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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 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 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
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 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 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