blood

blood, fluid pumped by the heart that circulates throughout the body via the arteries, veins, and capillaries (see circulatory system; heart). An adult male of average size normally has about 6 quarts (5.6 liters) of blood. The blood carries oxygen and nutrients to the body tissues and removes carbon dioxide and other wastes. The colorless fluid of the blood, or plasma, carries the red and white blood cells, platelets, waste products, and various other cells and substances.

Erythrocytes (Red Blood Cells)

The erythrocytes, or red blood cells, make up the largest population of blood cells, numbering from 4.5 million to 6 million per cubic millimeter of blood. They carry out the exchange of oxygen and carbon dioxide between the lungs and the body tissues. To effectively combine with oxygen, the erythrocytes must contain a normal amount of the red protein pigment hemoglobin, the amount of which in turn depends on the iron level in the body. A deficiency of iron and therefore of hemoglobin leads to anemia and poor oxygenation of the body tissues.

Erythrocytes are constantly developing from stem cells, the undifferentiated, self-regenerating cells that give rise to both erythrocytes and leukocytes in the bone marrow. In the fetus, red blood cells are produced in the spleen. As they mature, the erythrocytes lose their nuclei, become disk-shaped, and begin to produce hemoglobin. After circulating for about 120 days, the erythrocytes wear out and undergo destruction by the spleen. Although all red blood cells are essentially similar, certain structures on their surfaces vary from person to person. These serve as the basis for the classification into blood groups. There are four major blood groups, whose compatibility or incompatibility is an important consideration in successful blood transfusion.

Leukocytes (White Blood Cells)

The leukocytes, or white blood cells, defend the body against infecting organisms and foreign agents, both in the tissues and in the bloodstream itself (see immunity). Human blood contains about 5,000 to 10,000 leukocytes per cubic millimeter; the number increases in the presence of infection. An extraordinary and prolonged proliferation of leukocytes is known as leukemia. This overproduction suppresses the production of normal blood cells. Conversely, a sharp decrease in the number of leukocytes (leukopenia) strips the blood of its defense against infection and is an equally serious condition. A dramatic fall in levels of certain white blood cells occurs in persons with AIDS. Leukocytes as well as erythrocytes are formed from stem cells in the bone marrow. They have nuclei and are classified into two groups: granulocytes and agranulocytes.

Granulocytes

The granulocytes form in the bone marrow and account for about 70% of all white blood cells. Granulocytes include three types of cells: neutrophils, eosinophils, and basophils. Neutrophils constitute the vast majority of granulocytes. They travel about by ameboid movement and can surround and destroy bacteria and other foreign particles. The eosinophils, ordinarily about 2% of the granulocyte count, increase in number in the presence of allergic disorders and parasitic infestations. The basophils account for about 1% of the granulocytes. They release chemicals such as histamine and play a role in the inflammatory response to infection.

Agranulocytes

The agranulocytes include the monocytes and the lymphocytes. Monocytes are derived from the phagocytic cells that line many vascular and lymph channels, called the reticuloendothelial system. Monocytes ordinarily number 4% to 8% of the white cells. They move to areas of infection, where they are transformed into macrophages, large phagocytic cells that trap and destroy organisms left behind by the granulocytes and lymphocytes. In certain diseases of long duration (tuberculosis, malaria, and typhoid) the monocytes act as the main instrument of defense.

Lymphocytes, under normal conditions, make up about 20% to 35% of all white cells, but proliferate rapidly in the face of infection. There are two basic types of lymphocytes: the B lymphocytes and the T lymphocytes. B lymphocytes tend to migrate into the connective tissue, where they develop into plasma cells that produce highly specific antibodies against foreign antigens. Other B lymphocytes act as memory cells, ready for subsequent infection by the same organism. Some T lymphocytes kill invading cells directly; others interact with other immune system cells, regulating the immune response.

Other Constituents of Blood

The blood also contains platelets, or thrombocytes, and at least 15 other factors active in blood clotting. Platelets are tiny plate-shaped cytoplasmic bags of blood-clotting chemicals produced by megakaryocytes; if their production is hindered, as by AIDS or chemotherapy, there is an increased risk of bleeding. Also circulating in the plasma are the hormones that the endocrine glands secrete directly into the bloodstream. In addition, essential salts (such as those of sodium and potassium), essential plasma proteins (albumin, globulins, and fibrinogen), and metabolic wastes (such as urea) circulate in the plasma.

Serum, a straw-colored liquid, essentially composed of plasma without fibrinogen, makes up the liquid component of blood that separates from the clot. Serum is separated from whole blood by centrifuging and can serve various medical uses. Normal human serum is sometimes used to treat shock and the loss of fluid resulting from severe burns.

or white blood cell or white corpuscle

Any of several types of blood cells that help defend the body from infection. The different mature forms—granulocytes, including neutrophils (heterophils), basophils, and eosinophils; monocytes, including macrophages; and lymphocytes—have different functions, including ingesting bacteria, protozoans, or infected or dead body cells; producing antibodies; and regulating the action of other leukocytes. They act mostly in the tissues and are in the bloodstream only for transport. Blood normally contains 5,000–10,000 leukocytes per cu mm.

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or red blood cell or red blood corpuscle

Blood cell that carries oxygen from the lungs to the body tissues. Hemoglobin gives the cell—and whole blood—its colour. Red cells are small, round, flexible, and concave on both sides and lack a nucleus. They develop continuously in bone marrow in several stages and are stored in the spleen. The mature form lives 100–120 days. Adult human blood has about 5.2 million red cells per cu mm. Some conditions change their shape (e.g., pernicious anemia, sickle-cell anemia) or number (e.g., anemia, polycythemia).

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or low blood pressure

Condition in which blood pressure is abnormally low. It may result from reduced blood volume (e.g., from heavy bleeding or plasma loss after severe burns) or increased blood-vessel capacity (e.g., in syncope). Orthostatic hypotension—drop in blood pressure on standing—results from failure of the reflexes that contract muscles and constrict blood vessels in the legs to offset gravity as one rises. Hypotension is also a factor in poliomyelitis, shock, and barbiturate poisoning.

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or high blood pressure

Condition in which blood pressure is abnormally high. Over time, it damages the kidneys, brain, eyes, and heart. Hypertension accelerates atherosclerosis, increasing the risk of heart attack, stroke, and kidney failure. More common in the elderly and blacks, it usually has no symptoms but can be detected by a routine blood-pressure test. Secondary hypertension, caused by another disorder (most often kidney disease or hormone imbalance), accounts for 10percnt of cases. The other 90percnt have no specific cause (essential hypertension). A low-salt diet, weight loss, smoking cessation, limited alcohol intake, and exercise can prevent or treat hypertension or reduce medication if drug therapy proves necessary. Malignant hypertension, a severe, rapidly progressing form, requires emergency treatment with drugs to dilate the blood vessels.

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Vessel that carries blood to the heart. Except for the pulmonary veins, veins bear deoxygenated blood from capillaries, which converge into threadlike venules and then veins, finally emptying into the venae cavae (see cardiovascular system; vena cava). Blood moves through veins by contraction of the surrounding muscles. Backflow is prevented by valves in most veins' inner layer (tunica intima), which lacks the elastic membrane lining of arteries. The thin middle layer (tunica media) is mostly collagen fibres, and the thick outer layer (tunica adventitia) is mostly connective tissue. Seealso circulation; varicose vein.

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or varix

Twisted vein distended with blood. Varix also covers arteries and lymphatic vessels (see lymphatic system). Varicose veins occur mostly in the legs, when malfunctioning valves let blood pool in veins near the skin. Causes include hereditary valve and vein wall weakness and internal or external pressure on veins. Varices are common in pregnancy, suggesting that hormone abnormalities play a role. Symptoms include a heavy feeling, with leg cramps and swelling after standing a long time. Complications include skin ulcers and thrombosis. Treatment involves strong support hose, injection therapy, or surgery. Varices in the esophagus, which often occur in liver disease, can ulcerate and bleed. Seealso hemorrhoid.

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Classification of blood by inherited antigens associated with erythrocytes (red blood cells). The ABO blood-group system and Rh blood-group system are among those most commonly considered. Without identification of these factors, blood transfusion from an incompatible donor may result in destruction of red cells or coagulation. Blood typing also helps identify disorders such as erythroblastosis fetalis.

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Transfer of blood taken from one person into the circulation of another to restore blood volume, increase hemoglobin levels, or combat shock. Once the blood-group antigens and antibodies (see ABO blood-group system, Rh blood-group system) were discovered, blood typing of donors and recipients rendered transfusion safe. In exchange transfusion, all or most of the blood is removed and replaced with another's blood. Undesirable reactions to transfusion are not uncommon.

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Laboratory examination of the physical and chemical properties and components of a sample of blood. Analysis includes number of red and white blood cells (erythrocytes and leukocytes); red cell volume, sedimentation (settling) rate, and hemoglobin concentration; blood typing; cell shape and structure; hemoglobin and other protein structure; enzyme activity; and chemistry. Special tests detect substances characteristic of specific infections.

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or dextrose or grape sugar or corn sugar

Organic compound, a simple sugar (monosaccharide), chemical formula C₆H₁₂O₆. The product of photosynthesis in plants, it is found in fruits and honey. As the major circulating free sugar in blood, it is the source of energy in cell function and a major participant in metabolism. Control of its level and metabolism is of great importance (see insulin). Glucose and fructose make up sucrose. Glucose units in long chains make up polysaccharides (e.g., cellulose, glycogen, starch). Glucose is used in foods, medicine, brewing, and wine making and as the source of various other organic chemicals.

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Force originating when the heart's pumping pushes the blood against the walls of the blood vessels. Their stretching and contraction help maintain blood flow. Usually measured over an arm or leg artery in humans, blood pressure is expressed as two numbers; normal adult blood pressure is about 120/80 mm of mercury. The higher number (systolic) is measured when the heart's ventricles contract and the lower (diastolic) when they relax. Seealso hypertension, hypotension.

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or blood poisoning

Invasion of the bloodstream, after surgery or infectious disease, by microorganisms—typically gram-negative (see gram stain) bacteria—and the toxins they release. The latter trigger immune responses and widespread coagulation in blood vessels. High fever, chills, weakness, and sweating are followed by a drop in blood pressure. Multiple infections are often present, requiring broad-spectrum antibiotics as well as drainage of foci of infection. Without immediate treatment, septic shock follows, with a mortality rate over 50percnt. Invasive technology and antibiotic-resistant bacteria in hospitals have made septicemia more severe and more common. Seealso bacteremia.

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Electrically conducting medium in which there are roughly equal numbers of positively and negatively charged particles, produced when the atoms in a gas become ionized (see ionization). Plasma is sometimes called the fourth state of matter (the first three being solid, liquid, and gas). A plasma is unique in the way it interacts with itself, with electric and magnetic fields, and with its environment. It can be thought of as a collection of ions, electrons, neutral atoms and molecules, and photons in which some atoms are being ionized at the same time as electrons are recombining with other ions to form neutral particles, while photons are continuously being produced and absorbed. It is estimated that more than 99percnt of the matter in the universe exists in the plasma state.

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Process of forming a blood clot to prevent blood loss from a ruptured vessel. A damaged blood vessel stimulates activation of clotting factors, eventually leading to the formation of long, sticky threads of fibrin. These make a mesh that traps platelets, blood cells, and plasma. This meshwork soon contracts into a resilient clot that can withstand the friction of blood flow. Under abnormal circumstances, clots can form in an intact vessel and may block it. Seealso anticoagulant.

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Formation of a blood clot (thrombus) in the heart or a blood vessel. Contributing factors include injury to a blood vessel's lining from inflammation (thrombophlebitis) or atherosclerosis, blood flow that is turbulent (e.g., from an aneurysm) or sluggish (e.g., from prolonged bed rest), or coagulation abnormalities (e.g., from high numbers of platelets or excessive fats in the blood). Thrombosis, especially in deep veins of the leg, is a particular danger after major surgery. A thrombus can block blood flow at the point of clot formation or break free to block it elsewhere (embolism).

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Organization that collects, stores, processes, and supplies blood. Most blood donations are separated into components, which can be frozen and stored longer than whole blood and used by multiple patients. In hemapheresis, large amounts of one component can be separated from a single donor's blood and the rest returned to the donor. Before World War I, a physician had to find a compatible donor and give an immediate blood transfusion. Safe storage of blood and its components made possible innovations such as heart-lung machines.

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Circulatory fluid (see circulation) in multicellular animals. In many species it also carries hormones and disease-fighting substances. Blood picks up oxygen from the lungs and nutrients from the gastrointestinal tract and carries them to cells throughout the body for metabolism. It picks up carbon dioxide and other wastes from those cells and transports them to the lungs and excretory organs. Blood composition varies among species. Mammalian blood consists of plasma, red and white cells (erythrocytes and leukocytes), and platelets (thrombocytes). Blood disorders include polycythemia (abnormal increase in the number of circulating red blood cells), anemia, leukemia, and hemophilia. Seealso ABO blood-group system; blood analysis; blood bank; blood pressure; blood transfusion; blood typing; Rh blood-group system.

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Blood is a specialized bodily fluid that delivers necessary substances to the body's cells—such as nutrients and oxygen—and transports waste products away from those same cells.

In vertebrates it is composed of blood cells suspended in a liquid called blood plasma. Plasma, which comprises 55% of blood fluid, is mostly water (90% by volume), and contains dissolved proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), platelets and blood cells themselves. The blood cells present in blood are mainly red blood cells (also called RBCs or erythrocytes) and white blood cells, including leukocytes and platelets (also called thrombocytes).

The most abundant cells in vertebrate blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates transportation of oxygen by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is almost entirely transported extracellularly dissolved in plasma as bicarbonate ion.

Vertebrate blood is bright red when its hemoglobin is oxygenated. Some animals, such as crustaceans and mollusks, use hemocyanin to carry oxygen, instead of hemoglobin. Insects and some molluscs use a fluid called hemolymph instead of blood, the difference being that hemolymph is not contained in a closed circulatory system. In most insects, this "blood" does not contain oxygen-carrying molecules such as hemoglobin because their bodies are small enough that their tracheal system suffices for supplying oxygen.

Jawed vertebrates have an adaptive immune system, based largely on white blood cells. White blood cells help to resist infections and parasites. Platelets are important in the clotting of blood. Arthropods, using hemolymph, have hemocytes as part of their immune system.

Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals having lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.

Medical terms related to blood often begin with hemo- or hemato- (BrE: haemo- and haemato-) from the Greek word "αἷμα" for "blood." Anatomically and histologically, blood is considered a specialized form of connective tissue, given its origin in the bones and the presence of potential molecular fibers in the form of fibrinogen.

Functions

Blood performs many important functions within the body including:

• Supply of oxygen to tissues (bound to hemoglobin which is carried in red cells)
• Supply of nutrients such as glucose, amino acids and fatty acids (dissolved in the blood or bound to plasma proteins (eg blood lipids)
• Removal of waste such as carbon dioxide, urea and lactic acid
• Immunological functions, including circulation of white cells, and detection of foreign material by antibodies
• Coagulation, which is one part of the body's self-repair mechanism
• Messenger functions, including the transport of hormones and the signaling of tissue damage
• Regulation of body pH (the normal pH of blood is in the range of 7.35 - 7.45)
• Regulation of core body temperature
• Hydraulic functions

Constituents of human blood

Blood accounts for 7% of the human body weight, with an average density of approximately 1060 kg/m³, very close to pure water's density of 1000 kg/m³. The average adult has a blood volume of roughly 5 litres, composed of plasma and several kinds of cells (occasionally called corpuscles); these formed elements of the blood are erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets). By volume the red blood cells constitute about 45% of whole blood, the plasma constitutes about 55%, and white cells constitute a minute volume.

Whole blood (plasma and cells) exhibits non-Newtonian fluid dynamics; its flow properties are adapted to flow effectively through tiny capillary blood vessels with less resistance than plasma by itself. In addition, if all human hemoglobin was free in the plasma rather than being contained in RBCs, the circulatory fluid would be too viscous for the cardiovascular system to function effectively.

Cells

One microliter of blood contains:

• 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes: In mammals, mature red blood cells lack a nucleus and organelles. They contain the blood's hemoglobin and distribute oxygen. The red blood cells (together with endothelial vessel cells and other cells) are also marked by glycoproteins that define the different blood types. The proportion of blood occupied by red blood cells is referred to as the hematocrit, and is normally about 45%. 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.
• 4,000-11,000 leukocytes: 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. The cancer of leukocytes is called leukemia.
• 200,000-500,000 thrombocytes: thrombocytes, also called platelets, are responsible for blood clotting (coagulation). They change fibrinogen into fibrin. This fibrin creates a mesh onto which red blood cells collect and clot, which then stops more blood from leaving the body and also helps to prevent bacteria from entering the body.

Constitution of normal blood

Parameter	Value
Hematocrit	45 ± 7 (38-52%) for males 42 ± 5 (37-47%) for females
pH	7.35-7.45
base excess	-3 to +3
PO2	10-13kPa (80-100 mmHg)
PCO2	4.8kPa - 5.8kPa (35-45 mmHg)
HCO3-	21mM - 27mM
Oxygen saturation	Oxygenated: 98-99% Deoxygenated: 75%

Plasma

About 55% of whole blood is blood plasma, a fluid that is the blood's liquid medium, which by itself is straw-yellow in color. The blood plasma volume totals of 2.7-3.0 litres in an average human. It is essentially an aqueous solution containing 92% water, 8% blood plasma proteins, and trace amounts of other materials. Plasma circulates dissolved nutrients, such as, glucose, amino acids and fatty acids (dissolved in the blood or bound to plasma proteins), and removes waste products, such as, carbon dioxide, urea and lactic acid.

Other important components include:

• Serum albumin
• Blood clotting factors (to facilitate coagulation)
• Immunoglobulins (antibodies)
• lipoprotein particles
• Various other proteins
• Various electrolytes (mainly sodium and chloride)

The term serum refers to plasma from which the clotting proteins have been removed. Most of the proteins remaining are albumin and immunoglobulins.

The normal pH of human arterial blood is approximately 7.40 (normal range is 7.36-7.44), a weak alkaline solution. Blood that has a pH below 7.35 is too acidic, while blood pH above 7.45 is too alkaline. Blood pH, partial pressure of oxygen (PaO₂), partial pressure of carbon dioxide(PaCO₂) and HCO₃ are carefully regulated by a number of homeostatic mechanisms, which principally exert their influence through influence the respiratory system and the urinary system in the control the acid-base balance and respiration. Plasma also circulates hormones transmitting their messages to various tissues. The list of normal reference ranges for various blood electrolytes is extensive.

Physiology

Cardiovascular system

Blood is circulated around the body through blood vessels by the pumping action of the heart. In humans, blood is pumped from the strong left ventricle of the heart through arteries to peripheral tissues and returns to the right atrium of the heart through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air to all of the cells of the body, and venous blood carries carbon dioxide, a waste product of metabolism by cells, to the lungs to be exhaled. However, one exception includes pulmonary arteries which contains the most deoxygenated blood in the body, while the pulmonary veins contain oxygenated blood.

Additional return flow may be generated by the movement of skeletal muscles which can compress veins and push blood through the valves in veins towards the right atrium.

The blood circulation was famously described by William Harvey in 1628.

Production and degradation of blood cells

In vertebrates, the various cells of blood are made in the bone marrow in a process called hematopoiesis, which includes erythropoiesis, the production of red blood cells; and myelopoiesis, the production of white blood cells and platelets. During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs. In addition, during childhood, the thymus gland, found in the mediastinum, is an important source of lymphocytes. The proteinaceous component of blood (including clotting proteins) is produced predominantly by the liver, while hormones are produced by the endocrine glands and the watery fraction is regulated by the hypothalamus and maintained by the kidney.

Healthy erythrocytes have a plasma life of about 120 days before they 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.

Oxygen transport

About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with the Hgb. About 1.5% is physically dissolved in the other blood liquids and not connected to Hgb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below).

With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules and veins carry deoxygenated blood back to the heart.

Under normal conditions in humans at rest, hemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen. In a healthy adult at rest, deoxygenated blood returning to the lungs is still approximately 75% saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions. Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia. Sustained hypoxia,(oxygenation of less than 90%) is dangerous to health, and severe hypoxia (saturations of less than 30%) may be rapidly fatal.

A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 21% of the level found in an adult's lungs) and so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to function under these conditions.

Carbon dioxide transport

When blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is dissolved in the blood. Some carbon dioxide reacts with hemoglobin and other proteins to form carbamino compounds. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions through the action of RBC carbonic anhydrase. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

Carbon dioxide (CO₂), the main cellular waste product is carried in blood mainly dissolved in plasma, in equilibrium with bicarbonate (HCO₃^-) and carbonic acid (H₂CO₃). 86%-90% of CO₂ in the body is converted into carbonic acid, which can quickly turn into bicarbonate, the chemical equilibrium being important in the pH buffering of plasma. Blood pH is kept in a narrow range (pH between 7.35-7.45).

Transport of hydrogen ions

Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin binds most of the hydrogen ions as it has a much greater affinity for more hydrogen than does oxyhemoglobin.

Lymphatic system

In mammals, blood is in equilibrium with lymph, which is continuously formed in tissues from blood by capillary ultrafiltration. Lymph is collected by a system of small lymphatic vessels and directed to the thoracic duct, which drains into the left subclavian vein where lymph rejoins the systemic blood circulation.

Thermoregulation

Blood circulation transports heat throughout the body, and adjustments to this flow are an important part of thermoregulation. Increasing blood flow to the surface (e.g. during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss, while decreasing surface blood flow conserves heat.

Hydraulic functions

The restriction of blood flow can also be used in specialized tissues to cause engorgement resulting in an erection of that tissue; examples are the erectile tissue in the penis, nipples or clitoris.

Another example of a hydraulic function is the jumping spider, in which blood forced into the legs under pressure causes them to straighten for a powerful jump, without the need for bulky muscular legs.

Invertebrates

In insects, the blood (more properly called hemolymph) is not involved in the transport of oxygen. (Openings called tracheae allow oxygen from the air to diffuse directly to the tissues). Insect blood moves nutrients to the tissues and removes waste products in an open system.

Other invertebrates use respiratory proteins to increase the oxygen carrying capacity. Hemoglobin is the most common respiratory protein found in nature. Hemocyanin (blue) contains copper and is found in crustaceans and mollusks. It is thought that tunicates (sea squirts) might use vanabins (proteins containing vanadium) for respiratory pigment (bright green, blue, or orange).

In many invertebrates, these oxygen-carrying proteins are freely soluble in the blood; in vertebrates they are contained in specialized red blood cells, allowing for a higher concentration of respiratory pigments without increasing viscosity or damaging blood filtering organs like the kidneys.

Giant tube worms have unusual hemoglobins that allow them to live in extraordinary environments. These hemoglobins also carry sulfides normally fatal in other animals.

Color

Hemoglobin

Hemoglobin is the principal determinant of the color of blood in vertebrates. Each molecule has four heme groups, and their interaction with various molecules alters the exact color. In vertebrates and other hemoglobin-using creatures, arterial blood and capillary blood are bright red as oxygen imparts a strong red color to the heme group. Deoxygenated blood is a darker shade of red; this is present in veins, and can be seen during blood donation and when venous blood samples are taken. Blood in carbon monoxide poisoning is bright red, because carbon monoxide causes the formation of carboxyhemoglobin. In cyanide poisoning, the body cannot utilize oxygen, so the venous blood remains oxygenated, increasing the redness. While hemoglobin containing blood is never blue, there are several conditions and diseases where the color of the heme groups make the skin appear blue. If the heme is oxidized, methemoglobin, which is more brownish and cannot transport oxygen, is formed. In the rare condition sulfhemoglobinemia, arterial hemoglobin is partially oxygenated, and appears dark-red with a bluish hue (cyanosis).

Veins in the skin appear blue for a variety of reasons only weakly dependent on the color of the blood. Light scattering in the skin, and the visual processing of color play roles as well.

Skinks in the genus Prasinohaema have green blood due to a buildup of the waste product biliverdin.

Hemocyanin

The blood of most molluscs - including cephalopods and gastropods - as well as some arthropods, such as horseshoe crabs, is blue, as it contains the copper-containing protein hemocyanin at concentrations of about 50 grams per litre. Hemocyanin is colourless when deoxygenated and dark blue when oxygenated. The blood in the circulation of these creatures, which generally live in cold environments with low oxygen tensions, is grey-white to pale yellow, and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed. This is due to change in color of hemocyanin when is it oxidized. Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in mammals by hemoglobin in RBCs.

Vacuum

If human blood was exposed to vacuum, it would lose all oxygen (bound to the hemoglobin molecules in its red cells) as it does in part when it supplies oxygen to various organs as it passes through the body. This state of deoxyhemoglobin has a purplish colour.

Pathology

General medical disorders

• Disorders of volume
• Injury can cause blood loss through bleeding. A healthy adult can lose almost 20% of blood volume (1L) before the first symptom, restlessness, begins, and 40% of volume (2L) before shock sets in. Thrombocytes are important for blood coagulation and the formation of blood clots which can stop bleeding. Trauma to the internal organs or bones can cause internal bleeding, which can sometimes be severe.
• Dehydration can reduce the blood volume by reducing the water content of the blood. This would rarely result in shock (apart from the very severe cases) but may result in orthostatic hypotension and fainting.

• Disorders of circulation
• Shock is the ineffective perfusion of tissues, and can be caused by a variety of conditions including blood loss, infection, poor cardiac output.
• Atherosclerosis reduces the flow of blood through arteries, because atheroma lines arteries and narrows them. Atheroma tends to increase with age, and its progression can be compounded by many causes including smoking, high blood pressure, excess circulating lipids (hyperlipidemia), and diabetes mellitus.
• Coagulation can form a thrombosis which can obstruct vessels.
• Problems with blood composition, the pumping action of the heart, or narrowing of blood vessels can have many consequences including hypoxia (lack of oxygen) of the tissues supplied. The term ischemia refers to tissue which is inadequately perfused with blood, and infarction refers to tissue death (necrosis) which can occur when the blood supply has been blocked (or is very inadequate).

Hematological disorders

• Anemia
• Insufficient red cell mass (anemia) can be the result of bleeding, blood diseases like thalassemia, or nutritional deficiencies; and may 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.
• Sickle-cell anemia

• Disorders of cell proliferation
• Leukemia is a group of cancers of the blood-forming tissues.
• Non-cancerous overproduction of red cells (polycythemia vera) or platelets (essential thrombocytosis) may be premalignant.
• Myelodysplastic syndromes involve ineffective production of one or more cell lines.

• Disorders of coagulation
• 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.
• Ineffective or insufficient platelets can also result in coagulopathy (bleeding disorders).
• Hypercoagulable state (thrombophilia) results from defects in regulation of platelet or clotting factor function, and can cause thrombosis.

• Infectious disorders of blood
• Blood is an important vector of infection. HIV, the virus which causes AIDS, is transmitted through contact with blood, semen or other body secretions of an infected person. Hepatitis B and C are transmitted primarily through blood contact. Owing to blood-borne infections, bloodstained objects are treated as a biohazard.
• Bacterial infection of the blood is bacteremia or sepsis. Viral Infection is viremia. Malaria and trypanosomiasis are blood-borne parasitic infections.

Carbon monoxide poisoning

Substances other than oxygen can bind to hemoglobin; in some cases this can cause irreversible damage to the body. Carbon monoxide, for example, is extremely dangerous when carried to the blood via the lungs by inhalation, because carbon monoxide irreversibly binds to hemoglobin to form carboxyhemoglobin, so that less hemoglobin is free to bind oxygen, and less oxygen can be transported in the blood. This can cause suffocation insidiously. A fire burning in an enclosed room with poor ventilation presents a very dangerous hazard since it can create a build-up of carbon monoxide in the air. Some carbon monoxide binds to hemoglobin when smoking tobacco.

Medical treatments

Blood products

Blood for transfusion is obtained from human donors by blood donation and stored in a blood bank. There are many different blood types in humans, the ABO blood group system, and the Rhesus blood group system being the most important. Transfusion of blood of an incompatible blood group may cause severe, often fatal, complications, so crossmatching is done to ensure that a compatible blood product is transfused.

Other blood products administered intravenously are platelets, blood plasma, cryoprecipitate and specific coagulation factor concentrates.

Intravenous administration

Many forms of medication (from antibiotics to chemotherapy) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.

After severe acute blood loss, liquid preparations, generically known as plasma expanders, can be given intravenously, either solutions of salts (NaCl, KCl, CaCl₂ etc...) at physiological concentrations, or colloidal solutions, such as dextrans, human serum albumin, or fresh frozen plasma. In these emergency situations, a plasma expander is a more effective life saving procedure than a blood transfusion, because the metabolism of transfused red blood cells does not restart immediately after a transfusion.

Bloodletting

In modern evidence-based medicine bloodletting is used in management of a few rare diseases, including hemochromatosis and polycythemia. However, bloodletting and leeching were common unvalidated interventions used until the 19th century, as many diseases were incorrectly thought to be due to an excess of blood, according to Hippocratic medicine.

History

Classical Greek medicine

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.

Hippocratic medicine

In Hippocratic medicine, blood was considered to be one of the four humors, together with phlegm, yellow bile and black bile.

Myths, beliefs and religion

Due to its importance to life, blood is associated with a large number of beliefs. One of the most basic is the use of blood as a symbol for family relationships; to be "related by blood" is to be related by ancestry or descendance, rather than marriage. This bears closely to bloodlines, and sayings such as "blood is thicker than water" and "bad blood", as well as "Blood brother". Blood is given particular emphasis in the Jewish and Christian religions because Leviticus 17:11 says "the life of a creature is in the blood." This phrase is part of the Levitical law forbidding the drinking of blood, due to its practice in idol worship by surrounding societies.

Mythic references to blood can sometimes be connected to the living-giving nature of blood, seen in such events as childbirth, as contrasted with the blood of injury or death.

Indigenous Australians

In many indigenous Australian Aboriginal peoples' traditions, ochre (particularly red) and blood, both high in iron content and considered Maban, are applied to the bodies of dancers for ritual. As Lawlor states:

In many Aboriginal rituals and ceremonies, red ochre is rubbed all over the naked bodies of the dancers. In secret, sacred male ceremonies, blood extracted from the veins of the participant's arms is exchanged and rubbed on their bodies. Red ochre is used in similar ways in less secret ceremonies. Blood is also used to fasten the feathers of birds onto people's bodies. Bird feathers contain a protein that is highly magnetically sensitive.

Lawlor comments that blood employed in this fashion is held by these peoples to attune the dancers to the invisible energetic realm of the Dreamtime. Lawlor then connects these invisible energetic realms and magnetic fields, because iron is magnetic.

Indo-European paganism

Among the Germanic tribes (such as the Anglo-Saxons and the Norsemen), blood was used during their sacrifices; the Blóts. The blood was considered to have the power of its originator and after the butchering the blood was sprinkled on the walls, on the statues of the gods and on the participants themselves. This act of sprinkling blood was called bleodsian in Old English and the terminology was borrowed by the Roman Catholic Church becoming to bless and blessing. The Hittite word for blood, ishar was a cognate to words for "oath" and "bond", see Ishara. The Ancient Greeks believed that the blood of the Gods, ichor, was a mineral that was poisonous to mortals.

Judaism

In Judaism, blood cannot be consumed even in the smallest quantity (Leviticus 3:17 and elsewhere); this is reflected in Jewish dietary laws (Kashrut). Blood is purged from meat by salting and soaking in water.

Other rituals involving blood are the covering of the blood of fowl and game after slaughtering (Leviticus 17:13); the reason given by the Torah is: "Because the life of every animal is [in] his blood" (ibid 17:14).

Christianity

Some Christian churches, including Roman Catholicism, Eastern Orthodoxy, Anglicanism teach that, when consecrated, the Eucharistic wine actually becomes the blood of Jesus(symbolism). Thus in the consecrated wine, Jesus becomes spiritually and physically present. This teaching is rooted in the Last Supper as written in the four gospels of the Bible, in which Jesus stated to his disciples that the bread which they ate was his body, and the wine was his blood. "This cup is the new testament in my blood, which is shed for you." ().

Various forms of Protestantism, especially those of a Wesleyan or Presbyterian lineage, teach that the wine is no more than a symbol of the blood of Christ, who is spiritually but not physically present. Lutheran theology teaches that the body and blood is present together "in, with, and under" the bread and wine of the Eucharistic feast.

Christ's blood is also seen as the means for atonement for sins for Christians.

At the Council of Jerusalem, the apostles prohibited Christians from consuming blood, probably because this was a command given to Noah (Genesis 9:4, see Noahide Law). This command continued to be observed by the Eastern Orthodox.

Islam

Consumption of food containing blood is forbidden by Islamic dietary laws. This is derived from the statement in the Qur'an, sura Al-Ma'ida (5:3): "Forbidden to you (for food) are: dead meat, blood, the flesh of swine, and that on which hath been invoked the name of other than Allah."

Jehovah's Witnesses

Due to Bible-based beliefs, Jehovah's Witnesses do not eat blood or accept transfusions 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.

Chinese and Japanese culture

In Chinese popular 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 as well as in Japanese culture parodied in anime and manga. Characters, mostly males, will often be shown with a nosebleed if they have just seen someone nude or in little clothing, or if they have had an erotic thought or fantasy; this is based on the idea that a male's blood pressure will spike dramatically when aroused.

Blood libel

Various religious and other groups have been falsely accused of using human blood in rituals; such accusations are known as blood libel. The most common form of this is blood libel against Jews. Although there is no ritual involving human blood in Jewish law or custom, fabrications of this nature (often involving the murder of children) were widely used during the Middle Ages to justify Antisemitic persecution and some have persisted into the 21st century.

Vampire legends

Vampires are mythological beings which live forever by drinking the blood of the living. Stories of creatures of this kind are known all over the world. Most of these myths in Western culture originate from Eastern European folklore.

Entertainment

Art

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. Blood is also the main leitmotiv in Maligno Art.

Films

Blood is commonly associated with gore in motion pictures. Films with bloody scenes tend to receive a high rating by film licensing bodies, ranging from PG-13 to NC-17, depending on its depiction and its prominence. Blood in video games has led to video game censorship.

See also

• Blood donation
• Blood substitutes ("Artificial blood")
• Blood test
• List of human blood components
• Blood as food: see black pudding and tiết canh
• Taboo food and drink: Blood
• Hemophobia
• Autotransfusion

References

External links