Encyclopedia
Blood is a highly specialised circulating tissue consisting of several types of cell suspended in a fluid medium known as
plasma. The cellular constituents are:
red blood cells, which carry respiratory gases and give it its red color, white blood cells , which fight disease, and
platelets, cell fragments which play an important part in the clotting of the blood.
Medical terms related to blood often begin with
hemo- or
hemato- 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 of blood are:
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 and returned to the blood . The lymphatic circulation has been called the "second circulation".
Anatomy of mammalian blood
Blood is composed of several kinds of cells ; these
formed elements of the blood constitute about 45% of whole blood. The other 55% is
blood plasma, a fluid that is the blood's liquid medium, appearing yellow in color. The normal
pH of human arterial blood is approximately 7.40 , a weak alkaline solution. Blood that has a
pH below 7.35 is acidic, while blood
pH above 7.45 is alkaline. Blood
pH along with arterial carbon dioxide tension and HCO
3 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. Blood is about 7% of the human body weight , so the average adult has a blood volume of about 5 liters, of which 2.7-3 liters is plasma.
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.
The cells are:
- Red blood cells or erythrocytes . In mammals, mature red blood cells lack a nucleus and organelles. They contain the blood's hemoglobin and distribute oxygen. The red blood cells are also marked by glycoproteins that define the different blood types.
- White blood cells or leukocytes , are part of the immune system; they destroy infectious agents, pathogens.
- Platelet
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s or thrombocytes are responsible for blood clotting .
Blood plasma is essentially an
aqueous solution containing 92% water, 8% blood plasma
proteins, and trace amounts of other materials. Some components are:
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.
Physiology of blood
Production and degradation
Blood cells are produced in the
bone marrow; the process is termed
hematopoiesis. The proteinaceous component is produced overwhelmingly in the
liver, while hormones are produced by the
endocrine glands and the watery fraction 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. Erythrocytes usually live up to 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 saturation percentage. This is a non-linear 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 .
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, haemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen;
deoxygenated blood returning to the lungs is still approximately 75% saturated. A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures and so fetuses produce another form of hemoglobin with a much higher affinity for oxygen in order to extract as much oxygen as possible from this sparse supply.
Besides oxygen other substances can bind to the haemoglobin which can cause irreversible damage to the body.
Carbon monoxide for example is extremely dangerous when absorbed into the blood. When combined with the haemoglobin, it irreversibly makes carboxyhaemoglobin 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 . This damage can occur when smoking a
cigarette 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.
Insects
In
insects, the blood is not involved in the transport of oxygen. . Insect blood moves nutrients to the tissues and removes waste products in an open system.
Small invertebrates
In some small invertebrates like
insects, oxygen is simply dissolved in the plasma. Larger animals use respiratory proteins to increase the oxygen carrying capacity. Haemoglobin is the most common respiratory protein found in nature.
Hemocyanin contains
copper and is found in
crustaceans and
mollusks. It is thought that
tunicates might use
vanabins for 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 or damaging blood filtering organs like the
kidneys.
Deep sea invertebrates
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. 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 H+ than does oxyhemoglobin so it binds most of the hydrogen ions.
Color
In humans and other hemoglobin-using creatures, oxygenated blood is bright red. This is due to oxygenated iron 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. However, due to an optical effect caused by the way in which light penetrates through the skin, veins typically appear blue in color. This has led to a common misconception that venous blood is blue before it is exposed to air. Another reason for this misconception is that medical charts always show venous blood as blue in order to distinguish it from arterial blood which is depicted as red on the same chart.
Health and disease
Ancient medicine
Hippocratic medicine considered blood one of the
four humors . As many diseases were thought to be due to an excess of blood,
bloodletting and leeching were a common intervention until the
19th century .
In classical Greek medicine, blood was associated with air, springtime, and with a merry and gluttonous 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
See also blood diseasesProblems with blood circulation and composition play a role in many diseases.
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s 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 , 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
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compatible with that of the donor.
- 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.
- Infection of the blood is bacteremia or sepsis. 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 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.
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 where 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".
Indo-European paganism
Among the
Germanic tribes , 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 ; this is reflected in the
dietary laws. Blood is purged from
meat by salting and
pickling.
Other rituals involving blood are the covering of the blood of fowl and
game after slaughtering ; the reason given by the
Torah is: "Because the life of every animal is [in] his blood" , 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 , in the
Temple in Jerusalem.
Ironically,
Judaism has historically been the religion to be most affected by blood libels.
Christianity
- Main article: Eucharist
Some Christian churches, typically
Roman Catholicism,
Eastern Orthodoxy, some parts of
Anglicanism, and the Moravian Church, teach that when consecrated the
Eucharist wine becomes the Blood of
Jesus. 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." . Other forms of Protestantism, especially those of a Wesleyan or Presbyterian lineage, teach that Christ, while not physically present, is somehow still there.
Jehovah's Witnesses
Jehovah's Witnesses prohibit eating blood or accepting tranfusions of whole blood or its major components namely, red blood cells, white blood cells, platelets , and whole plasma. They are permitted to accept fractions, and the acute normovolemic hemodilution and autologous blood salvage procedures.
Vampire legends
Vampires are fictional beings thought to cheat death by drinking the blood of the living. This myth is mostly inspired by European folklore based on the stories regarding
Vlad III the Impaler and rare variations of dementia specific to the region.
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.
Art
Blood is one of the body fluids that has been used in art.
See also
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