Wikipedia

Blood

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Blood is a liquid bodily tissue found in all higher animals. 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.

In vertebrates, blood is red. It owes its colour to hemoglobin, a respiratory pigment containing iron, to which oxygen binds. Hemoglobin is the most efficient respiratory pigment found in nature. The blood of some other animals use respiratory pigments containing other elements. 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.

Blood moves in blood vessels and is circulated by the heart, a muscular pump. It passes to the lungs (or in fish, the gills) 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. In its deoxygenated state, its bright red colour fades. It then returns to the heart through the veins. See circulatory system for a more detailed description of this circulation in humans.

Blood also removes poisons and waste products to the liver, the kidneys, and the intestines to allow them to be rejected from the body in urine and feces.

Adult humans have roughly 60 millilitres of blood per kilogram of body weight.

Composition

Blood is composed of several kinds of corpuscles:

  • 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.

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. Blood plasma is essentially an aqueous solution of albumin, blood clotting factors, immunoglobulins and various other proteins. Together, plasma and corpuscles form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels.

Health

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.

Mythology

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.

Topics in blood, to be filled out

Biology

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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