Blood

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Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). Medical terms related to blood often begin in hemo- or hemato- (BE: haemo- and haemato-) from the Greek word for "blood".

Blood of different species
There are differences in blood between species.

Humans
Human blood is a liquid tissue; its major function is to transport oxygen necessary to life throughout the body. It also supplies the tissues with nutrients, removes waste products, and contains various components of the immune system defending the body against infection. Endocrine hormones also travel in the blood. There are about 6 quarts (or 5.6 liters) of blood in an average human body, accounting for ~8% of body mass. Adult humans have ~60 millilitres of blood per kilogram of body weight.

Human blood is red, ranging from bright red when oxygenated to dark red when not. It owes its colour to hemoglobin, a metalloprotein compound containing iron in the form of heme, to which oxygen binds. There exists a popular misconception that deoxygenated blood is blue and that blood only becomes red when it comes into contact with oxygen. Blood does not ever look blue. The veins appear blue because light is diffused by skin. Moreover, the blood inside is dark red and exhibit poor light reflection. From a physiological perspective, veins and arteries appear similar when skin is removed and are seen directly. Veins and arteries are actually whitish in color and slightly translucent. Arteries, because of the high pressure, have thicker walls than veins.

Blood moves in blood vessels and is circulated by the heart, a muscular pump. It passes to the lungs to be oxygenated, and then is circulated throughout the body by the arteries. It diffuses its oxygen by passing through tiny blood vessels called capillaries. It then returns to the heart through the veins. See circulatory system for a more detailed description of this circulation.

Blood also transports metabolic waste products, drugs and other foreign chemicals to the liver to be degraded and to the kidney to be excreted in urine.

A buffer of carbonic acid (H2CO3) and bicarbonate (HCO3-) is present in the human blood stream, in order to maintain a pH between 7.35 and 7.45.

See also:
 * Human anatomy
 * List of human blood components
 * List of distinct cell types in the adult human body

Insects
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.

Small invertebrates
In some small invertebrates, oxygen is simply dissolved in the plasma. All other animals use respiratory proteins to increase the oxygen carrying capacity. Hemoglobin is the most efficient respiratory protein found in nature. Hemocyanin (blue) contains copper and is used in crustaceans. Sea squirts, among others marine life, use a vanadium chromagen (bright green, blue, or orange) for its respiratory pigment.

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.

Anatomy of blood
Blood is composed of several kinds of corpuscles; these formed elements of the blood constitute about 45% of whole blood. The other 55% is blood plasma, a yellowish fluid that is the blood's liquid medium.

The corpuscles are:
 * Red blood cells or erythrocytes (96%). In mammals, these corpuscles lack a nucleus and organelles, so are not cells strictly speaking. They contain the blood's hemoglobin and distribute oxygen. The red blood cells also give rise to the system of blood types.
 * Platelets or thrombocytes (1.0%) are responsible for blood clotting or coagulation.
 * White blood cells or leukocytes (3.0%), are part of the immune system; they destroy infectious agents.

Blood plasma is essentially an aqueous solution containing 96% water, 4% blood plasma proteins, and trace amounts of following:
 * albumin
 * blood clotting factors
 * immunoglobulins (antibodies)
 * hormones
 * various other proteins
 * various salts

Together, plasma and corpuscles form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels.

The normal pH of arterial blood is approximately 7.40.

Health and disease
Blood is different in health and disease.

Wounds can cause major blood loss. The thrombocytes cause the blood to 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 cyanosis 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 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.

Leukaemia (more often called leukemia) is a group of 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 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.

Blood is an important vector of infection. One well-known example of a blood-borne illness is AIDS, whose virus, HIV, is transmitted through contact between blood, semen, or the bodily secretions of an infected person. Owing to blood-borne infections, bloodstained objects are treated as a biohazard.

Blood pressure is an important diagnostic tool.

Blood in mythology and religion
Due to its importance to life, blood is associated with a number of beliefs. One of the most basic is the use of blood as a symbol for family relationships; to be "related by blood" is to be related by ancestry or descendance, rather than marriage.

Blood and Blessing
Among the Germanic tribes (such as the Anglo-Saxons and the Vikings), blood was used during the 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 bleodsian in Old English and the terminology was borrowed by the Catholic Church becoming to bless and blessing.

Judaism
In Judaism, blood cannot be consumed even in the smallest quantity (Leviticus 3:17 and elsewhere); this is reflected in the dietary laws. Blood is purged from meat by salting and pickling.

Other rituals involving blood are the covering of the blood of fowl and game after slaughtering (Leviticus 17:13); the reason given by the Torah is: "Because the soul of every animal is [in] his blood" (ibid 17:14), although from its context in Leviticus 3:17 it would appear that blood cannot be consumed because it is to be used in the sacrificial service (known as the korbanot), in the Temple in Jerusalem.

Ironically, Judaism has historically been the religion to be most affected by Blood libels.

Christianity
Christians believe that the Eucharist wine either is or represents the blood of Jesus Christ shed for their salvation.

Vampire legends
Vampires are fictional beings thought to cheat death by drinking the blood of the living. Porphyrias, a group of inherited or acquired disorders, may have been the source of vampire legends.

Blood in ancient medicine
In the Greek theory of the four bodily humours, which dominated medicine until the 19th century, blood was associated with air, springtime, and with a merry and gluttonous (sanguine) personality. It was also beleived to have been produced by the liver, as opposed to the heart. An excess was removed by blood letting or leeching.

Physiology of blood
Blood has diverse physiological roles.

Transport of oxygen in blood
The amount of oxygen dissolved in blood is directly proportional to the PO2 of the blood.

The hemoglobin molecule is the primary transporter of oxygen. 98.5% of the oxygen is chemically combined with the Hb. Only 1.5% is physically dissolved.

Transport of carbon dioxide in blood
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 to form carbamino hemoglobin. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

Transport of hydrogen ions in blood
Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin has a much greater affinity for H+ than does oxyhemoglobin so it binds most of the hydrogen ions.

Cultural and historical aspects

 * bloodlines, "blood is thicker than water", "bad blood"
 * blood as one of the four bodily humours in Renaissance medicine
 * "Blood brother"
 * Jehovahs Witnesses' interpretation of the dietary laws as applied to blood tranfusions
 * blood as food, see black pudding and vampire
 * blood and video game censorship

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