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fluid pumped by the heart that circulates throughout the body via the arteries, veins, and capillaries (see circulatory systemcirculatory system,
group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the lungs, and the
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; heartheart,
muscular organ that pumps blood to all parts of the body. The rhythmic beating of the heart is a ceaseless activity, lasting from before birth to the end of life. Anatomy and Function

The human heart is a pear-shaped structure about the size of a fist.
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). 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 hemoglobinhemoglobin
, respiratory protein found in the red blood cells (erythrocytes) of all vertebrates and some invertebrates. A hemoglobin molecule is composed of a protein group, known as globin, and four heme groups, each associated with an iron atom.
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, the amount of which in turn depends on the iron level in the body. A deficiency of iron and therefore of hemoglobin leads to anemiaanemia
, condition in which the concentration of hemoglobin in the circulating blood is below normal. Such a condition is caused by a deficient number of erythrocytes (red blood cells), an abnormally low level of hemoglobin in the individual cells, or both these conditions
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 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 marrowbone marrow,
soft tissue filling the spongy interiors of animal bones. Red marrow is the principal organ that forms blood cells in mammals, including humans (see blood). In children, the bones contain only red marrow.
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. In the fetus, red blood cells are produced in the spleenspleen,
soft, purplish-red organ that lies under the diaphragm on the left side of the abdominal cavity. The spleen acts as a filter against foreign organisms that infect the bloodstream, and also filters out old red blood cells from the bloodstream and decomposes them.
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. 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 groupsblood 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.
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. There are four major blood groups, whose compatibility or incompatibility is an important consideration in successful blood transfusionblood 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.
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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 immunityimmunity,
ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
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). 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 leukemialeukemia
, cancerous disorder of the blood-forming tissues (bone marrow, lymphatics, liver, spleen) characterized by excessive production of immature or mature leukocytes (white blood cells; see blood) and consequently a crowding-out of red blood cells and platelets.
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. 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 AIDSAIDS
or acquired immunodeficiency syndrome,
fatal disease caused by a rapidly mutating retrovirus that attacks the immune system and leaves the victim vulnerable to infections, malignancies, and neurological disorders. It was first recognized as a disease in 1981.
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. 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.


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.


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 (tuberculosistuberculosis
(TB), contagious, wasting disease caused by any of several mycobacteria. The most common form of the disease is tuberculosis of the lungs (pulmonary consumption, or phthisis), but the intestines, bones and joints, the skin, and the genitourinary, lymphatic, and
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, malariamalaria,
infectious parasitic disease that can be either acute or chronic and is frequently recurrent. Malaria is common in Africa, Central and South America, the Mediterranean countries, Asia, and many of the Pacific islands.
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, and typhoidtyphoid fever
acute, generalized infection caused by Salmonella typhi. The main sources of infection are contaminated water or milk and, especially in urban communities, food handlers who are carriers.
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) 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 clottingblood 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.
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. 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 (albuminalbumin
[Lat.,=white of egg], member of a class of water-soluble, heat-coagulating proteins. Albumins are widely distributed in plant and animal tissues, e.g., ovalbumin of egg, myogen of muscle, serum albumin of blood, lactalbumin of milk, legumelin of peas, and leucosin of
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, globulinsglobulin,
any of a large family of proteins of a spherical or globular shape that are widely distributed throughout the plant and animal kingdoms. Many of them have been prepared in pure crystalline form.
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, and fibrinogen), and metabolic wastes (such as ureaurea
, organic compound that is the principal end product of nitrogen metabolism in most mammals. Urea was the first animal metabolite to be isolated in crystalline form; its crystallization was described in the early 18th cent.
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) 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.


See D. Starr, Blood (1998).


1. a reddish fluid in vertebrates that is pumped by the heart through the arteries and veins, supplies tissues with nutrients, oxygen, etc., and removes waste products. It consists of a fluid (see blood plasma) containing cells (erythrocytes, leucocytes, and platelets)
2. a similar fluid in such invertebrates as annelids and arthropods
3. Obsolete one of the four bodily humours


A fluid connective tissue consisting of the plasma and cells that circulate in the blood vessels.


The fluid that circulates in the blood vessels of the body. Blood consists of plasma and cells floating within it. The cells are derived from extravascular sites and then enter the circulatory system. They frequently leave the blood vessels to enter the extravascular spaces, where some of them may be transformed into connective tissue cells. The fluid part of the blood is in equilibrium with the tissue fluids of the body. The circulating blood carries nutrients and oxygen to the body cells, and is thus an important means of maintaining the homeostasis of the body. It carries hormones from their sites of origin throughout the body, and is thus the transmitter of the chemical integrators of the body. Blood plasma also circulates immune bodies and contains several of the components essential for the formation of blood clots. Finally, blood transports waste products to excretory organs for elimination from the body. Because of its basic composition (cells surrounded by a matrix), development, and ability to modify into other forms of connective tissues, blood can be regarded as a special form of connective tissue. See Connective tissue

Formed elements

The cells of the blood include the red blood cells and the white blood cells. In all vertebrates, except nearly all mammals, the red blood cells or corpuscles contain a nucleus and cytoplasm rich in hemoglobin. In nearly all mammals the nucleus has been extruded during the developmental stages.

In normal adult men the blood contains about 5,000,000 red blood corpuscles or erythrocytes per cubic millimeter; in normal adult women, about 4,500,000. Human erythrocytes are about 8 micrometers in diameter and about 2 μm at their thickest and have a biconcave shape. They contain hemoglobin, which imparts to them their color, and possess an envelope. When circulating in the blood vessels, the red blood cells are not evenly dispersed. In the capillaries the erythrocytes are often distorted. In certain conditions they may be densely aggregated. This is known as a sludge. The erythrocytes respond to changes in osmotic pressure of the surrounding fluid by swelling in hypotonic fluids and by shrinking irregularly in hypertonic fluids. Shrunken red blood cells are referred to as crenated cells. The average life of the mature red blood cells is surprisingly long, having a span of about 120 days. See Hemoglobin

In humans the white blood cells in the blood are fewer in number. There are about 5000–9000/mm3. In general, there are two varieties, agranular and granular. The agranular cells include the small, medium, and large lymphocytes and the monocytes (see illustration). The small lymphocytes are spherical, about the diameter of erythrocytes or a little larger, and constitute about 20–25% of the white blood cells. The medium and large lymphocytes are relatively scarce. In all lymphocytes the nucleus occupies nearly the whole volume of the cell, and the cytoplasm which surrounds it forms a thin shell. The typical monocyte is commonly as large as a large lymphocyte (12 μm), and constitutes 3–8% of the white blood cells. The nucleus is relatively small, eccentric, and oval or kidney-shaped. The cytoplasm is relatively larger in volume than that in lymphocytes.

Diagrammatic representation of human blood cellsenlarge picture
Diagrammatic representation of human blood cells

The granular leukocytes are of three varieties: neutrophil, eosinophil, and basophil. Their structure varies somewhat in different species, and the following applies to those of humans. The neutrophils make up 65–75% of the leukocytes. They are about as large as monocytes with a highly variable nucleus, consisting of three to five lobes joined together by threads of chromatin. The cytoplasm contains numerous minute granules which stain with neutral dyes and eosin. The eosinophils (also called acidophils) are about the same size as the neutrophils but are less numerous, constituting about 1% of the leukocytes. The nucleus commonly contains but two lobes joined by a thin thread of chromatin. The granules which fill the cytoplasm are larger than those of the neutrophils and stain with acid dyes. The basophils are about the same size as the other granular leukocytes. The nucleus may appear elongated or with one or more constrictions. The granules are moderately large, stain with basic dyes, and are water-soluble.

The functions of the leukocytes while they are circulating in the blood are not known. However, when they leave the blood vessels and enter the connective tissue, they constitute an important part of the defense mechanism and of the repair mechanism. Many of the cells are actively phagocytic and engulf debris and bacteria. Lymphocytes are of two major kinds, T cells and B cells. They are involved in the formation of antibodies and in cellular immunity.

The blood platelets are small spindle-shaped or rodlike bodies about 3 μm long and occur in large numbers in circulating blood. In suitably stained specimens they consist of a granular central portion (chromomere) embedded in a homogeneous matrix (hyalomere). They change their shape rapidly on contact with injured vessels or foreign surfaces and take part in clot formation. The platelets are not to be regarded as cells and are thought to be cytoplasmic bits broken off from their cells of origin in bone marrow, the megakaryocytes.


Plasma is the residual fluid of blood left after removal of the cellular elements. Serum is the fluid which is obtained after blood has been allowed to clot and the clot has been removed. Serum and plasma differ only in their content of fibrinogen and several minor components which are in large part removed in the clotting process. See Serum

The major constituents of plasma and serum are proteins. The total protein concentration of human serum is approximately 7 g/ml, and most other mammals show similar levels. By various methods it can be demonstrated that serum protein is a heterogeneous mixture of a large number of constituents. Only a few are present in higher concentrations, the majority being present in trace amounts. More than 60 protein components have been identified and characterized. Albumin makes up more than one-half of the total plasma proteins and has a molecular weight of 69,000. Because of its relatively small molecular size and its high concentration, albumin contributes to 75–80% of the colloid osmotic pressure of plasma. The immunoglobulins, which represent approximately one-sixth of the total protein, largely constitute the γ-globulin fraction. The immunoglobulins are antibodies circulating in the blood, and therefore are also called humoral antibodies. They are of great importance in the organism's defense against infectious agents, as well as other foreign substances. See Immunoglobulin

In addition to the proteins, many other important classes of compounds circulate in the blood plasma. Most of these are smaller molecules which diffuse freely through cell membranes and are, therefore, more similarly distributed throughout all the fluids of the body and not as characteristic for plasma or serum as the proteins. In terms of their concentration and their function, the electrolytes are most important. They are the primary factors in the regulation of the osmotic pressure of plasma, and contribute also to the control of the pH. The chief cations are sodium, potassium, calcium, and magnesium. The chief anions are chloride, bicarbonate, phosphate, sulfate, and organic acids. The circulating blood also contains the many small compounds which are transported to the sites of synthesis of larger molecules in which they are incorporated, or which are shifted as products of metabolic breakdown to the sites of their excretion from the body.


When mammalian blood is shed, it congeals rapidly into a gelatinous clot of enmeshed fibrin threads which trap blood cells and serum. Modern theories envision a succession of reactions leading to the formation of insoluble fibrin from a soluble precursor, fibrinogen (factor I). Blood also clots when it touches glass or other negatively charged surfaces, through reactions described as the intrinsic pathway. Several of the steps in this process are dependent upon the presence in blood of calcium ions and of phospholipids, the latter derived principally from blood platelets. The coagulation of blood can also be induced by certain snake venoms which either promote the formation of thrombin or clot fibrinogen directly, accounting in part for their toxicity.

Platelets, besides furnishing phospholipids for the clotting process, help to stanch the flow of blood from injured blood vessels by accumulating at the point of injury, forming a plug. Platelets participate in the phenomenon of clot retraction, in which the blood clot shrinks, expelling liquid serum. Although the function of retraction is unknown, individuals in whom this process is impaired have a bleeding tendency.

Hereditary deficiencies of the function of each of the protein-clotting factors have been described, notably classic hemophilia and Christmas disease, which are disorders of males and clinically indistinguishable. The various hereditary functional deficiencies are associated with a bleeding tendency with one inexplicable exception. Acquired deficiencies of clotting factors, sometimes of great complexity, are also recognized. Therapy for bleeding due to deficiencies of clotting factors often includes the transfusion of blood plasma or fractions of plasma rich in particular substances the patient may lack. See Human genetics

Clinical tests of the coagulability of the blood include (1) determination of the clotting time, that is, the time elapsing until shed blood clots; (2) the prothrombin time, the time elapsing until plasma clots in the presence of tissue thromboplastin (and therefore a measure of the extrinsic pathway of clotting); (3) the partial thromboplastin time, the time elapsing until plasma clots in the presence of crude phospholipid (and therefore a measure of the intrinsic pathway of clotting); (4) the enumeration of platelets; and (5) crude quantification of clot retraction and of the various plasma protein-clotting factors.

Heparin, a polysaccharide–sulfuric acid complex found particularly in the liver and lungs, impairs coagulation; its presence in normal blood is disputed. Both coumarin and heparin are used clinically to impede coagulation in thrombotic states, including thrombophlebitis and coronary heart disease. See Fibrinogen


humor effecting temperament of sanguineness. [Medieval Physiology: Hall, 130]


Popular culture and folk tradition present us with gentle and pastelcolored Easter symbols, such as the Easter Bunny, baby chicks, and colored Easter eggs. Alongside these mild images there resides a lesser-known religious symbol that is both vivid and somewhat frightening. This symbol is blood. In addition to celebrating Jesus' resurrection each year on Easter Sunday, Christians also commemorate Jesus'death on the cross on Good Friday. What's more, many churches remember Jesus' willingness to sacrifice himself for his followers every week in a ceremony called the Eucharist. In this ceremony worshipers partake of bread and wine, presented to them as Jesus' body and blood. Although this blood imagery may strike some people as gruesome, it was originally intended as an emblem of spiritual liberation and renewal. An exploration of the religious significance of blood to the ancient Jews and first Christians illuminates the meaning of this symbol.

Blood Symbolism among the Ancient Jews

The Hebrew scriptures known to Christians as the Old Testament teach that a creature's life force is contained in its blood (Leviticus 17:11). This belief turned blood into both a powerful symbol and a source of potential physical and spiritual contamination. The association of blood with violence and death may have inspired belief in the power of blood to contaminate.

The power of blood could be turned to good purposes, however. Jewish scripture relates several important instances where blood was used to seal a covenant, or agreement, between God and his faithful people. For example, the Book of Exodus recounts how Jewish families enslaved in Egypt obeyed the Lord's command, relayed to them by the prophet Moses, to sacrifice a lamb to God and smear its blood over their doorways. God used this mark to identify which homes to pass over when wreaking vengeance on the Egyptians for their cruelty and disobedience. Jews still commemorate these events in the yearly Passover festival. After the Jews escaped from Egypt Moses informed them of God's new rules for them. Then Moses signified the Jews' agreement to this plan by scattering the blood of sacrificed bulls on the people and the altar (Exodus 24:3-8).

The ancient Jews also sacrificed animals to God on other occasions. They made these sacrifices for a number of reasons, including as a way of showing reverence, offering thanks, and asking for reconciliation with God. The Bible sometimes refers to this process of reconciliation as redemption. A passage from Hebrew scripture explains the role of blood offerings in gaining God's forgiveness. In the book of the Bible called Leviticus, God informs the Jews that ". . . the life of the flesh is in the blood, and I have given it for you upon the altar to make atonement for your souls: for it is the blood that makes atonement, by reason of the life" (Leviticus 17:11). Animal sacrifices entailed sprinkling the blood of the slain animals on the altar. This blood represented the life force of the animal, and so some scholars believe that in the context of the sacrifice it served as a symbolic substitute for the life force of the person or group making the sacrifice. Thus in offering the blood of slain animals to God, faithful Jews of this period were symbolically offering their own lives to God. Other researchers contend that the ancient Jews believed that their sins dirtied the temple and that sacrifices could restore the house of God to a state of purity.

Blood Symbolism among the First Christians

Although the Christian religion grew out of Judaism, it did not recreate the ancient Jewish custom of animal sacrifice. Instead Christians grew to understand Jesus'death as a one-time-only sacrifice on behalf of all his followers and the world. Nevertheless, Christians interpreted Jesus'sacrifice in terms of the blood symbolism already established in Jewish religious culture. Jesus himself instructed them to do so.

On the night before Jesus died he shared a meal with his twelve disciples (see also Maundy Thursday). During this meal he passed bread and wine to his followers, telling them that the wine represented his blood and the bread his body. In Matthew's version of the story Jesus not only identifies the wine as his blood, but also as "the blood of the covenant, which is poured out for many for the forgiveness of sins" (Matthew 26:28). Jesus, aware that he will soon die, is interpreting his own death for his followers. He describes it as a blood sacrifice which will establish a new covenant, or relationship, between God and humanity. Moreover, just as the blood of a sacrificial animal could confer the forgiveness of sins, Jesus' blood - freely offered on behalf of all his followers - will cleanse them of their sins. Jesus' blood thereby became the vehicle which reconciles his followers with God, that is to say, re-establishes good will and harmony between them.

Christian scripture records that Jesus indeed suffered a bloody death on the cross the following day. Christians commemorate this terrible event each year on Good Friday. They also honor Jesus' words and deeds at the Last Supper with the Eucharist, celebrated every Sunday in many churches. This rite offers worshipers the opportunity to share in the soul-sustaining consequences of Jesus'sacrificial death. According to the Gospel of John, partaking of Jesus' body and blood imparts eternal life and a mystical connection to Christ (John 6:5456). This doctrine builds on the ancient Hebrew belief that the life force of all creatures resides in their blood. Therefore, by consuming Jesus' blood, his followers absorb some part of Jesus' divine nature and his connection to God. Regular participation in this ceremony strengthens their spiritual bond with Christ. This ritual sheds additional light on the meaning of Jesus' death, implying that Jesus sacrificed his own life so that others could share in it (see also Salvation).

Further Reading

"Blood." In Leland Ryken, James C. Wilhoit, and Tremper Longman III, eds. Dictionary of Biblical Imagery. Downers Grove, IL: InterVarsity Press, 1998. Mason, Steve A. "Sacrifices and Offerings." In David Noel Freedman, ed. Eerdmans Dictionary of the Bible. Grand Rapids, MI: William B. Eerdmans Publishing, 2000. Potts, Donald R. "Blood." In David Noel Freedman, ed. Eerdmans Dictionary of the Bible. Grand Rapids, MI: William B. Eerdmans Publishing, 2000. Roux, Jean-Paul. "Blood." In Mircea Elide, ed. The Encyclopedia of Religion. Volume 12. New York: Macmillan, 1987. "Sacrifice." In Leland Ryken, James C. Wilhoit, and Tremper Longman III, eds. Dictionary of Biblical Imagery. Downers Grove, IL: InterVarsity Press, 1998.



a fluid tissue that moves through the circulatory system of man and animals, providing for vital activities and the performance of various physiological functions of the body’s cells and tissues.

Overview. One of the principal functions of the blood is the transport of gases (O2 from the respiratory organs to the tissues and CO2 from the tissues to the respiratory organs). Blood is also involved in the transfer of glucose, amino acids, fatty acids, salts, and other nutrients from the digestive organs to the tissues. In addition, it carries the end products of metabolism, such as urea, uric acid, and creatinine, to the excretory organs. Blood takes part in the regulation of the body’s water-salt metabolism and acid-base equilibrium and plays an important role in maintaining constant body temperature. A protective function is carried out by the antibodies, antitoxins, and lysines present in the blood and by the white blood cells, or leukocytes, which are capable of absorbing microorganisms and foreign bodies. A very important protective adaptation, one that prevents blood loss from the body, is the arrest of blood flow as a result of coagulation.

Blood contains a number of chemical compounds for which the demand changes with the functional activities of the tissues. However, the blood’s chemical composition, the active reaction of its medium (pH), and other physical and chemical constants maintain relative stability. This is ensured by the mechanisms of homeostasis, which include the blood’s rate of flow (regulating the entry of nutrient substances into the tissues), the capacity of the excretory organs to remove metabolic products, and the maintenance of the water balance (achieved by the exchange of fluid between the blood and the lymph). Homeostasis is also maintained by regulation of the metabolism (matter and energy) of biologically active substances (histamine, serotonin, acetylcholine) and by hormones transported by the blood from site of formation to site of activity.

In unicellular organisms and in many invertebrates (protozoans, sponges, coelenterates), oxygen is supplied by means of diffusion from the external environment through the body surface. In some primitive metazoans there is a system of canals communicating with the external environment (the gastrovascular system) through which hydrolymph circulates. Hydrolymph delivers nutrients to the cells and removes their metabolic products, but it does not, as a rule, bind and transport oxygen. Only in a few invertebrates does the hydrolymph contain protein pigments that are capable of carrying oxygen. In more developed animals (mollusks and arthropods) there is an open system of blood circulation, filled with hemolymph and communicating with the intertissular spaces. In a number of invertebrates, all vertebrates, and man, the circulatory system is closed and the blood is separated from the interstitial fluid and lymph.

Only in a few relatively inactive animals is blood (or hemolymph) able to carry sufficient oxygen in a dissolved state without the participation of respiratory pigments (chromoproteins). With the appearance at a certain stage of animal evolution of respiratory pigments, the ability of the blood to bind oxygen and to release it to the tissues increases sharply. These pigments include hemoglobin, chlorocruorin, and hemerythrin, which contain iron in the nonprotein parts of their molecules, and hemocyanin, which contains copper. The pigments are either dissolved in hemolymph or carried in blood corpuscles. The green pigment chlorocruorin is dissolved in the plasma of polychaetes. Hemerythrin, a violet pigment, is contained in the blood corpuscles of polychaetes, sipunculids, and brachiopods. The blood of many mollusks and coelenterates is blue because of the hemocyanin dissolved in it. Hemoglobin is the most widely distributed. This red pigment is dissolved in the perivisceral fluid or blood of many invertebrates, and it is found in the erythrocytes of all vertebrates, including man.

In invertebrates the ratio of the mass of the fluid that performs the function of blood to the mass of the body is significantly higher than in vertebrates. Whereas hemolymph constitutes 30 percent of the mass of the mollusk Anodonta and 20 percent in many insects, blood constitutes only 2–8 percent of the body mass in vertebrates (in fish, about 3 percent; in amphibians, to 6 percent; in reptiles, 6.5 percent; and in birds and mammals, to 8 percent). Blood averages 6.8 percent of the body weight in man (about 5 liters for a weight of 70 kg). The decrease in the relative volume of the blood in vertebrates may be explained by the development of the closed system of blood circulation and the appearance of respiratory pigments that effectively bind oxygen.

In vertebrates, the blood is a thick, homogeneous, red fluid that consists of a liquid part, or plasma, and formed elements— the erythrocytes, which give blood its red color, the leukocytes, and the thrombocytes, or blood platelets. The volume of formed elements in the lower vertebrates (fish, amphibians, and reptiles) is 15–40 percent. In higher vertebrates (birds and mammals) it is 35–54 percent. Of the formed elements, the erythrocytes are the most numerous. Their number and size differ for different vertebrates. Thus, among the ungulates, 1 cu mm of llama blood contains 15.4 million erythrocytes, and 1 cu mm of goat blood contains 13 million; in the reptiles, the range is from 500,000 to 1.65 million; and among the chondrosteans, from 90,000 to 130,000. The smallest erythrocytes are found in mammals; in the musk deer, the diameter is about 2.5 microns (μm) and in the goat, about 4.0 μm. The largest erythrocytes are found in amphibians (70 μm, in the urodele Amphiumd).

The erythrocytes in all vertebrates except the mammals are elliptical and have a nucleus. In mammals the erythrocytes are nonnucleated and are shaped like biconcave disks (with the exception of the camel, in which the erythrocytes are oval and lentiform). Increasing the number of erythrocytes and decreasing their size improves the oxygen supply to the body. In the lower vertebrates, 100 ml of blood contains 5–10 g of hemoglobin; in fish, 6–11 g; and in the mammals, 10–15 g. In man, 1 cu mm of blood normally contains 4.5–5.5 million erythrocytes (in men, 4.5–5.5 million; in women, 4–4.5 million). The constancy of the number of erythrocytes in the blood is the result of an equilibrium between their formation in the bone marrow (hematopoiesis) and the destruction of old erythrocytes in the cells of the reticuloendothelial system. The average hemoglobin content in men is 13.3–18 g percent; in women, 11.7–15.8 g percent. The diameter of the human erythrocyte is 7.2 μm; the thickness, 2 μm; and the volume, 88 cu μm. The shape of the biconcave disk ensures passage of the erythrocytes through the narrow lumina of the capillaries. According to A. L. Chizhevskii, the flow of blood is a single, structured, dynamic system comprising a huge number of elements. The movement of the erythrocyte through the vascular channel is not chaotic; this is a consequence both of limited space and of electrostatic, hydro-dynamic, and other forces that inhibit approach and contact between erythrocytes.

The principal function of erythrocytes, the transport of O2 and CO2, can be accomplished thanks to the large content of hemoglobin (about 265 million molecules of hemoglobin in every erythrocyte), the high activity of the enzyme carboanhydrase, the large concentration of 2,3-diphosphoglyceric acid, and the presence of ATP and ADP (adenosine phosphoric acids). These compounds (mainly 2,3-diphosphoglyceric acid) bond with deoxyhemoglobin and decrease its affinity for O2, promoting the liberation of oxygen to the tissues. Erythrocytes have an important role in water-salt metabolism, in the regulation of the body’s acid-base equilibrium, and in the regulation (by absorption) of the body’s amino acid and polypeptide content. Erythrocytes are also the carriers of the properties that define blood groups.

The leukocytes are nucleated cells; they are subdivided into granular leukocytes, or granulocytes (comprising the neutrophils, the eosinophils, and the basophils), and agranular leukocytes, or agranulocytes. Neutrophils are characterized by their ability to move and to penetrate from the loci of hematopoiesis to the peripheral blood and tissues. They are capable of capturing (phagocytizing) microbes and other foreign particles that enter the body. Agranulocytes participate in immunological reactions and in the processes of regeneration and inflammation. There are 6,000–8,000 leukocytes per cu mm in the blood of an adult human.

Thrombocytes, or blood platelets, play an important role in stemming blood flow (clotting). There are 200,000–400,000 thrombocytes per cu mm of human blood. Thrombocytes are nonnucleated. Analogous functions in the blood of all other vertebrates are performed by nucleated spindle cells. The relative constancy of the number of formed elements in the blood is regulated by complex nervous (central and peripheral) and humoral-hormonal mechanisms.

Physicochemical properties. The density and viscosity of blood depend mainly on the number of formed elements. Normally they vary within narrow limits. In man, the density of whole blood is 1.05–1.06 g/cm3; of plasma, 1.02–1.03 g/cm3; and of formed elements, 1.09 g/cm3. The difference in densities makes it possible to separate whole blood into plasma and formed elements by centrifugation. The erythrocytes constitute 44 percent, and the leukocytes and thrombocytes, 1 percent, of the total volume of blood. The osmotic pressure of blood, equal to 740 kilonewtons/m2 (7.63 atmospheres) at 37°C, is determined predominantly by electrolytes—that is, in the plasma, by Na and Cl ions, and in the erythrocytes, by K and Cl ions. The oncotic pressure is determined by the proteins present in the blood. The hydrogen ion concentration is slightly alkaline (pH 7.26–7.36) and is maintained at a constant level by the blood’s buffering systems—bicarbonate, phosphate, and protein—and by the activity of the respiratory and excretory organs.

Chemical composition. There are 18–24 g of dry residue and 77–82 g of water per 100 ml of blood. The water makes up more than half the mass of the erythrocytes and 90–92 percent of the plasma. The blood plasma contains the intermediate and end products of metabolism, as well as salts, hormones, vitamins, and enzymes. A substantial portion of the blood is composed of proteins. These proteins are represented principally by respiratory pigments, the proteins of the erythrocytic stroma, and the proteins of the other formed elements. Proteins dissolved in the plasma (6.5–8.5 percent of the 9–10 percent dry residue of plasma) are formed predominantly in the cells of the liver and reticuloendothelial system. Plasma proteins do not penetrate capillary walls, so that their content in the plasma is significantly higher than in the interstitial fluid. This leads to the retention of water by the plasma proteins. Despite the fact that oncotic pressure accounts for only a small fraction (about 0.5 percent) of the total osmotic pressure, it is the oncotic pressure that causes the preponderance of the osmotic pressure of blood over that of interstitial fluid. The high hydrodynamic pressure in the circulatory system would otherwise cause water to infiltrate the tissues and produce edema of the various organs and subcutaneous tissue.

Proteins also determine the blood’s viscosity, which is five or six times higher than that of water and plays an important role in maintaining the circulatory system’s hemodynamic relationships. Plasma proteins perform a transport function, participate in the regulation of the blood’s acid-base equilibrium, and serve as a nitrogen reserve. A significant portion of the serum calcium, iron, and magnesium are bound to plasma proteins. Fibrinogen, prothrombin, and other proteins participate in blood coagulation. Other plasma proteins play an important role in immune processes.

Plasma proteins are separated by means of electrophoresis into the albumin fraction, the globulin group (α1, α2, β, and γ), and fibrinogen (which participates in blood coagulation). The protein fractions of plasma are not homogeneous; using modern chemical and physicochemical separation methods, it has been possible to discover about 100 protein components of plasma.

Albumins account for 55–60 percent of the proteins of plasma. Because of their relatively small molecules, their high concentration in the plasma, and their hydrophilic properties, the proteins of the albumin group play an important role in maintaining oncotic pressure. Albumins transport organic compounds, such as cholesterol and bile pigments, and serve as a source of nitrogen for the building of proteins. The free sulfhydryl (—SH) group of albumin binds heavy metals, such as mercury compounds, which are deposited in the kidneys until being eliminated from the body. Albumins are capable of combining with certain medicinal substances, such as penicillin and salicylates, and of binding calcium, magnesium, and manganese.

Globulins are an extremely varied group of proteins, differing in physical and chemical properties and in functional activity. With paper electrophoresis they can be separated into α1, α2, β, and γ-globulins. Most of the proteins of the α- and α-globulin fractions are bound to carbohydrates (glycoproteins) or lipids (lipoproteins). Sugar or amino sugars are usually among the components of the glycoproteins. Blood lipoproteins, which are synthesized in the liver, are separated by their electrophoretic mobility into three principal fractions, differing in lipid composition. The physiological role of lipoproteins consists in delivering water-insoluble lipids, steroid hormones, and fat-soluble vitamins to the tissues.

The α2-globulin fraction comprises several proteins that participate in blood coagulation, including prothrombin, an inactive precursor of the enzyme thrombin that produces the conversion of fibrinogen to fibrin. The fraction also includes haptoglobin, whose content in the blood increases with age. Haptoglobin forms a complex with hemoglobin that is absorbed by the reticuloendothelial system. This prevents the body’s loss of iron, a component of hemoglobin. In addition, the α2-globulins include the glycoprotein ceruloplasmin, which contains 0.34 percent copper (that is, almost all plasma copper). Ceruloplasmin catalyzes the oxidation by oxygen of ascorbic acid and aromatic diamines.

The α2-globulin fraction of the plasma contains the polypep-tides bradykininogen and kallidinogen, which are activated by proteolytic enzymes of the plasma and tissues. Their active forms, bradykinin and kallidin, form the kinin system, which regulates the permeability of the capillary walls and activates the coagulation system of the blood.

The glycoproteins of the β-globulin fraction include transferrin, the carrier of iron in the body. The β1- and β2-globulin fractions contain several of the coagulation factors of the plasma, including antihemophilic globulin.

Fibrinogen migrates between β- and γ-globulins. The plasma proteins that migrate with the γ-globulins include the various antibodies, such as the antibodies to diphtheria, whooping cough, measles, scarlet fever, and poliomyelitis.

The nonprotein nitrogen of the blood is contained mainly in the end or intermediate products of nitrogenous metabolism: urea, ammonia, polypeptides, amino acids, creatine, creatinine, uric acid, and purine bases. The blood that flows away from the intestinal tract carries amino acids through the portal vein to the liver, where the amino acids undergo deamination, reamination, and other transformations (to the formation of urea) and where they are used for the biosynthesis of protein.

Blood carbohydrates are represented mainly by glucose and the intermediate products of its conversions. The glucose content of the blood varies in man from 80 to 100 mg percent. The blood also contains small amounts of glycogen and fructose and a significant amount of glucosamine. The products of the digestion of carbohydrates and proteins—glucose, fructose, other monosaccharides, amino acids, and low-molecular-weight peptides, as well as salts and water—are absorbed directly into the blood flowing through the capillaries of the intestine and delivered to the liver. Some of the glucose is transported to the organs and tissues, where it breaks down, releasing energy. The rest is converted to glycogen in the liver. When the intake of carbohydrates with the food is insufficient, this glycogen breaks down to form glucose. These processes are regulated by the enzymes of carbohydrate metabolism, by the central nervous system, and by the endocrine glands.

The blood contains a complex mixture of lipids that consist of neutral fats and free fatty acids and the products of their decomposition, free and bound cholesterol, steroid hormones, and other substances. Neutral fats, glycerin, and fatty acids are absorbed from the mucosa of the intestine partially into the blood but mainly into the lymph. The amount of lipids in the blood is not constant and depends on the composition of the diet and the stage of digestion. The blood transports lipids in the form of various complexes; a large proportion of both the plasma lipids and the cholesterol in the blood is found in the form of lipo-proteins, bound with α- and β-globulins. Free fatty acids are transported in the form of complexes with albumins, which are soluble in water. Triglycerides form compounds with phosphatides and proteins. The blood transports a fatty emulsion to the depots of adipose tissue, where it is deposited in the form of reserve fat and can, when needed, again be transferred to the blood plasma (fats and the products of their decomposition are used for the energy requirements of the body). The principal organic components of the blood are shown in Table 1.

Mineral substances maintain the blood’s active reaction (pH) and the constancy of the blood’s osmotic pressure and influence the state of blood colloids and intracellular metabolism. Sodium (Na) and chlorine (Cl) are the principal plasma minerals, and potassium (K) is found predominantly in the erythrocytes. Sodium participates in water metabolism, holding water in the tissues by swelling colloidal matter. Chlorine, which penetrates readily from the plasma into the erythrocytes, participates in the maintenance of the blood’s acid-base equilibrium. Calcium (Ca), which is necessary for clotting, is found in the plasma, mostly as ions or bound to proteins. HCO3- ions and dissolved carbonic acid form the bicarbonate buffer system. HPO4- and H2PO4- ions form the phosphate buffer system. A number of other anions and cations, including those of microelements, are found in the blood.

In addition to the compounds that are transported by the blood to the various organs and tissues to be used for biosynthesis, energy, and other body needs, metabolic products are continuously entering the blood and being excreted by the kidneys with the urine (mainly urea and uric acid). The products of hemoglobin decomposition (mainly bilirubin) are excreted with the bile.


Chizhevskii, A. L. Strukturnyi analiz dvizhushcheisia krovi Moscow, 1959.
Korzhuev, P. A. Gemoglobin. Moscow, 1964.
Haurowitz, F. Khimiia i funktsiia belkov. Moscow, 1965. (Translated from English.)
Table 1. Most important organic components of human whole blood, plasma, and erythrocytes
ComponentsWhole bloodPlasmaErythrocytes
Water (%). . . . . . . . . .75-8590-9157-68
Dry residue (%). . . . . . . . . .15-259-1032-43
Hemoglobin (%). . . . . . . . . .13-1630-41
Total protein (%). . . . . . . . . .6.5-8.5
Fibrinogen (%). . . . . . . . . .0.2-0.4
Globulins (%). . . . . . . . . .2.0-3.0
Albumins (%). . . . . . . . . .4.0-5.0
Residual nitrogen (nitrogen of nonprotein compounds; mg % ). . . . . . . . . .25-3520-3030-40
Glutathione (mg %). . . . . . . . . .35-45traces75-120
Urea (mg %). . . . . . . . . .20-3020-3020-30
Uric acid (mg %). . . . . . . . . .3-44-52-3
Creatinine (mg %). . . . . . . . . .1-21-21-2
Creatine (mg %). . . . . . . . . .3-51-1.56-10
Amino-acid nitrogen (mg %). . . . . . . . . .6-84-68
Glucose (mg %). . . . . . . . . .80-10080-120
Glucosamine (mg %). . . . . . . . . .70-90
Total lipids (mg %). . . . . . . . . .400-720385-675410-780
Neutral fats (mg %). . . . . . . . . .85-235100-25011-150
Total cholesterol (mg %). . . . . . . . . .150-200150-250175
Indican (mg %). . . . . . . . . .0.03-0.1
Kinins (mg %). . . . . . . . . .1-20
Guanidine (mg %). . . . . . . . . .0.3-0.5
Phospholipids (mg %). . . . . . . . . .220-400
Lecithin (mg %). . . . . . . . . .about 200100-200350
Ketone bodies (mg %). . . . . . . . . .0.8-3.0
Acetoacetic acid (mg %). . . . . . . . . .0.5-2.0
Acetone (mg %). . . . . . . . . .0.2-0.3
Lactic acid (mg %). . . . . . . . . .10-20
Pyruvic acid (mg %). . . . . . . . . .0.8-1.2
Citric acid (mg %). . . . . . . . . .2.0-3.0
Ketoglutaric acid (mg %). . . . . . . . . .0.8
Succinic acid (mg %). . . . . . . . . .0.5
Bilirubin (mg %). . . . . . . . . .0.25-1.5
Choline (mg %). . . . . . . . . .18-30
Rapoport, S. M. Meditsinskaia khimiia. Moscow, 1966. (Translated from German.)
Prosser, L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.)
Vvedenie v klinicheskuiu biokhimiiu. Edited by I. I. Ivanov. Leningrad, 1969.
Kassirskii, I. A., and G. A. Alekseev. Klinicheskaia gematologiia, 4th ed. Moscow, 1970.
Semenov, N. V. Biokhimicheskie komponenty i konstanty zhidkikh sred i tkanei cheloveka. Moscow, 1971.
Biochimie médicale, 6th ed., fasc. 3. Paris, 1961.
The Encyclopedia of Biochemistry. Edited by R. J. Williams and E. M. Lansford. New York, 1967.
Brewer, G. J., and J. W. Eaton. “Erythrocyte Metabolism.” Science, 1971, vol. 171, p. 1205.
The Red Cell: Metabolism and Function. Edited by G. J. Brewer. New York-London, 1970.
Pathology. The blood reflects, to a degree, shifts in the functioning of certain organs and systems and pathological processes. With metabolic disturbances and diseases of the endocrine glands, kidneys, liver, and certain other organs, there are observable chemical changes in the blood’s composition, such as increased protein content (hyperproteinemia), decreased protein content (hypoproteinemia), increased nonprotein nitrogen (azotemia or, more correctly, hyperazotemia), increased plasma lecithin (hyperlecithinemia), and increased plasma sugar (hyperglycemia). One of the most characteristic indexes of pathology is the blood’s hemoglobin content, which may fall with anemias and a number of other diseases. A change in the color index of the blood (the degree of coloration of the erythrocytes, which depends on their hemoglobin content) toward increased color (hyperchromatism) or decreased color (hypochromatism) is a symptom of some anemias. An increase in the hemoglobin content of the blood (polyglobulia) is observed with an increase in the number of erythrocytes (polycythemias). In congenital anomalies and diseases of the hematopoietic apparatus (hemoglobinoses, or hemoglobinopathies), anomalous hemoglobins, which differ from normal hemoglobins in structure and physico-chemical properties (solubility, resistance to denaturation), appear in the blood. Physiological increase in the number of erythrocytes (erythrocytosis) may occur as a compensatory phenomenon in hypoxia, or oxygen starvation of the tissues (for example, during ascents to high altitudes). Decrease in the number of erythrocytes (oligocythemia, erythropenia) is found with blood loss, anemias, and chronic wasting diseases.
When erythrocytes regenerate after blood loss or in the presence of intensive erythrocytic decomposition (hemolysis), altered erythrocytes and reticulocytes (erythrocytes with a granuloreticulate substance) appear in the peripheral blood. A sharp intensification of new growth of erythrocytes leads to the appearance of young forms: normoblasts, erythroblasts, and, in severe cases, megaloblasts.
The number of white cells in the blood (leukocytes) may also change. Cases of increased leukocytes, called leukocytoses, appear with certain physiological conditions and in various pathological states. Decreased leukocyte content is called leukopenia. It is seen mainly with a suppression of hematopoiesis in the bone marrow. Change in the amount of various types of leukocytes in the blood plays an important role in the diagnosis and prognosis of disease.
The thrombocyte content of the blood increases (thrombocytosis) after bleeding, in certain diseases of the blood system, such as myeloleukosis, polycythemia, and hemorrhagic thrombocythemia, and with certain tumor diseases. A decrease in the number of thrombocytes (thrombocytopenia) occurs under the influence of radiation and chemical factors, with immunoaggressive diseases, and in certain diseases of the blood system. A low thrombocyte count is manifested in the form of thrombocytopenic purpura, or Werlhof’s disease. The normal course of blood coagulation, in which the thrombocytes participate (along with other factors), depends on the balance between the clotting and anticlotting systems of the blood. Disruption of this balance may produce increased susceptibility to bleeding, which is observed in hemophilia, hemorrhagic diatheses, disturbances of vitaminK absorption (obstructive jaundices), and thromboembolic disease (increased thrombogenesis).
In a number of pathological states there is a change in blood volume. An increase in blood volume (hypervolemia) may occur without change in the proportion between the volumes of plasma and erythrocytes, or it may occur chiefly through an increase in the cell mass (true plethora, or polycythemic hypervolemia). A decrease in blood volume (hypovolemia) occurs as a result of loss of plasma (with intractable vomiting, diarrhea, or overheating) or loss of erythrocytic mass (as a result of hemorrhage).
Blood changes may be reactive—that is, they may arise as a responsive physiological reaction to stresses: blood loss, infection (bacterial, viral, or parasitic), or the entry into the body of toxic substances or allergens of external or internal origin. Pathological (nonreactive) changes in the blood arise in connection with diseases of the blood system or of the hematopoietic system. The etiologies of a number of these diseases (in particular, of leukemias) remain unelucidated.
In anthropology. The investigation of many of the hereditary features of the blood has great significance for anthropology. These features reveal genetic polymorphism (hereditary diversity) in the majority of peoples of the world and clearly expressed ethnic variations in the frequency of the genes determining them. The most thoroughly studied hereditary aspects of the blood are the variations of the erythrocytic blood groups of various systems (ABO, MNS5, Rh, or rhesus factor), anomalous hemoglobins (hemoglobinopathies), serum proteins (haptoglobins, transferrins, immunoglobulins), and certain blood enzymes.
Complex analysis of these blood factors makes it possible to distinguish several large population groups that do not coincide entirely with the large races but are found to be in definite correlation with them. Serological differences are being traced between the Caucasoid, Negroid, Australoid, and Mongoloid (including American Indian) populations.
Various serological complexes that are characteristic of certain populations arise and change in the course of time as a result of mutations, prolonged isolation, and racial mixing. However, the blood is qualitatively equivalent in all peoples and races, and no blood group has advantages over any other.
Many serological features are also being studied from the point of view of physiological anthropology, including such widely varying indexes as the levels in the blood of proteins, lipids (particularly cholesterol), carbohydrates, and enzymes. The quantitative content of these components, unlike blood groups, is closely associated with living conditions.
Fossil bone material is also being investigated in order to elucidate group serological characteristics and the connections between different groups of prehistoric and modern man and animals. The blood groups of monkeys are being studied for this purpose. In addition, the genetically determined blood factors in primates are being compared in terms of evolution. These investigations have made substantial contributions to primate taxonomy.


Cheboksarov, N. N., and I. A. Cheboksarova. Narody, rosy, kul’tury. Moscow, 1971.
Biologiia cheloveka. Moscow, 1968. (Translated from English.)



It is the life-giving, vital part of our physiology and it may symbolize our strengths and weaknesses and our physical and mental health. If you are currently experiencing a very difficult time in your life, you may have dreams with bloody and frightening images. Don’t worry: you may be venting your fears! Some believe that when you see blood in your dream, the distressing situation in your life, which is at the root of the dream, has come to an end, and the worst is over. Consider the details and the relationships between all of the symbols in your dream before making an interpretation.
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