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'Blood' is a specialized biological fluid consisting of
red blood cells (also called RBCs or erythrocytes),
white blood cells (also called leukocytes) and
platelets (also called thrombocytes) suspended in a complex fluid medium known as
blood plasma.
By far the most abundant cells in blood are red blood cells. These contain
hemoglobin which gives blood its red color. The
iron-containing heme portion of Hemoglobin facilitates hemoblobin-bound transportation of
oxygen and
carbon dioxide by selectively binding to these
respiratory gasses and greatly increasing their solubility in blood. White blood cells help to resist infections, and platelets are important in the
clotting of blood.
Blood is circulated and recirculated around the body through
blood vessels by the pumping action of the
heart. Blood is pumped from the strong left ventricle of the heart through
arteries to peripheral
tissues and returns to the right atrium of the heart through
veins, blood then enters the right ventricle and is pumped through the
pulmonary artery to the
lungs and returns to the left atrium through the
pulmonary veins, blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air in the lungs to all of the cells of the body, and venous blood carries carbon dioxide, produced as a waste product of
metabolism by
cells, to the lungs to be exhaled.
Medical terms related to blood often begin with '''hemo-''' or '''hemato-''' (
BE: ''haemo-'' and ''haemato-'') from the
Greek word "''haima''" for "blood." Anatomically, blood is considered a
connective tissue from both its origin in the bones and its function.
Functions
★ Supply of
oxygen to tissues (bound to
hemoglobin which is carried in red cells)
★ Supply of nutrients such as
glucose,
amino acids and
fatty acids (dissolved in the blood or bound to plasma proteins)
★ Removal of waste such as
carbon dioxide,
urea and
lactic acid
★ Immunological functions, including circulation of white cells, and detection of foreign material by
antibodies
★
Coagulation, which is one part of the body's self-repair mechanism
★ Messenger functions, including the transport of
hormones and the signalling of
tissue damage
★ Regulation of body
pH (the normal pH of blood is in the range of 7.35 - 7.45)
★ Regulation of core
body temperature
★
Hydraulic functions
Problems with blood composition or circulation can lead to downstream tissue dysfunction. The term ''ischaemia'' refers to tissue which is inadequately perfused with blood.
The blood is circulated around the
lungs and body by the
pumping action of the
heart. Additional return pressure may be generated by gravity and the actions of skeletal muscles. In mammals, blood is in equilibrium with
lymph, which is continuously formed from blood (by capillary ultrafiltration) and returned to the blood (via the
thoracic duct). The lymphatic circulation may be thought of as the "second circulation".
Anatomy of mammalian blood
Blood accounts for 7% of the human body weight
[1], with an average density of approximately 1060 kg/m³.
[2] The average adult has a blood volume of roughly 5
litres, composed of plasma (see below) and several kinds of cells (occasionally called ''corpuscles''); these ''formed elements'' of the blood are ''erythrocytes'' (
red blood cells), ''leukocytes'' (
white blood cells), and ''thrombocytes'' (
platelets). The red blood cells constitute about 45% of whole blood by volume.
Each litre of blood contains:
★ '5 ×
1012 erythrocytes' (45.0% of blood volume) : In mammals, mature red blood cells lack a
nucleus and
organelles. They contain the blood's
hemoglobin and distribute oxygen. The red blood cells (together with
endothelial vessel cells and some other cells) are also marked by
glycoproteins that define the different
blood types. The proportion of blood occupied by red blood cells is referred to as the
hematocrit. The combined surface area of all the red cells in the human body would be roughly 2,000 times as great as the body's exterior surface.
[3]
★ '9 ×
109 leukocytes' (1.0% of blood volume) : White blood cells are part of the
immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (
pathogens) and foreign substances.
★ '3 ×
1011 thrombocytes' (<1.0% of blood volume) : Platelets are responsible for blood clotting (
coagulation). They change fibrinogen into fibrin. This fibrin creates a mesh onto which red blood cells collect and clot. This clot stops more blood from leaving the body and also helps to prevent bacteria from entering the body.
The other 55% (making up a total of 2.7-3.0 litres in an average human) is
blood plasma, a fluid that is the blood's liquid medium, appearing golden-yellow in color.
Blood plasma is essentially an
aqueous solution containing 92% water, 8% blood plasma
proteins, and trace amounts of other materials. Some components are:
★
Serum albumin
★ Blood clotting factors (to facilitate
coagulation)
★
Immunoglobulins (antibodies)
★
Hormones
★ Carbon dioxide
★ Various other
proteins
★ Various
electrolytes (mainly
sodium and
chloride)
Together, plasma and cells form a
non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels. The term ''serum'' refers to plasma from which the clotting proteins have been removed. Most of the protein remaining is albumin and
immunoglobulins.
The normal
pH of human arterial blood is approximately 7.40 (normal range is 7.35-7.45), a weak alkaline solution. Blood that has a pH below 7.35 is considered overly
acidic, while blood pH above 7.45 is too
alkaline. Blood pH along with arterial carbon dioxide tension (PaCO
2) and
HCO3 readings are helpful in determining the acid-base balance of the body. The
respiratory system and
urinary system normally control the acid-base balance of blood as part of
homeostasis.
Physiology of blood
Production and degradation
Blood cells are produced in the
bone marrow; this process is termed
hematopoiesis. The proteinaceous component (including clotting proteins) is produced overwhelmingly in the
liver, while hormones are produced by the
endocrine glands and the watery fraction is regulated by the
hypothalamus and maintained by the
kidney and indirectly by the
gut.
Blood cells are degraded by the
spleen and the
Kupffer cells in the liver. The liver also clears some proteins, lipids and
amino acids. The kidney actively secretes waste products into the
urine. Healthy erythrocytes have a plasma
half-life of 120 days before they are systematically replaced by new erythrocytes created by the process of hematopoiesis.
Transport of oxygen
Blood oxygenation is measured in several ways, but the most important measure is the hemoglobin (Hb) saturation percentage. This is a non-linear (sigmoidal) function of the
partial pressure of oxygen. About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at normal pressure is chemically combined with the Hb. Only 1.5% is physically dissolved in the other blood liquids and not connected to Hb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below).
With the exception of
pulmonary and
umbilical arteries and their corresponding veins,
arteries carry oxygenated blood away from the
heart and deliver it to the body via
arterioles and
capillaries, where the oxygen is consumed; afterwards,
venules and
veins carry deoxygenated blood back to the heart.
Differences in infrared absorption between oxygenated and deoxygenated blood form the basis for realtime oxygen saturation measurement in hospitals and ambulances.
Under normal conditions in humans at rest, hemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen. In a healthy adult at rest, ''deoxygenated'' blood returning to the lungs is still approximately 75% saturated.
[4][5] Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions.
[6] Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia): "As a general rule, any condition which leads to a sustained mixed venous saturation of less than 50% will be poorly tolerated and a mixed venous saturation of less than 30% should be viewed as a medical emergency."
[7]
A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 21% of the level found in an adult's lungs) and so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to function under these conditions.
[8]
Substances other than oxygen can bind to the hemoglobin; in some cases this can cause irreversible damage to the body.
Carbon monoxide for example is extremely dangerous when absorbed into the blood. When combined with the hemoglobin, it irreversibly makes
carboxyhemoglobin which reduces the volume of oxygen that can be carried in the blood. This can very quickly cause suffocation, as oxygen is vital to many organisms (including humans). This damage can occur when smoking a
cigarette (or similar item) or in event of a fire. Thus carbon monoxide is considered far more dangerous than the actual fire itself because it reduces the oxygen carrying content of the blood.
Invertebrates
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 an open system.
Other invertebrates use respiratory proteins to increase the oxygen carrying capacity. Hemoglobin is the most common respiratory protein found in nature.
Hemocyanin (
blue) contains
copper and is found in
crustaceans and
mollusks. It is thought that
tunicates (sea squirts) might use
vanabins (
proteins containing
vanadium) for respiratory pigment (bright
green, blue, or
orange).
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 or damaging blood filtering organs like the
kidneys.
Giant tube worms have extraordinary hemoglobins that allow them to live in extraordinary environments. These hemoglobins also carry sulfides normally fatal in other animals.
Transport of carbon dioxide
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 and other proteins to form
carbamino compounds. The remaining carbon dioxide is converted to
bicarbonate and
hydrogen ions through the action of RBC
carbonic anhydrase. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.
Transport of hydrogen ions
Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin has a much greater affinity for hydrogen ion (H
+) than does oxyhemoglobin so it binds most of the hydrogen ions.
Thermoregulation
Blood circulation transports
heat through the body, and adjustments to this flow are an important part of
thermoregulation. Increasing blood flow to the surface (e.g. during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss, while decreasing surface blood flow conserves heat.
Hydraulic functions
The restriction of blood flow can also be used in specialized tissues to cause engorgement resulting in an
erection of that tissue. Examples of this would occur in a mammalian
penis,
clitoris or
nipple.
Another example of a hydraulic function is the
jumping spider, in which blood forced into the legs under pressure causes them to straighten for a powerful jump.
Color
Droplets of human blood have a distinctive red color.
In humans and other hemoglobin-using creatures, oxygenated blood is bright red. This is due to oxygenated iron-containing hemoglobin found in the red blood cells. Deoxygenated blood is a darker shade of red, which can be seen during blood donation and when venous blood samples are taken.
The blood of most
molluscs, and some
arthropods such as
horseshoe crabs, is blue. This is a result of its high content of copper-based
hemocyanin instead of the iron-based
hemoglobin found, for example, in
mammals. While mammalian blood is never blue, there is a rare condition (
sulfhemoglobinemia) that results in green blood.
Skinks in the genus ''
Prasinohaema'' have green blood due to a buildup of the waste product
biliverdin.
Health and disease
Ancient medicine
Hippocratic medicine considered blood one of the
four humors (together with
phlegm,
yellow bile and
black bile). As many diseases were thought to be due to an excess of blood,
bloodletting and
leeching were a common intervention until the 19th century (it is still used for some rare blood disorders).
In classical Greek medicine, blood was associated with air, springtime, and with a merry and gluttonous (''sanguine'') personality. It was also believed to be produced exclusively by the
liver.
Diagnosis
Blood pressure and
blood tests are amongst the most commonly performed diagnostic investigations that directly concern the blood.
Pathology
Problems with blood circulation and composition play a role in many diseases.
★
Wounds can cause major blood loss (see
bleeding). 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
ischemia 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.
★
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.
★ Overproduction of red blood cells is called
polycythemia.
★ Blood is an important vector of infection.
HIV, the
virus which causes
AIDS, is transmitted through contact between blood,
semen, or the bodily secretions of an infected person.
Hepatitis B and
C are transmitted primarily through blood contact. Owing to
blood-borne infections, bloodstained objects are treated as a
biohazard.
★ Bacterial infection of the blood is
bacteremia or
sepsis. Viral Infection is viremia.
Malaria and
trypanosomiasis are blood-borne parasitic infections.
Treatment
Blood transfusion is the most direct therapeutic use of blood. It is obtained from human donors by
blood donation. As there are different
blood types, and transfusion of the incorrect blood may cause severe complications,
crossmatching is done to ascertain the correct type is transfused.
Other blood products administered
intravenously are platelets, blood plasma, cryoprecipitate and specific coagulation factor concentrates.
Many forms of medication (from
antibiotics to
chemotherapy) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.
As stated above, some diseases are still treated by removing blood from the circulation, eg.
haemochromatosis.
It is the fluid part of the blood that saves lives where severe blood loss occurs, other preparations can be given such as ringers atopical plasma volume expander as a non-blood alternative, and these alternatives where used are rivalling blood use when used.
Mythology and religion
Due to its importance to life, blood is associated with a large 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. This bears closely to
bloodlines, and sayings such as "blood is thicker than water" and "bad blood", as well as "
Blood brother". Blood is given particular emphasis in the Jewish and Christian religions because
Leviticus 17:11 says "the life of a creature is in the blood." This phrase is part of the Levitical law forbidding the drinking blood, due to its practice in idol worship by surrounding societies.
Mythic references to blood may be connected to the
childbirth or
menstruation, both bloody but life-affirming events, as opposed to the blood of injury or death.
Indigenous Australians
In many
indigenous Australian Aboriginal peoples' traditions
ochre (particularly red) and blood, both high in
iron content and considered
Maban, are applied to the bodies of dancers for ritual. As Lawlor states:
In many Aboriginal rituals and ceremonies, red ochre is rubbed all over the naked bodies of the dancers. In secret, sacred male ceremonies, blood extracted from the veins of the participant's arms is exchanged and rubbed on their bodies. Red ochre is used in similar ways in less secret ceremonies. Blood is also used to fasten the feathers of birds onto people's bodies. Bird feathers contain a protein that is highly magnetically sensitive.
[9]
Lawlor comments that blood employed in this fashion is held by these peoples to attune the dancers to the invisible energetic realm of the Dreamtime. Lawlor then draws information from different disciplines charting a relationship between these invisible energetic realms and
magnetic fields. Iron and
magnetism having a marked relationship.
Indo-European paganism
Among the
Germanic tribes (such as the
Anglo-Saxons and the
Norsemen), 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
Roman Catholic Church becoming ''to bless'' and ''blessing''. The
Hittite word for blood, ''ishar'' was a cognate to words for "oath" and "bond", see
Ishara.
The
Ancient Greeks believed that the blood of the Gods, ''
ichor'', was a mineral that was poisonous to mortals.
Judaism
In
Judaism, blood cannot be consumed even in the smallest quantity (
Leviticus 3:17 and elsewhere); this is reflected in Jewish
dietary laws (
Kashrut). Blood is purged from
meat by
salting and soaking in water.
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 life 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. Blood (the blood of a lamb) was also the means for atonement of sins for the Jews.
Christianity
Main articles: Eucharist
Some Christian churches, including
Roman Catholicism,
Eastern Orthodoxy, branches of
Anglicanism, and the
Moravian Church, teach that when consecrated the
Eucharist wine ''becomes'' the material Blood of
Jesus. Thus in the consecrated wine (now the Most Precious Blood of Christ), Jesus becomes spiritually and physically present. This teaching is rooted in
the Last Supper as written in the four gospels of the
Bible, in which Jesus stated to his
disciples that the bread which they ate was his body, and the wine was his blood. ''"This cup is the new testament in my blood, which is shed for you." ()''. Various forms of Protestantism, especially those of a
Wesleyan or
Presbyterian lineage, teach that the wine is no more than a symbol of the blood of Christ, who is not physically but spiritually present. Blood (the blood of Jesus Christ) is also seen as the means for atonement for sins for Christians.
Lutheran theology teaches that the body and blood is present together "in, with, and under" the bread and wine of the eucharist feast.
Islam
Consumption of food containing blood is forbidden by
Islamic dietary laws.
Jehovah's Witnesses
Main articles: Jehovah's Witnesses and blood
Due to Bible-based beliefs,
Jehovah's Witnesses do not eat blood or accept tranfusions of whole blood or its four major components namely, red blood cells, white blood cells, platelets (thrombocytes), and whole plasma. Members are instructed to personally decide whether or not to accept fractions, and medical procedures that involve their own blood.
Vampire legends
Vampires are mythological beings which live forever by drinking the blood of the living. Stories of creatures of this kind are known all over the world. Most known of this myth in Western culture comes from Eastern European folklore.
Chinese and Japanese culture
In Chinese culture, it is often said that if a man's nose produces a small flow of blood, this signifies that he is experiencing sexual desire. This often appears in
Chinese-language and
Hong Kong films.
This is also evident in
Japanese culture and is parodied in
anime and
manga. Male characters will often be shown with a
nosebleed if they have just seen a female
nude or in little clothing, or if they have had an erotic thought or fantasy.
Blood libel
Main articles: Blood libel
Various religious and other groups have been falsely accused of using human blood in rituals; such accusations are known as
blood libel. The most common form of this is
blood libel against Jews. Although there is no ritual involving human blood in Jewish law or custom, fabrications of this nature (often involving the murder of children) were widely used during the Middle Ages to justify
anti-Semitic persecution and some have persisted into the 21st century.
Art
Blood is one of the body fluids that has been used in art.
[10] In particular, the performances of
Viennese Actionist Hermann Nitsch,
Franko B,
Lennie Lee,
Ron Athey,
Yang Zhichao and
Kira O' Reilly along with the photography of
Andres Serrano, have incorporated blood as a prominent visual element.
Marc Quinn has made sculptures using frozen blood, including a cast of his own head made using his own blood.
See also
★
Blood substitutes often called Artificial blood
★
List of human blood components
★ Blood as
food: see
black pudding and
tiết canh
★ Blood and
video game censorship
★
Taboo food and drink: Blood
★
blood phobia
★
Medical technologist
Notes
1.
2. Density of Blood
3. Martini, Frederic, et al (2006). ''Human Anatomy.'' 5th ed. Page 529. San Francisco, California: Pearson Education, Inc. ISBN 0-8053-7211-3
4. Ventilation and Endurance Performance
5. Transplant Support- Lung, Heart/Lung, Heart MSN groups
6. J Physiol. 2005 July 1
7. The 'St George' Guide To Pulmonary Artery Catheterisation
8. Oxygen Carriage in Blood - High Altitude
9. Lawlor, Robert (1991). ''Voices Of The First Day: Awakening in the Aboriginal dreamtime.'' Page 102-3. Rochester, Vermont: Inner Traditions International, Ltd. ISBN 0-89281-355-5
10. "Nostalgia" Artwork in blood
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