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Blood

Blood

Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). Medical terms related to blood often begin in hemo- or hemato- (BE: haemo- and haemato-) from the Greek word "haima" for "blood". The main function of blood is to supply nutrients (oxygen, glucose) and constitutional elements to tissues and to remove waste products (such as carbon dioxide and lactic acid). Blood also enables cells (leukocytes, abnormal tumor cells) and different substances (amino acids, lipids, hormones) to be transported between tissues and organs. Problems with blood composition or circulation can lead to downstream tissue dysfunction.

Anatomy of blood

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 normal pH of human arterial blood is approximately 7.40. Blood is about 7% of the human body weight [http://www.bloodcenters.org/aboutblood/bloodfacts.htm], 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 erythrocytes in the human anatomy would be roughly 2,000 times as great as the body's exterior surface. The corpuscles are:
- Red blood cells or erythrocytes (96%). In mammals, 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 (together with endothelial vessel cells and some other cells) are also marked by proteins that define different blood types.
- White blood cells or leukocytes (3.0%), are part of the immune system; they destroy infectious agents.
- Platelets or thrombocytes (1.0%) are responsible for blood clotting (coagulation) Blood plasma is essentially an aqueous solution containing 96% water, 4% blood plasma proteins, and trace amounts of other materials. Some components are:
- albumin
- blood clotting factors
- immunoglobulins (antibodies)
- hormones
- various other proteins
- various electrolytes (mainly sodium and chlorine) Together, plasma and corpuscles form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels.

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 maintained by the gut and the kidney. Blood cells are degraded by the spleen and the Kupffer cells in the liver. The liver also clears proteins and amino acids (the kidney secretes many small proteins 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 with the partial pressure of oxygen. 98.5% of the oxygen is chemically combined with the Hb. Only 1.5% is physically dissolved. 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. Under normal conditions in humans, hemoglobin in blood leaving the lungs is about 96-97% saturated with oxygen; 'deoxygenated' blood returning to the lungs is still approximately 75% saturated.[http://home.hia.no/~stephens/ventphys.htm][http://groups.msn.com/TransplantSupportLungHeartLungHeart/oxygen2.msnw] A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 20% 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 extract as much oxygen as possible from this sparse supply.[http://members.aol.com/Bio50/LecNotes/lecnot20.html]

Color

In humans and other hemoglobin-using creatures, oxygenated blood is a bright red in colour. 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 skin, veins typically appear blue.[http://www.people.virginia.edu/~rjh9u/blueblud.html] This has led to a common misconception that venous blood itself is blue.

Insects

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.

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. 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.

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 to form carbamino hemoglobin. 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.

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

See also blood diseases 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.
- 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 (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.

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 (such as the Anglo-Saxons and the Vikings), 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.

Judaism

In Judaism, blood cannot be consumed even in the smallest quantity (Leviticus 3:17 and elsewhere); 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 (Leviticus 17:13); the reason given by the Torah is: "Because the soul 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. Ironically, Judaism has historically been the religion to be most affected by blood libels.

Christianity

Christians believe that the Eucharist wine is, or represents, the blood of Jesus. This belief 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 his blood. "This cup is the new testament in my blood, which is shed for you." (Luke 22:20, KJV). The accepted Christian belief is that Jesus' blood atoned for the sins of the people.

Jehovah's Witnesses

Jehovah's Witnesses are prohibited from eating blood and accepting tranfusions of whole blood or any of red cells, white cells, platelets or plasma. They are permitted to accept fractions, and the acute normovolemic hemodilution (ANH) and autologous blood salvage (cell saver) procedures.

Vampire legends

Vampires are fictional beings thought to cheat death by drinking the blood of the living.

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.

Biological tissue

Biological tissue is a substance made up of cells that perform a similar function. The study of tissues is known as histology, or, in connection with disease, histopathology. The classical tools for studying the tissues are the wax block, the tissue stain, and the optical microscope, though developments in electron microscopy, immunofluorescence, and frozen sections have all added to the sum of knowledge in the last couple of decades. With these tools, the classical appearances of the tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis.

Animal Tissues

There are four basic types of tissue in the body of all animals, including the human body and lowar multicellular organisms such as insects. These compose all the organs, structures and other contents.
- Epithelium - Tissues composed of layers of cells that cover organ surfaces such as surface of the skin and inner lining of digestive tract. The tissues serve for protection, secretion, and absorption.
- Connective tissue - As the name suggests, connective tissue holds everything together. Blood is considered a connective tissue.
- Muscle tissue - Muscle cells contain contractile filaments that move past each other and change the size of the cell.
- Nervous tissue - Cells forming the brain, spinal cord and peripheral nervous system.

Plant Tissues

Examples of tissue in other multicellular organisms are vascular tissue in plants, such as xylem and phloem. Plant tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue.
- Epidermis - Cells forming the outer surface of the leaves and of the young plant body.
- Vascular tissue - The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally.
- Ground tissue - Ground tissue is less differentiated than other tissues. Ground tissue manufactures nutrients by photosynthesis and stores reserve nutrients.

References

- Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (1986). Biology of Plants (4th ed.). New York: Worth Publishers. ISBN 0-87901-315-X. Category:Anatomy Category:Tissues ms:Tisu biologi ja:組織 (生物学) simple:Tissue (biological)

Blood plasma

Blood plasma is the liquid component of blood, in which the blood cells are suspended. Serum is the same as blood plasma except that clotting factors (such as fibrin) have been removed. Plasma resembles whey in appearance (transparent with a faint straw colour). It is mainly composed of water, blood proteins, and inorganic electrolytes. It serves as transport medium for glucose, lipids, hormones, metabolic end products, carbon dioxide and oxygen. (Oxygen transport capacity of plasma is much lower than that of the hemoglobin in the red blood cells; it may increase under hyperbaric conditions.) Plasma is the storage and transport medium of clotting factors and its protein content is necessary to maintain the oncotic pressure of the blood.

Laboratory use of plasma and serum

For purposes of laboratory tests, plasma is obtained from whole blood. To prevent clotting, an anticoagulant such as citrate or heparin is added to the blood specimen immediately after it is obtained. (Usually the anticoagulant is already in the evacuated blood collection tube (e.g. Vacutainer or Vacuette®) when the patient is bled.) The specimen is then centrifuged to separate plasma from blood cells. Plasma can be frozen below -80°C nearly indefinitely for subsequent analysis or use. This blood product derivative is known as fresh frozen plasma (FFP). For many biochemical laboratory tests, plasma and blood serum can be used interchangeably. Serum resembles plasma in composition but lacks the coagulation factors. It is obtained by letting a blood specimen clot prior to centrifugation. For this purpose, a serum-separating tube (SST) can be used which contains an inert catalyst (such as glass beads or powder) to facilitate clotting as well as a portion of gel with a density designed to sit between the liquid and cellular layers in the tube after centrifugation, making separation more convenient. Tests of coagulation (such as the INR and aPTT) require all clotting factors to be preserved. Serum, therefore, is inappropriate for these tests. A citrated evacuated blood collection tube (e.g. Vacutainer or Vacuette) is usually used, as the anticoagulant effects of citrate is dependent upon concentration and can be reversed for testing. Serum is preferred for many tests as the anticoagulants in plasma can sometimes interfere with the results. Different anticoagulants interfere with different tests; using serum means the same sample can be used for many tests. In protein electrophoresis, using plasma causes an additional band to be seen, which might be mistaken for a paraprotein.

Fresh frozen plasma

Fresh frozen plasma (FFP) is prepared from a single unit of blood. It is frozen after collection and can be stored for one year from date of collection. FFP contains all of the coagulation factors and proteins present in the original unit of blood. It is used to treat coagulopathies from warfarin overdose, liver disease, or dilutional coagulopathy. FFP that has been stored more than a standard length of time is re-classified as simply "frozen plasma", which is identical except that the coagulation factors are no longer considered completely viable.

Dried plasma

liver disease Dried plasma was developed and first used during WWII. Prior to the United States involvement in the war, liquid plasma and whole blood were used. The "Blood for Britain" program during the early 1940s was quite successful (and popular stateside). Nontheless the decision was made to develop a dried plasma package for the armed forces because it reduced breakage and made transport, packaging, and storage much simpler. [http://history.amedd.army.mil/booksdocs/wwii/blood/chapter1.htm] The resulting Army-Navy dried plasma package came in two tin cans containing 400 cc bottles. One bottle contained enough distilled water to completely reconstitute the dried plasma contained in the other bottle. In about three minutes, the plasma would be ready to use and could stay fresh for around four hours. [http://history.amedd.army.mil/booksdocs/wwii/blood/chapter7.htm] By the end of the war the American Red Cross had provided enough blood for over six million plasma packages. Most of the surplus plasma was returned stateside for civilian use. Serum albumin replaced dried plasma for combat use during the Korean War. [http://history.amedd.army.mil/booksdocs/wwii/blood/chapter11.htm] Category:Blood ko:혈장 ja:血漿

Cell (biology)

(red) and DNA (green)]] The cell is the structural and functional unit of all living organisms, and are sometimes called the "building blocks of life." Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100,000 billion or 10¹⁴ cells). The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells and that cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. The word cell comes from the Latin cella, a small room. The name was chosen by Robert Hooke when he compared the cork cells he saw to small rooms monks lived in.

Overview

Properties of cells

cork Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities. All cells share several abilities:
- Reproduction by cell division.
- Metabolism, including taking in raw materials, building cell components, creating energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is derived from metabolic pathways.
- Synthesis of proteins, the functional workhorses of cells, such as enzymes. A typical mammalian cell contains up to 10,000 different proteins.
- Response to external and internal stimuli such as changes in temperature, pH or nutrient levels.
- Traffic of vesicles.

Types of cells

vesicle One way to classify cells is whether they live alone or in groups. Organisms vary from single cells (called single-celled or unicellular organisms) that function and survive more or less independently, through colonial forms with cells living together, to multicellular forms in which cells are specialized. 220 types of cells and tissues make up the multicellular human body. Cells can also be classified into two categories based on their internal structure.
- Prokaryotic cells are structurally simple. They are found only in single-celled and colonial organisms. In the three-domain system of scientific classification, prokaryotic cells are placed in the domains Archaea and Eubacteria.
- Eukaryotic cells have organelles with their own membranes. Single-celled eukaryotic organisms such as amoebae and some fungi are very diverse, but many colonial and multicellular forms such as plants, animals, and brown algae also exist.

Subcellular components

brown alga (2) nucleus (3) ribosome (4) vesicle,(5) rough endoplasmic reticulum (ER), (6) Golgi apparatus, (7) Cytoskeleton, (8) smooth ER, (9) mitochondria, (10) vacuole, (11) cytoplasm, (12) lysosome, (13) centrioles]] centriole All cells whether prokaryotic or eukaryotic have a membrane, which envelopes the cell, separates its interior from its environment, controls what moves in and out, and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes and RNA, which contain the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article will list these primary components of the cell then briefly describe their function.

Cell membrane - a cell's protective coat

Main article: Cell membrane The cytoplasm of a eukaryotic cell is surrounded by a plasma membrane. A form of plasma membrane is also found in prokaryotes, but is usually referred to as the cell membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (fat-like molecules) and proteins. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell.

Cytoskeleton - a cell's scaffold

Main article: Cytoskeleton The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell; and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell's structure by directing, bundling, and aligning filaments.

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long term information storage, but some viruses (retroviruses) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g. mRNA) and enzymatic functions (e.g. ribosomal RNA) in organisms that use RNA for the genetic code itself. Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory). A human cell, e.g. has genetic material in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). The nuclear genome is divided into 46 linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some important proteins. Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is.

Organelles

Main article: Organelle The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs", called organelles, that are adapted and/or specialized for carrying out one or more vital functions. Membrane-bound organelles are only found in eukaryotes.
- Cell nucleus - a cell's information center: The cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occur. The nucleus is spheroid in shape and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm.
- Ribosomes - the protein production machine: Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an mRNA. The process of converting an mRNA's genetic code into the exact sequence of amino acids that make up a protein is called translation. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell.
- Mitochondria and chloroplasts - the power generators: Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mentioned earlier, mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process involves a number of complex metabolic pathways. Chloroplasts are larger than mitochondria, and convert solar energy into a chemical energy ("food") via photosynthesis. Like mitochondria, chloroplasts have their own genome. Chloroplasts are found only in photosynthetic eukaryotes like plants and algae. There are a number of plant organelles that are modified chloroplasts; they are broadly called plastids and are often involved in storage.
- Endoplasmic reticulum and Golgi apparatus - macromolecule managers:: The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface, and the smooth ER, which lacks them. Translation of the mRNA for those proteins that will either stay in the ER or be exported from the cell occurs at the ribosomes attached to the rough ER. The smooth ER is important in lipid synthesis, detoxification and as a calcium reservoir. The Golgi apparatus, sometimes called a Golgi body or Golgi complex is the central delivery system for the cell and is a site for protein processing, packaging, and transport. Both organelles consist largely of heavily folded membranes.
- Lysosomes and peroxisomes - the cellular digestive system: Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.
- Centrioles - They help in the formation of mitotic appratus. Two centrioles are present in the animal cells. They are also found in some fungi and algae cells.
- Vacuoles-They store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane.

Anatomy of cells

Prokaryotic cells

Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack most of the intracellular organelles and structures that are characteristic of eukaryotic cells (an important exception is the ribosomes, which are present in both prokaryotic and eukaryotic cells). Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili—proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:
- The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
- Most prokaryotes have a cell wall (some exceptions are Mycoplasma (a bacterium) and Thermoplasma (an archaeon)). It consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" from osmotic pressure against a hypotonic environment. A cell wall is also present in some eukaryotes like fungi, but has a different chemical composition
- A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Even without a real nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.

Eukaryotic cells

There are two types of cells, eukaryotic and prokaryotic. Eukaryotic cells are usally found in multi-cellular organisms, while prokaryotic cells are usually on their own. Eukaryotic cells are about 10 times the size of a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true nucleus." Other differences include:
- The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
- The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are highly condensed (i.e. folded around histones). All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles can contain some DNA.
- Eukaryotes can move using cilia or flagella. The flagella are more complex than those of prokaryotes.

Cell functions

Cell growth and metabolism

Main articles: Cell growth, Cell metabolism Between successive cell divisions cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions; catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, where the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into a less chemically complex sugar molecule called glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form of energy, via two different pathways. The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy. The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.

Making new cells

Main article: Cell division Cell divisions (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.]] Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells. DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.

Protein synthesis

Main article: Protein biosynthesis Protein synthesis is the process in which the cell builds proteins. DNA transcription refers to the synthesis of a messenger RNA (mRNA) molecule from a DNA template. This process is very similar to DNA replication. Once the mRNA has been generated, a new protein molecule is synthesized via the process of translation. The cellular machinery responsible for synthesizing proteins is the ribosome. The ribosome consists of structural RNA and about 80 different proteins. When the ribosome encounters an mRNA, the process of translating an mRNA to a protein begins. The ribosome accepts a new transfer RNA, or tRNA—the adaptor molecule that acts as a translator between mRNA and protein—bearing an amino acid, the building block of the protein. Another site binds the tRNA that becomes attached to the growing chain of amino acids, forming the a polypeptide chain that will eventually be processed to become a protein.

Origins of cells

Main article: Origin of life The origin of cells has to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.

Origin of first cell

If life is viewed from the point of view of replicators, that is DNA molecules in the organism, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. The latter is fundamental for the evolution of biological complexity. If freely-floating DNA molecules that code for enzymes that are not enclosed into cells, the enzymes that advantage a given DNA molecule (for example, by producing nucleotides) will automatically advantage the neighbouring DNA molecules. This might be viewed as "parasitism by default". Therefore the selection pressure on DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: all molecules in a given neighbourhood are almost equally advantaged. If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules won't benefit from a positive mutation in a neighbouring molecule: this in turn means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it. (Note. This is more a metaphor given for simplicity than complete accuracy, since the earliest molecules of life, probably up to the stage of cellular life, were most likely RNA molecules, acting both as replicators and enzymes: see RNA world hypothesis . But the core of the reasoning is the same.) Biochemically, cell-like spheroids formed by proteinoids are observed by heating amino acids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth. Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water tend to burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The Phospholipid is a good example of a common oily compound prevalent in the prebiotic seas. Phospholipids can be constructed in ones mind as a hydrophilic head on one end, and a hydrophobic tail on the other. Phospholipids also possess an important characteristic, that is being able to link together to form a bilayer membrane. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Phospholipids.html] can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multi-celluar organisms could be achieved. This theory is expanded upon in the book, "The Cell: Evolution of the First Organism" by Joseph Panno Ph.D.

Origin of eukaryotic cells

The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell – a theory termed the endosymbiotic theory. There is still considerable debate on if organelles like the hydrogenosome predated the origin of mitochondria, or viceversa : see the hydrogen hypothesis for the origin of eukaryotic cells.

History

- 1632-1723: Antony van Leeuwenhoek teaches himself to grind lenses, builds a microscope and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth.
- 1665 : Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope. ::...I could exceedingly plainly perceive it to be all perforated and porous, much like honeycomb...these pores or cells, were not very deep, but consisted of a great many little boxes... – Hooke describing his observations on a thin slice of cork.
- 1839 : Theodor Schwann and Matthias Jakob Schleiden elucidate the principal that plants and animals are made of cells, concluding that cells are a common unit of structure and development, thus founding the Cell Theory.
- The belief that life forms are able to occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822-1895).
- Rudolph Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula).
- 1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of Berlin. By 1935 he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1953: Watson and Crick made their first announcement on the double-helix structure for DNA on February 28.
- 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory.

References

- Category:Cell biology Category:Biology ko:세포 ms:Sel ja:細胞 simple:Cell th:เซลล์ (ชีววิทยา)

Red blood cell

Red blood cells are the most common type of blood cell and are the vertebrate body's principal means of delivering oxygen from the lungs or gills to body tissues via the blood. Red blood cells are also known as RBCs or erythrocytes (from Greek erythros for "red" and kytos for "hollow," nowadays translated as "cell"). A schistocyte is a red blood cell undergoing fragmentation, or a fragmented part of a red blood cell.

Vertebrate erythrocytes

Erythrocytes consist mainly of hemoglobin, a complex molecule containing heme groups whose iron molecules temporarily link to oxygen molecules in the lungs or gills and release them throughout the body. Hemoglobin also carries some of the waste product carbon dioxide back from the tissues. (In humans, less than 2% of the total oxygen, and most of the carbon dioxide, are held in solution in the blood plasma). A related compound, myoglobin, acts to store oxygen in muscle cells. The color of erythrocytes is due to the heme group of hemoglobin. The blood plasma is straw-colored alone, but the red blood cells change colors due to the state of the hemoglobin: when combined with oxygen the resulting oxyhemoglobin is scarlet and when oxygen has been released, the resulting deoxyhemogloben is darker, appearing bluish through the blood vessel walls. The keeping of oxygen-binding proteins in cells (rather than having them dissolved in body fluid) was an important step in the evolution of vertebrates; it allows for less viscous blood and longer transport ways of oxygen.

Mammalian erythrocytes

Erythrocytes in mammals are anucleate when mature, meaning that they don't have a cell nucleus and thus no DNA. (The erythrocytes of nearly all other vertebrates have nuclei; the only known exception is salamanders of the Batrachoseps genus.) Erythrocytes also lose their other organelles including their mitochondria and produce energy by fermentation, via glycolysis of glucose followed by lactic acid production. Like most cell types, red cells do not have an insulin receptor and thus glucose uptake is not regulated by insulin. Mammalian erythrocytes have a biconcave shape: flattened and depressed in the center. This shape (as well as the loss of organelles) optimizes the cell for the exchange of oxygen with its surroundings. The cells are flexible so as to fit through tiny capillaries, where they release their oxygen load. Erythrocytes are circular, except in the camel family Camelidae, where they are oval. In large blood vessels, red blood cells sometimes occur as a stack, flat side next to flat side. This is known as rouleaux formation, and it occurs more often if the levels of certain serum proteins are elevated, as for instance during inflammation. The spleen acts as a reservoir of red blood cells, but this effect is somewhat limited in humans. In some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells that are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.

Human erythrocytes

The diameter of a typical human erythrocyte is 6–8 µm; they are thus much smaller than most other human cells. A typical erythrocyte contains about 270 million hemoglobin molecules, with each carrying four heme groups. Adult humans have roughly 2–3 × 10¹³ red blood cells at any given time (women have about 4 million to 5 million erythrocytes per cubic millimeter (microliter) of blood and men about 5 million to 6 million; people living at high altitudes with low oxygen concentration will have more). Red blood cells are thus much more common than the other blood particles: There are about 4,000–11,000 white blood cells and about 150,000–400,000 platelets in a cubic millimeter of human blood. The red blood cells store collectively about 3.5 grams of iron; that's more than five times the iron stored by all the other tissues combined. The process by which red blood cells are produced is called erythropoiesis. Erythrocytes are continuously being produced in the red bone marrow of large bones. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO), which is used for doping in sports. Erythrocytes develop in about 7 days and live a total of about 120 days. The aging cells swell up to a sphere-like shape and are engulfed by phagocytes, destroyed and their materials are released into the blood. The main sites of destruction are the liver and the spleen. The heme constituent of hemoglobin is eventually excreted as bilirubin. The blood types of humans are due to variations in surface glycoproteins of erythrocytes. Red blood cells can be separated from blood plasma by centrifugation. During plasma donation, the red blood cells are pumped back into the body right away, and the plasma is collected. Some athletes have tried to improve their performance by doping their blood: First about 1 liter of their blood is extracted, then the red blood cells are isolated, frozen and stored, to be reinjected shortly before the competition. (Red blood cells can be conserved for 5 weeks at −78 °C.) This practice is hard to detect but may endanger the human cardiovascular system which is not equipped to deal with blood of the resulting higher viscosity.

Diseases and diagnostic tools

viscosity Blood diseases involving the red blood cells include:
- Anemias (or anaemias) are diseases characterized by low oxygen transport capacity of the blood, because of low red cell count or some abnormality of the red blood cells or the hemoglobin.
- Iron deficiency anemia is the most common anemia; it occurs when the dietary intake or absorption of iron is insufficient, and hemoglobin, which contains iron, cannot be formed
- Sickle-cell disease is a genetic disease which leads to mis-shaped red blood cells.
- Thalassemia is a genetic disease that results in the production of abnormal hemoglobin molecules.
- Spherocytosis is a genetic disease that causes a defect in the red blood cell's cytoskeleton, causing the RBCs to be small, sphere-shaped, and fragile instead of donut-shaped and flexible.
- Pernicious anemia is an autoimmune disease wherein the body lacks intrinsic factor, required to absorb vitamin B12 from food. Vitamin B12 is needed for the production of hemoglobin.
- Aplastic anemia is caused by the inability of the bone marrow to produce blood cells.
- Hemolysis is the general term for excessive breakdown of red blood cells. It can have several causes.
- The malaria parasite spends part of its life-cycle in red blood cells, feeds on their hemoglobin and then breaks them apart, causing fever. Both sickle-cell disease and thalassemia are more common in malaria areas, because these mutations convey some protection against the parasite.
- Polycythemias (or erythrocytoses) are diseases characterized by a surplus of red blood cells. The increased viscosity of the blood can cause a number of symptoms.
- In polycythemia vera the increased number of red blood cells results from an abnormality in the bone marrow. Several blood tests involve red blood cells, including the RBC count (the number of red blood cells per volume of blood) and the hematocrit (percentage of blood volume occupied by red blood cells). The blood type needs to be determined to prepare for a blood transfusion or an organ transplantation.

History

In 1658, the Dutch Jan Swammerdam was the first to describe red blood cells; he had used an early microscope.

External links

- [http://www.genomesize.com/cellsize/ Database of vertebrate erythrocyte sizes] Category:Blood cells Category:Hematology ko:적혈구 ja:赤血球

White blood cell

White blood cells (also called leukocytes or immune cells) are a component of blood. They help to defend the body against infectious disease and foreign materials as part of the immune system. There are normally between 4x10⁹ and 11x10⁹ white blood cells in a litre of healthy adult blood - about 7 000 to 25 000 white blood cells per drop. In conditions such as leukemia this may rise to as many as 50 000 white blood cells in a single drop of blood. As well as in the blood, white cells are also found in large numbers in the lymphatic system, the spleen, and in other body tissues.

Nomenclature

The name "white cells" derives the from the fact that after centrifugation of a blood sample, the white cells are found in the Buffy coat, a small fraction between the hematocrit and the blood plasma, which is white in color (or sometimes green, if there are large amounts of neutrophils in the sample, which are high in green myeloperoxidase). Any of various blood cells that have a nucleus and cytoplasm, separate into a thin white layer when whole blood is centrifuged, and help protect the body from infection and disease. White blood cells include neutrophils, eosinophils, basophils, lymphocytes, and monocytes. Also called leukocyte, white cell, white corpuscle.

Types

There are three major types of white blood cells.

Granulocytes

Granulocytes are a category of white blood cells, characterised by the fact that all types have differently staining granules in their cytoplasm on light microscopy. There are three types of granulocytes: neutrophils, basophils, and eosinophils (named according to their staining properties).

Lymphocytes

Lymphocytes are much more common in the lymphatic system, and include the so-called "killer T-cells". The blood has three types of lymphocytes: B cells, T cells and natural killer cells. B cells make antibodies that bind to pathogens to enable their destruction. CD4+ (helper) T cells co-ordinate the immune response (they are what becomes defective in an HIV infection). CD8+ (cytotoxic) T cells and natural killer cells are able to kill cells of the body that are infected by a virus.

Monocytes

Monocytes share the 'vacuum cleaner' function of neutrophils, but are much longer lived as they have an additional role. They present pieces of pathogens to T cells so that the pathogens may be recognised again and killed, or so that an antibody response may be mounted. Monocytes are also known as macrophages after they leave the bloodstream and enter tissue.

Diseases

- Leukopenia is a disease symptom defined as a lower than normal number of white blood cells in the blood.
- Leukocytosis refers to an increase in the number of white blood cells in the blood.
- Leukemia and lymphoma are two types of cancer in which white blood cells multiply out of control.

Other tissue cells

- Histiocytes, found in the lymphatic system and other body tissues, but not normally in blood:
- Macrophages
- Dendritic cells
- Mast cells

External links

Category:Blood cells ko:백혈구 ja:白血球 simple:White blood cell

Platelet

Platelets or thrombocytes are the blood cell fragments that are involved in the cellular mechanisms that lead to the formation of blood clots. Low levels or dysfunction predisposes for bleeding, while high levels - although usually asymptomatic - may increase the risk of thrombosis.

Anatomy

Like red blood cells, platelets are anuclear and discoid; they measure 1.5-3.0 μm in diameter. The body has a very limited reserve of platelets and so they can be rapidly depleted. They contain RNA, a canalicular system, and several different types of granules; lysosomes (containing acid hydrolases), dense bodies (containing ADP, ATP serotonin and calcium) and alpha granules (containing fibrinogen, factor V, vitronectin, thrombospondin and von Willebrand factor), the contents of which are released upon activation of the platelet. These granule contents play an important role in both hemostasis and in the inflammatory response.

Physiology

Production

Platelets are produced in the bone marrow; the progenitor cell for platelets is the megakaryocyte. This large, multinucleated cell sheds platelets into the circulation. Thrombopoietin (c-mpl ligand) is a hormone, mainly produced by the liver, that stimulates platelet production. It is bound to circulating platelets; if platelet levels are adequate, serum levels remain low. If the platelet count is decreased, more thrombopoeitin circulates freely and increases marrow production.

Circulation

The circulating life of a platelet is 9-10 days. After this it is sequestered in the spleen. Decreased function (or absence) of the spleen may increase platelet counts, while hypersplenism (overactivity of the spleen, e.g. in Gaucher's disease or leukemia) may lead to increased elimination and hence low platelet counts.

Function

Platelets are activated when brought into contact with collagen (which is exposed when the endothelial blood vessel lining is damaged), thrombin (primarily through PAR-1), ADP, with receptors expressed on white blood cells or the endothelial cells of the blood vessels, among other activators. Once activated, they release a number of different coagulation factors and platelet activating factors, they also provide a catalytic phospholipid surface (with the charge provided by phosphatidylserine and phosphatidylethanolamine) for the tenase and prothrombinase complexes. The platelets adhere to each other via adhesion receptors or integrins, and to the endothelial cells in the wall of the blood vessel forming a haemostatic plug in conjunction with fibrin. The high concentration of myosin and actin filaments in platelets are stimulated to contract during aggregation, further reinforcing the plug. The most common platelet adhesion receptor is glycoprotein (GP) IIb/IIIa this is a calcium dependent receptor for fibrinogen, fibronectin, vitronectin, thrombospondin and von Willebrand factor (vWF). Other receptors include GPIb-V-IX complex (vWF) and GPVI (collagen)

Activators

There are many known platelet activators. They include
- Collagen, especially with von Willebrand factor which is exposed when endothelial blood vessel lining is damaged and binds to GPVI on the platelet,
- thrombin primarily through cleavage of the extracellular domain of PAR1 and PAR4,
- Thromboxane A₂ (TxA₂) which binds to TP,
- ADP through creation of TxA₂, and it can be blocked by conversion of ADP to cAMP,
- Human neutrophil elastase (HNE) cleaves the α_IIbβ₃ integrin on the platelet surface,
- P-selectin which binds to PSGL-1 on endothelial cells and white blood cells, and
- Convulxin (a purified protein from snake venom) which binds to GPVI.

Inhibitors

- Prostacyclin opposes the actions of Thromboxane A₂
- Nitric oxide
- Clotting factors II, IX, X, XI, XII
- Nucleotidases by breaking down ADP

Role in disease

High and low counts

A normal platelet count in a healthy person is between 150 and 400 (x 10⁹/L of blood). Both thrombocytopenia (or thrombopenia) and thrombocytosis may present with coagulation problems. Generally, low platelet counts increase bleeding risks (although there are exceptions, e.g. Immune heparin-induced thrombocytopenia) and thrombocytosis (high counts) may lead to thrombosis (although this is mainly when the elevated count is due to myeloproliferative disorder). Low platelet counts are generally not corrected by transfusion unless the patient is bleeding or the count has fallen below 5 (x 10⁹/L); it is contraindicated in thrombotic thrombocopenic purpura (TTP) as it fuels the coagulopathy. In patients having surgery, a level below 50 (x 10⁹/L) is associated with abnormal surgical bleeding, and regional anaesthetic procedures such as epidurals are avoided for levels below 80-100. Note however that the actual platelet count is only part of the story, since they may not all be functioning normally. For example, aspirin irreversibly prevents platelets from working correctly and so normal hemostasis may not return until the aspirin is ceased and the affected platelets have been replaced by new ones, which may take over a week.

Diseases

Disorders leading to a reduced platelet count:
- Thrombocytopenia
- Idiopathic thrombocytopenic purpura
- Thrombotic thrombocytopenic purpura
- Drug-induced thrombocytopenia, e.g. heparin-induced thrombocytopenia (HIT)
- Gaucher's disease
- Aplastic anemia Disorders leading to platelet dysfunction or reduced count:
- HELLP syndrome
- Hemolytic-uremic syndrome
- Chemotherapy Disorders featuring an elevated count:
- Thrombocytosis, including benign essential thrombocytosis (elevated counts, either reactive or as an expression of myeloproliferative disease); may feature dysfunctional platelets Disorders of platelet adhesion or aggregation:
- Bernard-Soulier syndrome
- Glanzmann's thrombasthenia
- Scott's syndrome
- von Willebrand disease Disorders of platelet metabolism
- Decreased cyclooxygenase activity, induced or congenital
- Storage pool defects, acquired or congenital

Transfusion

Platelets are separated from donated blood using an apheresis blood separator. This is necessary because platelets will not survive at the low temperatures used to store red blood cells, so they must be stored separately using porous storage bags that allow oxygen to flow in and carbon dioxide to flow out. Typical storage is between 20 and 24 °C and continuously agitated to promote gas exchange. Because of the higher risks of bacterial growth at this temperature, platelets are generally only stored for up to 5 days. A bag of platelets can be separated from multiple bags of whole blood or from a single donor connected to the separator for less than two hours. By drawing and returning blood repeatedly, a bag of high quality platelets can be prepared in about 90 minutes. Platelets collected from a single donor can reduce the infection rates of blood-transmitted diseases. People with few platelets or platelets that are dysfunctional may benefit from a platelet transfusion, however patients with autoimmune disorders that affect platelets may not.

British English

British English (BrE) is a term used to differentiate the form of the written English language in the United Kingdom from other forms of the English language. It is also used by some, particularly Americans, to describe the spoken versions of English used within England. The term is rarely heard within the United Kingdom. British people say that they speak English - but never British - and that others speak English with an accent, such as a 'South African accent'. When speaking, they will often drop the word "accent" and simply say Canadian, American, Jamaican and so on. A less ambiguous term is English English. Although British English can describe the formal written English used in the United Kingdom, the forms of spoken English used in the United Kingdom vary considerably more than in most other areas of the world where English is spoken. Dialects and accents vary not only within regions of the UK, for example in Scotland, Northern Ireland and Wales, but also within England. The written form of the language, as taught in schools, is universally Commonwealth English with a slight emphasis on a few words that might be more common in some areas than in others. For example, although the words "wee" and "small" are interchangeable, one is more likely to see "wee" written by a Scot than by a Londoner. For historical reasons dating back to the rise of London in the 9th century, the variety of language spoken in London and the East Midlands became the standard English within the Court and thus the form of language generally accepted for use in the law, government, literature and education of the British Isles. Like other forms of languages, the English used in Britain changes over time. Although British English is often used in the United States to denote the English spelling and lexicon used outside the US, the term Commonwealth English is more accurate for this purpose. The British spellings were most famously recorded in Samuel Johnson's A Dictionary of the English Language (1755). Historically, the widespread usage of English across the world is attributed to the power once held by the British Empire, and hence the most common form of English used by the British ruling class was the English used in south-east England (in the area around the capital city London, and the main English university towns of Oxford and Cambridge). This form of the language is associated with Received Pronunciation (RP), which is still regarded by many people outside the UK (especially in the United States) as "the British accent". From the second half of the 20th century to the present day, the preeminence of the English language has largely been linked to the economic, military and political dominance of the United States in world affairs, and American English is often regarded as the most prominent form of English in the world today, especially with the large amount of U.S. cultural products (such as films, books, and music) around the world, which is not matched in volume by those from other English-speaking nations. The form of English spoken and particularly written in the United Kingdom still has a major cultural influence on the English used in many Commonwealth countries, including Australia, South Africa, and India, as well as in the European Union. Although British English is taught and used in the former British colonies of Hong Kong, Singapore and Malaysia, American English is often taught in Chinese and Japanese schools, and in other schools throughout Asia.

-ise versus -ize

Words of the sort organize/organise and their derivatives can be spelt with either s or z in British English. The -ize forms are promoted by the Oxford English Dictionary. British English with -ize is sometimes known as OED spelling, and may be marked by the registered IANA language tag 'en-GB-oed'. It is the spelling used by the Encyclopaedia Britannica, by the United Nations, and by many international organizations and academic publications. The -ize forms were used by the London Times until the mid-1980s. The -ise forms are now generally used by the British government, by the European Union and mostly taught in the British school system. They are far more prevalent in common usage. Pam Peters (2004, -ize/-ise) relates that British National Corpus data indicates the ratio of popularity for -ise forms to -ize forms in Britain is 3:2.

Greek language

Greek (Greek Ελληνικά, IPA – "Hellenic") is an Indo-European language with a documented history of 3,500 years. Today, it is spoken by 15 million people in Greece, Cyprus, the former Yugoslavia, particularly The Former Yugoslav Republic of Macedonia, Bulgaria, Albania and Turkey. There are also many Greek emigrant communities around the world, such as those in Melbourne, Australia which is the third-largest Greek-populated city in the world, after Athens and Thessaloniki. Greek has been written in the Greek alphabet, the first true alphabet, since the 9th century B.C. and before that, in Linear B and the Cypriot syllabaries. Greek literature has a long and rich tradition.

History

This article does not cover the reconstructed history of Greek prior to the use of writing. For more information, see main article on Proto-Greek language. Greek has been spoken in the Balkan Peninsula since the 2nd millennium BC. The earliest evidence of this is found in the Linear B tablets dating from 1500 BC. The later Greek alphabet (q.v.) is unrelated to Linear B, and was derived from the Phoenician alphabet (abjad); with minor modifications, it is still used today. Greek is conventionally divided into the following periods:
- Mycenean Greek: the language of the Mycenean civilisation. It is recorded in the Linear B script on tablets dating from the 16th century BC onwards.
- Classical Greek (also known as Ancient Greek): In its various dialects was the language of the Archaic and Classical periods of Greek civilisation. It was widely known throughout the Roman empire. Classical Greek fell into disuse in western Europe in the Middle Ages, but remained known in the Byzantine world, and was reintroduced to the rest of Europe with the Fall of Constantinople and Greek migration to Italy.
- Hellenistic Greek (also known as Koine Greek): The fusion of various ancient Greek dialects with Attic (the dialect of Athens) resulted in the creation of the first common Greek dialect, which gradually turned into one of the world's first international languages. Koine Greek can be initially traced within the armies and conquered territories of Alexander the Great, but after the Hellenistic colonisation of the known world, it was spoken from Egypt to the fringes of India. After the Roman conquest of Greece, an unofficial diglossy of Greek and Latin was established in the city of Rome and Koine Greek became a first or second language in the Roman Empire. Through Koine Greek it is also traced the origin of Christianity, as the Apostles used it to preach in Greece and the Greek-speaking world. It is also known as the Alexandrian dialect, Post-Classical Greek or even New Testament Greek (after its most famous work of literature).
- Medieval Greek: The continuation of Hellenistic Greek during medieval Greek history as the official and vernacular (if not the literary nor the ecclesiastic) language of the Byzantine Empire, and continued to be used until, and after the fall of that Empire in the 15th century. Also known as Byzantine Greek.
- Modern Greek: Stemming independently from Koine Greek, Modern Greek usages can be traced in the late Byzantine period (as early as 11th century). Two main forms of the language have been in use since the end of the medieval Greek period: Dhimotikí (Δημοτική), the Demotic (vernacular) language, and Katharévousa (Καθαρεύουσα), an imitation of classical Greek, which was used for literary, juridic, and scientific purposes during the 19th and early 20th centuries. Demotic Greek is now the official language of the modern Greek state, and the most widely spoken by Greeks today. It has been claimed that an "educated" speaker of the modern language can understand an ancient text, but this is surely as much a function of education as of the similarity of the languages. Still, Koinē , the version of Greek used to write the New Testament and the Septuagint, is relatively easy to understand for modern speakers. Greek words have been widely borrowed into the European languages: astronomy, democracy, philosophy, thespian, etc. Moreover, Greek words and word elements continue to be productive as a basis for coinages: anthropology, photography, isomer, biomechanics etc. and form, with Latin words, the foundation of international scientific and technical vocabulary. See English words of Greek origin, and List of Greek words with English derivatives.

Classification

Greek is an independent branch of the Indo-European language family. The ancient languages which were probably most closely related to it, Ancient Macedonian language (which may be regarded as a dialect of Greek) and Phrygian, are not well enough documented to permit detailed comparison. Among living languages, Armenian seems to be the most closely related to it.

Geographic distribution

Modern Greek is spoken by about 15 million people mainly in Greece and Cyprus. There are also Greek-speaking populations in Georgia, Ukraine, Egypt, Turkey, Albania, Former Yugoslav Republic of Macedonia and Southern Italy. The language is spoken also in many other countries where Greeks have settled, including Armenia, Australia, Austria, Belgium, Bulgaria, Canada, Denmark, France, Germany, Netherlands, Sweden, United Kingdom, and the United States.

Official status

Greek is the official language of Greece where it is spoken by about 99.5% of the population. It is also, alongside Turkish, the official language of Cyprus. Due to the membership of Greece and Cyprus, Greek is one of the 20 official languages of the European Union.

Phonology

This section generally describes the post-Classic phonology of the Greek language. :All phonetic transcriptions in this section use the International Phonetic Alphabet

Vowel sounds

Greek has 5 vowel sounds, all phonemic:

Glucose

Glucose (Glc), a monosaccharide, is one of the most important carbohydrates. The cell uses it as a source of energy and metabolic intermediate. Glc is one of the main products of photosynthesis and starts cellular respiration. The natural form (D-glucose) is also referred to as dextrose, especially in the food industry. This article deals with the D-form of Glc (see Isomers-section bellow)

Structure

cellular respiration cellular respiration Glc contains six carbon atoms and an aldehyde group and is therefore refered to as an aldohexose. Glc molecule can exist in an open-chain (acyclic) and ring (cyclic) form, the latter being the result of a intramolecular reaction between the aldehyde C atom and the C-5 hydroxyl group to form an intramolecular hemiacetal. In water solution both forms are in equilibrium, and at pH 7 the cyclic one is the predominant. As the ring contains 5 carbon and one oxygen atoms, which resembles the structure of pyran, the cyclic form of Glc is also refered to as glucopyranose. In this ring, each carbon is linked to hydroxyl side group with the exception of the fifth atom, which links to a sixth carbon atom outside the ring, forming a CH₂OH group.

Isomers

Glc has 4 optic centers which means that in theory Glc can have 15 optical stereoisomers. In living organisms only 7 of them are found, of which Gal and Man are the most important. These eight isomers (including Glc) are all