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Blood is a liquid found in all higher animals whose main function is to supply the tissues with nutrient and to remove waste products.

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Human blood

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 other nutrients, removes waste products, and contains various components of the immune system defending the body against infection. Hormones also travel in the blood.

Adult humans have roughly 60 millilitres of blood per kilogram of body weight. This normally amounts to about 4-5 litres (roughly a gallon) of blood in an adult.

Human blood is red, ranging from bright red when oxygenated to dark red when not. It owes its colour to hemoglobin, a respiratory pigment[?] containing iron, to which oxygen binds.

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 in humans.

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


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 (about 99%). 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.
  • Thrombocytes or platelets (0.6 - 1.0%) are responsible for blood clotting or coagulation.
  • Leukocytes or white blood cells (0.2%), are part of the immune system; they destroy infectious agents.

Blood plasma is essentially an aqueous solution of

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


Several health problems can involve blood.

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.

Hemophilia is a genetic illness that causes a dysfunction in the clotting mechanism. This can allow even minor wounds to spill so much blood that the patient's life can be endangered.

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 and the blood, semen, or 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.


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.

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

Vampires are fictional beings thought to cheat death by drinking the blood of the living.

In the medieval theory of the four bodily humours, blood was associated with fire and with a merry and gluttonous (sanguine) personality.

Blood of non-human animals

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

In other animals, the main function of blood is the transport of oxygen from the lungs or gills to the tissues. In some small invertebrates, oxygen is simply dissolved in the plasma. All other animals use respiratory pigments to increase the oxygen carrying capacity. Hemoglobin is the most efficient respiratory pigment 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 respiratory pigments are freely dissolved in the blood; in vertebrates they are contained in specialized red blood cells, allowing for a higher concentration of respiratory pigments without increasing viscosity.

Topics in blood, to be filled out


Cultural and historic aspects

  • bloodlines, "blood is thicker than water", "bad blood"
  • blood as one of the four bodily humours in Renaissance medicine
  • "Blood brother"
  • "The blood is the life"
  • Kosher

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