anatomically, blood is considered a [[connective tissue]] from both its origin in the bones and its function.

== Functions ==
 
* 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)
* Removal of waste such as [[carbon dioxide]], [[urea]] and [[lactic acid]]
* Immunological functions, including circulation of white cells, and detection of foreign material by [[antibodies]]
* [[Coagulation]], which is one part of the body's self-repair mechanism
* Messenger functions, including the transport of [[hormones]] and the signalling of [[tissue (biology)|tissue]] damage
* Regulation of body [[pH]] (the normal pH of blood is in the range of 7.35 - 7.45)
* Regulation of core [[body temperature]]
* Hydraulic functions

Problems with blood composition or circulation can lead to downstream tissue dysfunction. The term ''ischaemia'' refers to tissue which is inadequately perfused with blood.

The blood is circulated around the [[lungs]] and body by the pumping action of the [[heart]].  Additional return pressure may be generated by gravity and the actions of skeletal muscles. In mammals, blood is in equilibrium with [[lymph]], which is continuously formed from blood (by capillary ultrafiltration) and returned to the blood (via the [[thoracic duct]]). The lymphatic circulation may be thought of as the "second circulation".

== Constituents of human blood ==
Blood accounts for 7% of the human body weight<ref name="alberts_table">{{Cite web|url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&rid=mboc4.table.4143|title=Leukocyte functions and percentage breakdown|publisher=NCBI Bookshelf|year=2005|author=Alberts, Bruce|work=Molecular Biology of the Cell}}</ref>, with an average density of approximately 1060 kg/m³, very close to pure water's density of 1000 kg/m<sup>3</sup><ref>{{cite web | url = http://hypertextbook.com/facts/2004/MichaelShmukler.shtml | title = Density of Blood | year = 2004 | publisher = [http://hypertextbook.com/facts/ The Physics Factbook] }}</ref> The average adult has a blood volume of roughly 5 litres, composed of plasma (see below) and several kinds of cells (occasionally called ''corpuscles''); these ''formed elements'' of the blood are ''erythrocytes'' ([[red blood cell]]s), ''leukocytes'' ([[white blood cell]]s), and ''thrombocytes'' ([[platelet]]s). The red blood cells constitute about 45% of whole blood by volume.  
[[Image:SEM blood cells.jpg|thumb|left|230px|A [[scanning electron microscope]] (SEM) image of normal circulating human blood. One can see [[red blood cell]]s, several [[white blood cell]]s including knobby [[lymphocyte]]s, a [[monocyte]], a [[neutrophil]], and many small disc-shaped [[platelet]]s.]]
Each litre of blood contains:<ref name="alberts_table"/>
*'''5 × 10<sup>12</sup> erythrocytes''' (45.0% of blood volume) : In mammals, mature red blood cells lack a [[Cell nucleus|nucleus]] and [[organelle]]s. They contain the blood's [[hemoglobin]] and distribute oxygen. The red blood cells (together with [[endothelial]] vessel cells and some 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]]. The combined surface area of all the red cells in the human body would be roughly 2,000 times as great as the body's exterior surface.<ref>Martini, Frederic, et al (2006). ''Human Anatomy.'' 5th ed. Page 529. San Francisco, California: Pearson Education, Inc.  ISBN 0-8053-7211-3</ref> 
*'''9 × 10<sup>9</sup> leukocytes''' (1.0% of blood volume) : White blood cells are part of the [[immune system]]; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents ([[pathogens]]) and foreign substances.
*'''3 × 10<sup>11</sup> thrombocytes''' (<1.0% of blood volume) : Platelets are responsible for blood clotting ([[coagulation]]). They change fibrinogen into fibrin. This fibrin creates a mesh onto which red blood cells collect and clot. This clot stops more blood from leaving the body and also helps to prevent bacteria from entering the body.

The other 55% (making up a total of 2.7-3.0 litres in an average human) is [[blood plasma]], a fluid that is the blood's liquid medium, appearing golden-yellow in color.  [[Blood plasma]] is essentially an [[water|aqueous]] solution containing 92% water, 8% blood plasma [[protein]]s, and trace amounts of other materials. Some components are:

* [[Serum albumin]]
* Blood clotting factors (to facilitate [[coagulation]])
* [[Immunoglobulins]] (antibodies)
* [[Hormone]]s
* Carbon dioxide
* Various other [[protein]]s
* Various [[electrolyte]]s (mainly [[sodium]] and [[chloride]])

Together, plasma and cells form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels. The term ''serum'' refers to plasma from which the clotting proteins have been removed. Most of the protein remaining is albumin and [[antibody|immunoglobulins]].

The normal [[pH]] of human arterial blood is approximately 7.40 (normal range is 7.35-7.45), a weak alkaline solution. Blood that has a pH below 7.35 is considered overly acidic, while blood pH above 7.45 is too alkaline. Blood pH along with arterial carbon dioxide tension (PaCO<sub>2</sub>) and [[Bicarbonate|HCO<sub>3</sub>]] readings are helpful in determining the acid-base balance of the body. The [[respiratory system]] and [[urinary system]] normally control the acid-base balance of blood as part of [[homeostasis]].

== Physiology ==
===Production and degradation===
Blood cells are produced in the [[bone marrow]]; this process is termed [[hematopoiesis]]. The proteinaceous component (including clotting proteins) is produced overwhelmingly in 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]] and indirectly by the [[gut]].

Blood cells are degraded by the [[spleen]] and the [[Kupffer cell]]s in the liver. The liver also clears some proteins, lipids and [[amino acid]]s. The kidney actively secretes waste products into the [[urine]]. Healthy erythrocytes have a plasma [[half-life]] of 120 days before they are systematically replaced by new erythrocytes created by the process of hematopoiesis.

===Transport of oxygen===
{{see|Oxygen transportation}}

Blood oxygenation is measured in several ways, but the most important measure is the hemoglobin (Hb) saturation percentage. This is a non-linear (sigmoidal) function of the [[partial pressure]] of oxygen. About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at normal pressure is chemically combined with the Hb. Only 1.5% is physically dissolved in the other blood liquids and not connected to Hb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below).

With the exception of [[Pulmonary artery|pulmonary]] and [[Umbilical artery|umbilical arteries]] and their corresponding veins, [[artery|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 [[vein]]s carry deoxygenated blood back to the heart. 

Differences in infrared absorption between oxygenated and deoxygenated blood form the basis for realtime oxygen saturation measurement in hospitals and ambulances.

Under normal conditions in humans at rest, hemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen.  In a healthy adult at rest, ''deoxygenated'' blood returning to the lungs is still approximately 75% saturated.<ref>[http://home.hia.no/~stephens/ventphys.htm Ventilation and Endurance Performance]</ref><ref>[http://groups.msn.com/TransplantSupportLungHeartLungHeart/oxygen2.msnw Transplant Support- Lung, Heart/Lung, Heart] MSN groups</ref>  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.<ref>[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1464731 J Physiol. 2005 July 1]</ref>  Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia): "As a general rule, any condition which leads to a sustained mixed venous saturation of less than 50% will be poorly tolerated and a mixed venous saturation of less than 30% should be viewed as a medical emergency."<ref>[http://www.manbit.com/PAC/chapters/P30.cfm The 'St George' Guide To Pulmonary Artery Catheterisation]</ref>

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.<ref>[http://members.aol.com/Bio50/LecNotes/lecnot20.html Oxygen Carriage in Blood - High Altitude]</ref>

Substances other than oxygen can bind to the hemoglobin; in some cases this can cause irreversible damage to the body. [[Carbon monoxide]] for example is extremely dangerous when absorbed into the blood. When combined with the hemoglobin, it irreversibly makes [[carboxyhemoglobin]] which reduces the volume of oxygen that can be carried in the blood. This can very quickly cause suffocation, as oxygen is vital to many organisms (including humans). This damage can occur when smoking a [[cigarette]] (or similar item) or in event of a fire. Thus carbon monoxide is considered far more dangerous than the actual fire itself because it reduces the oxygen carrying content of the blood.

====Invertebrates====
In insects, the blood (more properly called hemolymph) is not involved in the transport of oxygen. (Openings called [[invertebrate trachea|trachea]]e 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 ([[protein]]s 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 cell]]s, allowing for a higher concentration of respiratory pigments without increasing viscosity or damaging blood filtering organs like the [[kidneys]].

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

===Transport of carbon dioxide===
When systemic arterial blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is dissolved in the blood. Some carbon dioxide reacts with hemoglobin and other proteins to form [[carbamino]] compounds. The remaining carbon dioxide is converted to [[bicarbonate]] and [[hydrogen ion]]s through the action of RBC [[carbonic anhydrase]]. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

===Transport of hydrogen ions===
Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin has a much greater affinity for hydrogen ion (H<sup>+</sup>) than does oxyhemoglobin so it binds most of the hydrogen ions.

===Thermoregulation===
Blood circulation transports heat through 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, while decreasing surface blood flow conserves heat.

===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 of this would occur in a mammalian [[penis]], [[clitoris]] or [[nipple]]. 

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.

===Color===
[[Image:Blooddroplets.jpg|left|250px|thumb|Droplets of human blood have a distinctive red color.]]
In humans and other hemoglobin-using creatures, oxygenated blood is bright red. This is due to oxygenated iron-containing hemoglobin found in the red blood cells. Deoxygenated blood is a darker shade of red, which can be seen during blood donation and when venous blood samples are taken. 

The blood of most molluscs, and some arthropods such as horseshoe crabs, is blue. This is a result of its high content of copper-based hemocyanin instead of the iron-based [[hemoglobin]] found, for example, in mammals. While mammalian blood is never blue, there is a rare condition (sulfhemoglobinemia) that results in green blood. Skinks in the genus ''Prasinohaema'' have green blood due to a buildup of the waste product biliverdin.

==Health and disease==
===Ancient medicine===
[[Hippocrates|Hippocratic]] medicine considered blood one of the [[four humors]] (together with [[phlegm]], [[yellow bile]] and [[black bile]]). As many diseases were thought to be due to an excess of blood, [[bloodletting]] and leeching were a common intervention until the 19th century (it is still used for some rare blood disorders).

In classical Greek medicine, blood was associated with air, springtime, and with a merry and gluttonous (''sanguine'') personality. It was also believed to be produced exclusively by the [[liver]].

===Diagnosis===
[[Blood pressure]] and [[blood test]]s are amongst the most commonly performed diagnostic investigations that directly concern the blood.

===Pathology===
{{seealso|Blood diseases}}

Problems with blood circulation and composition play a role in many diseases.

* [[Injury|Wounds]] can cause major blood loss (see [[bleeding]]). The [[thrombocyte]]s cause the blood to [[coagulation|coagulate]], blocking relatively minor wounds, but larger ones must be repaired at speed to prevent [[exsanguination]]. Damage to the internal organs can cause severe [[internal bleeding]], or [[hemorrhage]].
* Circulation blockage can also create many medical conditions from [[ischemia]] in the short term to tissue [[necrosis]] and [[gangrene]] in the long term.
* [[Hemophilia]] is a 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.
* [[Leukemia]] is a group of [[cancer (medicine)|cancers]] of the blood-forming tissues.
* Major blood loss, whether traumatic or not (e.g. during surgery), as well as certain blood diseases like [[anemia]] and [[thalassemia]], can require [[blood transfusion]]. 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.
* Overproduction of red blood cells is called [[polycythemia]].
* Blood is an important vector of infection. [[HIV]], the [[virus]] which causes [[AIDS]], is transmitted through contact between blood, [[semen]], or the bodily 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.

===Treatment===
[[Blood transfusion]] is the most direct therapeutic use of blood. It is obtained from human donors by [[blood donation]]. As there are different [[blood type]]s, and transfusion of the incorrect blood may cause severe complications, [[crossmatching]] is done to ascertain the correct type is transfused.

Other blood products administered [[intravenous]]ly are platelets, blood plasma, cryoprecipitate and specific coagulation factor concentrates.

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

As stated above, some diseases are still treated by removing blood from the circulation, eg. [[haemochromatosis]].

It is the fluid part of the blood that saves lives where severe blood loss occurs, other preparations can be given such as ringers atopical plasma volume expander as a non-blood alternative, and these alternatives where used are rivalling blood use when used.

==See also==
* [[Blood substitutes]] often called Artificial blood
* [[List of human blood components]]
* [[blood phobia]]
* [[Medical technologist]]

== References ==
<div class="references-small">{{reflist|2}}</div>


{{blood}}
{{cardiovascular_system}}
{{transfusion_medicine}}


[[Category:Blood]]
[[Category:Hematology]]
[[Category:Tissues]]

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