Blood is a liquid found in all higher animals whose main function is to supply the tissues with nutrients and oxygen and to remove waste products. Medical terms related to blood often begin in hemo- or hemato- from the Greek word for "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 nutrients, removes waste products, and contains various components of the immune system defending the body against infection. Several hormones also travel in the blood.
Adult humans have roughly 60 milliliters of blood per kilogram of body weight. This normally amounts to about 4-5 liters (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 color to hemoglobin, a respiratory protein containing iron in the form of heme, 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.
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.
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
* blood clotting factors,
* immunoglobulins (antibodies)
* 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.
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 humors, 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 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.
A blood type is a description of certain characteristics of blood which depend on certain substances present on the surface of red blood cells. There are 46 known antigens, each of which is described by its own system.
Two important classifications to describe blood types in humans are ABO and Rh factor. Blood transfusions from incompatible groups can cause an immunological "transfusion reaction", resulting in hemolysis, anemia, renal failure, shock, and death.
Individuals with type A blood, have red blood cells with substance A on their surface and antibodies against substance B in their blood serum.
Individuals with type B blood have the opposite arrangement, substance B in the cell and antibodies to substance A in their serum. Type O people have neither substance but can form antibodies against both types. Type AB people have both substances. Because of this arrangement, type O can be safely given to any person with any ABO blood type. Type AB people can safely receive any ABO type blood.
The precise reason why people are born with antibodies against an antigen they have never been exposed to is unknown. It is thought that some bacterial antigens are similar enough to the A and B glycoproteins, that antibodies created against the bacteria will react to ABO-incompatible blood cells.
Austrian scientist Karl Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930 for his work in discovering ABO blood types.
Another characteristic of blood is Rhesus factor or Rh factor. Someone either has or does not have the Rh factor on the surface of their red blood cells. This is indicated as + or -. This is often combined with the ABO type. Type O+ blood is most common, though in some areas type A prevails, and there are other areas in which as many as 80 percent of the people are type B.
Rh factor is named after the rhesus monkey where the factor was first identified.
Blood groups are inherited from both parents. They are controlled by a single gene with 3 alleles: i, A, B.
A allele gives type A, B gives type B, and i gives type O. A and B are dominant over i, so ii people have type O, AA or Ai have A, BB or Bi have type B. AB people have both phenotypes because A and B express a special dominance relationship: codominance. Thus, it is usually impossible for a type AB parent to have a type O child.
When a type AB parent has a type O child, or when one type A and one type O parent produce a type AB child, it is sometimes mistakenly assumed that the child MUST be illegitimate.
Another possible explanation is that the child or parent who tests as type O has the very rare Bombay phenotype: they have inherited two recessive alleles of the H gene, (their blood group is Oh and their genotype is "hh"), and so do not produce the "H" protein that is the precursor to the "A" and "B" antigens. It then no longer matters whether the A or B enzymes are present or not, as no A or B antigen can be produced since the precursor antigen is not present.
The rare individuals with Bombay phenotype do not express H substance on their red blood cells and therefore do not bind A or B antigens. Instead, they produce antibodies to H substance (which is present on all red cells except those of hh phenotype) as well as to both A and B antigens and are therefore compatible only with other hh donors.
Individuals with Bombay phenotype blood groups can only be transfused with blood from other Bombay phenotype individuals. Given that this condition is very rare to begin with, a person with this blood group who needs an urgent blood transfusion, may be simply out of luck, as it would be quite unlikely that any blood bank would have any in stock.
Rh is inherited the same way, except that it has two alleles and Rh is dominant. Rh Disease is caused by an Rh negative mother having an Rh positive child. The antibodies in the mother's blood destroy the infant's blood. At first, this was treated by transfusing the blood of infants who survived.
Other Blood Types
Other blood type systems exist to describe the presence or absence of each of the antigens. Diego positive blood is found only among East Asians and Native Americans. MNS systems gives blood types of M, N, and MN. It has use in tests of maternity or paternity. Duffy negative blood gives partial immunity to malaria. The Lutheran system describes a set of 21 antigens. Other systems include Colton, Hh or Bombay, Kell, Kidd, Lewis, Landsteiner-Wiener, P, Yt or Cartwright, XG, Scianna, Dombrock, Chido/Rodgers, Kx, Gerbich, Cromer, Knops, Indian, Ok, Raph, and JMH.
Artificial blood is supposed to replace biological blood, especially in humans. As of 2003, there is no working artificial blood, though several attempts are made to create it.
Artificial blood, produced in large quantities, would remove the need for blood donors. It could also be equipped with special properties, like the ability to transport an increased amount of oxygen, which would be useful for someone who suffered massive blood loss. Artificial blood would also most likely have a universal blood type, so it can be used on anyone.
Apart from attempts to create blood by genetic engineering, work is being conducted on nanotechnology-based blood.
The United States Army is experimenting with varieties of dried blood, which takes up less room, weighs less and can be used much longer than blood plasma. These properties make it ideal for first aid during combat. Water has to be added prior to use.
See also: Respirocytes: A Mechanical Artificial Red Cell
Blood diseases * hemoglobinopathies - sickle cell disease - thalassemia - methemoglobinemia * decreased numbers of cells - anemia + iron-deficiency anemia + megaloblastic anemias - vitamin B12 or folate deficiency + hemolytic anemia + myelodysplastic syndromes + hereditary elliptocytosis - neutropenia - thrombocytopenia * increased numbers of cells - polycythemia vera - leukocytosis - thromocytosis * cancers of blood and immune cells - leukemia - Hodgkins disease - Burkitts lymphoma * coagulopathies - disorders of clotting proteins + Hemophilia + Christmas disease - disorders of platelets + Glanzman's thrombasthenia
Blood-borne diseases * AIDS * Hepatitis * West Nile virus * parasites - malaria - babesiosis
Blood is one of the four bodily humors in Renaissance medicine.