Blood

A human blood smear showing red blood cells (a) and three types of white blood cells (neutrophil, eosinophil, and lymphocyte) (b-d).

Blood is a highly specialized circulating tissue that consists of several types of cells suspended in a fluid medium. Along with the heart (a pump) and blood vessels (a series of conduits), blood forms an essential part of the circulatory system, a system of gas exchange in all terrestrial vertebrates.

There are two main components of blood: its cellular components (collectively known as the hematocrit) and an aqueous medium for transporting the cellular materials (called the plasma). The hematocrit consists of three types of cells responsible for the blood's crucial functions:

• Red blood cells (erythrocytes) transport respiratory gases. Erythrocytes give vertebrate blood its distinctive color due to the presence of hemoglobin, an iron-containing protein that binds oxygen in the lungs and transports it to tissues in the body.
• White blood cells (leukocytes) serve an immunological function, defending the body against foreign materials.
• Platelets (thrombocytes) are cell fragments that play an important part in the clotting of the blood.

In addition to the roles outlined above, blood performs a range of functions, supplying nutrients such as glucose, amino acids and fatty acids to various tissues; removing waste products such as carbon dioxide, urea and lactic acid; transporting hormones and signaling tissue damage; and regulating the body pH and core body temperature required for a stable internal environment (homeostasis).

The term "blood" may also be used to refer to a related substance in insects called the hemolymph. explain

Components and properties

Blood contributes about 7% of the human body weight (Alberts, 2005), with an average density of approximately 1060 kg/m³ (Shmukler, 2004). The average adult has a blood volume of roughly 5 liters. Blood cells constitute about 45% of whole blood by volume, with the other 55% represented by plasma, a fluid that is the blood cells' liquid medium, appearing golden-yellow in color.

A scanning electron microscope (SEM) image of normal circulating human blood. Visible components include red blood cells; several types of white blood cells; and many small, disc-shaped platelets.

There are three types of blood cells

Each liter of blood contains the following cellular components:

• 5 × 10¹² erythrocytes (45.0% of blood volume): Red blood cells (erythrocytes) are shaped like discs, giving them a large surface for gas exchange; their flexibility also enables them to squeeze through capillaries. 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 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.
• 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 are activated when they come into contact with exposed collagen fibers, and work in tandem with components in the blood plasma (called blood clotting factors).

Blood plasma represents over half the volume of blood

Blood plasma is essentially an aqueous solution containing 92% water, 8% blood plasma proteins, and trace amounts of other materials. It includes a variety of materials, from gases and ions (mostly sodium and chloride ions, which give blood its salty taste) to nutrients, hormones, and immunoglobulins (antibodies). The most abundant blood plasma protein in humans and other mammals is serum albumin, which is essential for maintaining the osmotic pressure needed for proper distribution of body fluids.

Properties of blood vary by species

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 (PaCO₂) 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.

In humans and other hemoglobin-using creatures, oxygenated blood is bright red, due to the 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. However, due to skin pigments, blood vessel coverings and an optical effect caused by the way in which light penetrates through the skin, veins typically appear blue in color.

The blood of horseshoe crabs is blue, which is a result of its high content of copper-based hemocyanin instead of the iron-based hemoglobin found, for example, in humans. Members of the lizard-family called skinks, which belong to the genus Prasinohaema, have green blood due to a buildup of the waste product biliverdin.

Other functions

In addition to the respiratory and immunological roles described above, blood often has additional functions, including:

• 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 blood flow to the surface conserves heat.
• Hydraulic function. The restriction of blood flow can also be used in specialized tissues (such as the mammalian penis, clitoris, or nipple to cause engorgement resulting in an erection of that tissue. Another example of blood's hydraulic function is found in the jumping spider, in which blood forced into the legs under pressure causes them to straighten for a powerful jump.

Properties and functions of blood in invertebrates

In insects, the blood (which is 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 their oxygen-carrying capacity. While hemoglobin is the most common respiratory protein found in nature, [hemocyanin]] (blue) is found in crustaceans and mollusks. It is thought that tunicates (sea squirts) might use vanabins (proteins containing vanadium) for respiratory pigmentation; they can be bright green, blue, or orange in color.

In many invertebrates, oxygen-carrying proteins are freely soluble in the blood, whereas 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 particularly complex hemoglobins that allow them to live in especially harsh aquatic environments. These hemoglobins carry sulfides that are normally fatal in other animals.

Physiology of blood

Production and degradation

Blood cells are produced by specialized stem cells located in the bone marrow through a process termed termed hematopoiesis. The protein components of plasma (including clotting proteins) are produced overwhelmingly in the liver, while hormones are produced by the endocrine glands. 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

The blood is circulated around the lungs and body through the pumping action of the heart. Additional return pressure may be generated by gravity and by 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."

Blood-related diseases and disorders

Medical terms related to blood often begin with hemo- or hemato- from the Greek word for blood (haima). The study of blood, the blood-forming organs, and blood-related diseases is referred to as hematology, a branch of biology (physiology), pathology, clinical laboratory, internal medicine, and pediatrics. Hematology includes the study of etiology, diagnosis, treatment, prognosis, and prevention of blood diseases, which affect the production of blood and its components, such as blood cells, haemoglobin, blood proteins, and the mechanism of coagulation.

The most common disorder of the blood is anemia, a deficiency of red blood cells (RBCs) and/or hemoglobin. This results in a reduced ability of blood to transfer oxygen to the tissues, causing tissue hypoxia. There are several types of anemia, produced by a variety of underlying causes. Since all human cells depend on oxygen for survival, varying degrees of anemia can have a wide range of clinical consequences. The three main classes of anemia include excessive blood loss (acutely such as a hemorrhage or chronically through low-volume loss), excessive blood cell destruction (hemolysis) or deficient red blood cell production (ineffective hematopoiesis). In menstruating women, dietary iron deficiency is a common cause of deficient red blood cell production.

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 refers to a group of cancers of the blood-forming tissues; it is characterized by an abnormal proliferation (production by multiplication) of blood cells, usually white blood cells (leukocytes).

Blood is also an important vector of infection. HIV, the virus that causes AIDS, is transmitted through contact with blood, semen, or the bodily secretions of an infected person. Hepatitis B and C are transmitted primarily through blood contact. Malaria and trypanosomiasis are blood-borne parasitic infections.

Blood in ancient medicine

Hippocratic medicine considered blood to be 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.

Blood type

Blood type (or blood group) is determined, in part, by the ABO blood group antigens present on red blood cells.

A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins or glycolipids, depending on the blood group system, and some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens, that stem from one allele (or very closely linked genes), collectively form a blood group system.

Animals and bacteria have cell surface antigens referred to as a blood type. Antigens from the human ABO blood group system are also found in apes such as chimpanzees, bonobos and gorillas. Other animal blood sometimes agglutinates (to varying levels of intensity) with human blood group reagents, but the structure of the blood group antigens in animals is not always identical to those typically found in humans. The classification of most animal blood groups therefore uses different blood typing systems to those used for classification of human blood.

If an individual is exposed to a blood group antigen that is not recognised as self, the immune system will produce antibodies that can specifically bind to that particular blood group antigen and an immunological memory against that antigen is formed. The individual will have become sensitized to that blood group antigen. These antibodies can bind to antigens on the surface of transfused red blood cells (or other tissue cells) often leading to destruction of the cells by recruitment of other components of the immune system. When IgM antibodies bind to the transfused cells, the transfused cells can clump. It is vital that compatible blood is selected for transfusions and that compatible tissue is selected for organ transplantation. Transfusion reactions involving minor antigens or weak antibodies may lead to minor problems. However, more serious incompatibilities can lead to a more vigorous immune response with massive RBC destruction, low blood pressure, and even death.

The role of blood in religion, myth, and culture

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.

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 (Lawlor, 1991).

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.

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.

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." (Luke 22:20). 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. Mormons believe that before Adam and Eve ate the forbidden fruit, blood was not present in their bodies. It is said to have formed after the Fall when they became mortal.

Consumption of food containing blood is forbidden by Islamic dietary laws.

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.

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. European versions of this myth are mostly inspired by folklore based on the stories regarding Vlad Dracula.^{[citation needed]}

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.^{[citation needed]}

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.

Blood is one of the body fluids that has been used in art. 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.

References

ISBN links support NWE through referral fees

• Alberts, B. 2005. Leocyte functions and percentage breakdown. From Molecular Biology of the Cell. NCBI Bookshelf. Retrieved April 14, 2007.
• Lawlor, R. 1991. Voices Of The First Day: Awakening in the Aboriginal Dreamtime. Rochester, VT: Inner Traditions International. ISBN 0892813555.
• Martini, F. et al. 2006. Human Anatomy, 5th ed. San Francisco, CA: Pearson. ISBN 0805372113.
• Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. Life: The Science of Biology, 7th edition. Sunderland, MA: Sinauer. ISBN 0716766728.
• Shmukler, M. 2004. Density of Blood. The Physics Factbook. Retrieved October 4, 2006.

External links

Wikimedia Commons has media related to:

Blood

• Blood Online, a journal published by the American Society of Hematology

Cardiovascular system
Blood  |  Heart → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava → Heart → Pulmonary arteries → Lungs → Pulmonary veins → Heart

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