white blood cell: see blood.

red blood cell: see blood.

high blood pressure: see hypertension.

dragon's blood, name for a red resin obtained from a number of different plants. It was held by early Greeks, Romans, and Arabs to have medicinal properties; Dioscorides and other early writers described it. A chief source was Dracaena cinnabari, a tree of the agave family. Voyagers to the Canary Islands in the 15th cent. obtained it from another species, D. draco. The resin, exuding beautiful garnet-colored drops when the tree is wounded, was well known as the source of varnish for 18th-century Italian violinmakers. Later, dragon's-blood varnishes and medicines were obtained chiefly from the immature fruits of a palm (Daemonorops draco) native to Malaya. Although still sometimes used in photoengraving processes, dragon's blood as a coloring material has largely been replaced by synthetics.

blood transfusion, transfer of blood from one person to another, or from one animal to another of the same species. Transfusions are performed to replace a substantial loss of blood and as supportive treatment in certain diseases and blood disorders. When whole blood is not needed, or when it is not available, plasma, the fluid of the blood without the blood cells, can be given. Alternately, such components of the blood as red cells, white cells, or platelets may be given for particular deficiencies. Blood substitutes, which are under development, are expected ultimately to ease the chronic short supply of blood and to alleviate certain storage and compatibility problems.

In whole-blood transfusions, the blood of the donor must be compatible with that of the recipient. Blood is incompatible when certain factors in red blood cells and plasma differ in donor and recipient; when that occurs, agglutinins (i.e., antibodies) in the recipient's blood will clump with the red blood cells of the donor's blood. The most frequent blood transfusion reactions are caused by substances of the ABO blood group system and the Rh factor system. In the ABO system, group AB individuals are known as universal recipients, because they can accept A, B, AB, or O donor blood. Persons with O blood are sometimes called universal donors, since their red cells are unlikely to be agglutinated by the blood of any other group. In the Rh factor system, agglutinins are not produced spontaneously in an individual but only in response to previous exposure to Rh antigens, as in some earlier transfusion. Transfusion reactions involving incompatibility eventually cause hemolysis, or disruption of donor cells. The resulting liberation of hemoglobin into the circulatory system, causing jaundice and kidney damage, can be lethal.

In addition to providing for the compatibility of blood groups in transfusion, it is necessary to determine that the donor's blood is free of organisms that might cause syphilis, malaria, serum hepatitis, or HIV, the virus believed to cause AIDS. Allergic reactions to transfusions may occur in cases where allergic antibodies have been transmitted from the donor's blood, possibly because of some type of food recently ingested by the donor. These problems have increased the popularity of autologous transfusions, transfusions using a person's own blood, which has been donated ahead of time. See blood bank.

blood test, examination of blood routinely or as an aid in diagnosing a suspected disease. Tests may be performed on whole blood or on the plasma portion only. Blood typing identifies the proteins at specific sites on red blood cells, a necessity in determining compatibility for blood transfusion. Human Lymphocyte Antigens (HLA) is a form of white blood cell typing prerequisite for organ and bone marrow transplants. The Coulter Cell Counter is widely used in electronic counts of red blood cells for the diagnosis of anemia and polycythemia. White cell counts are vital in detecting infections or in confirming leukemia. Serum or plasma may be collected, cultured, and inoculated with bacteria or other substances for the purpose of detecting the body's reaction to infections, cancer, or HIV, the virus that causes AIDS. Plasma may also be examined for evidence of functional disorders, e.g., for blood sugar in testing for diabetes mellitus. Blood tests for tumor markers, such as prostate-specific antigen, are effective in detecting cancer in high risk groups. Almost all blood tests are now performed by electronic equipment, and results are evaluated and printed out by computer.

blood substitute, substance that mimics the function of blood. Blood substitutes typically concentrate only on reproducing the function of hemoglobin, the molecule that carries oxygen through the body, and do not attempt to replicate the blood's other functions. Blood donated by humans must be refrigerated, can be contaminated by such diseases as AIDS and hepatitis, and is often in short supply. Designers of blood substitutes hope to eliminate these problems and develop genetically engineered or chemical products that will be tolerated by people of all blood types. Some blood substitutes under development use hemoglobin derived from human blood that is too old for use in blood transfusions.

blood pressure, force exerted by the blood upon the walls of the arteries. The pressure in the arteries originates in the pumping action of the heart, and pressure waves can be felt at the wrist and at other points where arteries lie near the surface of the body (see pulse). Since the heart can pump blood into the large arteries more quickly than it can be absorbed and released by the tiny arterioles and capillaries, considerable inner pressure always exists in the arteries. The contraction of the heart (systole) causes the blood pressure to rise to its highest point, and relaxation of the heart (diastole) brings the pressure down to its lowest point.

Blood pressure is strongest in the aorta, where the blood leaves the heart. It diminishes progressively in the smaller blood vessels and reaches its lowest point in the veins (see circulatory system). Blood pressure manifests itself dramatically when an artery is severed or pierced and the blood (under pressure) ejects in spurts.

Since blood pressure varies in different arteries, the pressure in the brachial artery of the forearm serves as a standard. A sphygmomanometer measures blood pressure in millimeters of mercury; blood pressure gauges that do not use mercury also produce readings that are expressed in terms of millimeters of mercury. Normal blood pressure readings for healthy young people should be below 120 mm for systolic pressure and 80 mm for diastolic pressure, commonly written as 120/80 and read as "one-twenty over eighty." With age, and the constriction of the small arteries and then the larger ones, blood pressure increases, so that at 50 years, a person may typically have a systolic pressure between 140 and 150, and a diastolic pressure of about 90.

Factors other than heart action and the condition of the arteries also influence blood pressure. Temporary high blood pressure usually occurs during or following physical activity, nervous strain, and periods of rage or fear. Therapy for persistent high blood pressure, sometimes called hypertension, consists of sufficient rest, a diet low in salt and alcohol, reduction in weight where there is obesity, and increased exercise. Drug therapy may include diuretics, beta-blockers, calcium-channel blockers, or ACE inhibitors. Low blood pressure (hypotension) has not been studied as extensively as high blood pressure. If not caused by disease or injury, it is generally considered to be a benign or even advantageous condition; however, studies have linked hypotension with feelings of tiredness or faintness and minor psychiatric conditions in some people.

blood poisoning: see septicemia.

blood groups, differentiation of blood by type, classified according to immunological (antigenic) properties, which are determined by specific substances on the surface of red blood cells. Blood groups are genetically determined and each is characterized by the presence of a specific complex carbohydrate. About 200 different blood group substances have been identified and placed within 19 known blood group systems. The most commonly encountered blood group system is the ABO, or Landsteiner, system. Individuals may contain the A, B, or both A and B antigenic substances, or else lack these substances (type O). In the ABO system an individual who lacks one or more of these antigens will spontaneously develop the corresponding antibodies (agglutinins) shortly after birth. Thus a person with A type blood will naturally produce anti-B agglutinins, a person with B blood will produce anti-A agglutinins, and a person with O blood will produce anti-A and anti-B agglutinins; but a person with AB blood will not produce any agglutinins in this blood group system. Since these agglutinins are always present in the blood, in blood transfusion the donor blood must be compatible with the recipient's blood, i.e., the donor's blood must not contain antigen corresponding to the recipient's antibody. Other blood group systems, such as the MNSs, Lewis, Lutheran, and P systems, are not as important in transfusion because they act like true antigen-antibody systems, i.e., antibodies do not appear in blood plasma until the individual has been immunized by exposure to the other blood group antigens as in previous transfusions. In general, blood group substances are weak antigens, and antibody formation after transfusion occurs less than 3% of the time. Immunization can occur by pregnancy as well as by transfusion. Thus, in the Rh factor blood group system, an Rh-negative mother carrying an Rh-positive fetus produces anti-Rh antibodies against fetal red blood cells that cross the placenta. Since blood type is a genetic trait that is easy to test and the blood type of an individual is related to his or her parent's blood types by the laws of Mendelism (see under Mendel, Gregor), blood group typing is used legally to establish paternity. Anthropologists use the frequency of occurrence of various blood groups as tools to study racial or tribal origins.

blood feud: see vendetta.

blood count, method for determining the number of red (erythrocytes) and white (leukocytes) blood cells in a certain volume of blood. This test can be used as a preliminary step in diagnosing some diseases. Leukemia, for instance, causes an increase in the white blood cell count, while a decrease in the number of red blood cells is associated with anemia. A determination of the percentage of various types of white blood cells (neutrophils, lymphocytes, eosinophils, basophils, and monocytes) is called a differential blood count. A complete blood count (CBC) is an extended series of tests to determine such additional factors as the hemoglobin concentration of red blood cells and the amount of platelets in the blood.

blood clotting, process by which the blood coagulates to form solid masses, or clots. In minor injuries, small oval bodies called platelets, or thrombocytes, tend to collect and form plugs in blood vessel openings. To control bleeding from vessels larger than capillaries a clot must form at the point of injury. The coagulation of the blood is also initiated by the blood platelets. The platelets produce a substance that combines with calcium ions in the blood to form thromboplastin, which in turn converts the protein prothrombin into thrombin in a complex series of reactions. Thrombin, a proteolytic enzyme, converts fibrinogen, a protein substance, into fibrin, an insoluble protein that forms an intricate network of minute threadlike structures called fibrils and causes the blood plasma to gel. The blood cells and plasma are enmeshed in the network of fibrils to form the clot. Blood clotting can be initiated by the extrinsic mechanism, in which substances from damaged tissues are mixed with the blood, or by the intrinsic mechanism, in which the blood itself is traumatized. More than 30 substances in blood have been found to affect clotting; whether or not blood will coagulate depends on a balance between those substances that promote coagulation (procoagulants) and those that inhibit it (anticoagulants). Prothrombin, a substance essential to the clotting mechanism, is produced by the liver in the presence of vitamin K. When the body is deficient in this vitamin, bleeding is more difficult to control. In hemophiliacs, or "bleeders," the blood's coagulation time is greatly prolonged (see hemophilia). The coagulation of blood within blood vessels in the absence of injury can cause serious illness or death, especially when a clot forms in the coronary arteries (thrombosis) or cerebral arteries (stroke or apoplexy). To prevent coagulation of the blood in persons with known tendency to clot formation, and also as prophylaxis before performing surgery or blood transfusion, the blood's natural anticlotting substance, heparin, is reinforced by an additional amount of an anticoagulant such as Dicumarol injected into the body.

blood bank, site or mobile unit for collecting, processing, typing, and storing whole blood, blood plasma and other blood constituents. Most hospitals maintain their own blood reserves, and the American Red Cross provides a nationwide collection and distribution service. The Red Cross collects about 50% of the blood for the nation's blood banks. The Food and Drug Administration licenses blood banks.

Whole blood may be preserved for up to 21 days without losing its usefulness in blood transfusions; an anticoagulant is added to prevent clotting. Blood plasma, the fluid portion of the blood, may be frozen and/or dried and stored indefinitely. Blood and donors are screened for hepatitis, AIDS, malaria, and other infectious diseases. The potential risk of acquiring AIDS or hepatitis through transfusions has made it a common practice among patients anticipating surgery to "bank" their own blood before it is needed.

Many blood banks also have facilities for apheresis, bone marrow donations, and related procedures. Some centers save umbilical cord blood (blood that is especially rich in stem cells) for use in treatments; however, the cost of preparing and storing such blood is much higher than that of normal blood. Sometimes parents store their newborn's cord blood at a private cord blood bank in case the child has need of it, but the use of one own's cord blood is ineffective or undesirable in many diseases where such blood is used as a treatment.

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.

Blood Blackfoot: see Blackfoot.

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.

Learn more about leukocyte with a free trial on Britannica.com.

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).

Learn more about erythrocyte with a free trial on Britannica.com.

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.

Learn more about hypotension with a free trial on Britannica.com.

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.

Learn more about hypertension with a free trial on Britannica.com.

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.

Learn more about vein with a free trial on Britannica.com.

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.

Learn more about varicose vein with a free trial on Britannica.com.

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.

Learn more about blood typing with a free trial on Britannica.com.

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.

Learn more about blood transfusion with a free trial on Britannica.com.

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.

Learn more about blood analysis with a free trial on Britannica.com.

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.

Learn more about glucose with a free trial on Britannica.com.

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.

Learn more about blood pressure with a free trial on Britannica.com.

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.

Learn more about septicemia with a free trial on Britannica.com.

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.

Learn more about plasma with a free trial on Britannica.com.

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.

Learn more about coagulation with a free trial on Britannica.com.

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).

Learn more about thrombosis with a free trial on Britannica.com.

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.

Learn more about blood bank with a free trial on Britannica.com.

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.

Learn more about blood with a free trial on Britannica.com.