..... Click the link for more information. ; heart heart, 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.
..... Click the link for more information. ). 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 hemoglobin (hē`məglō'bĭn)
..... Click the link for more information. , 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 anemia (ənē`mēə), condition in which the concentration of hemoglobin in the circulating blood is below normal.
..... Click the link for more information. 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 Bone marrow transplantation is a technique that infuses healthy bone marrow into a patient whose bone marrow is defective. The transplant can be autologous, consisting of bone marrow removed from the patient, treated, and then reinserted, or it can be allogeneic, consisting of
..... Click the link for more information. . In the fetus, red blood cells are produced in the spleen spleen, 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
..... Click the link for more information. . 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 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.
..... Click the link for more information. . There are four major blood groups, whose compatibility or incompatibility is an important consideration in successful blood transfusion 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.
..... Click the link for more information. .
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 immunity, 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.
..... Click the link for more information. ). 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 leukemia (l
..... Click the link for more information. . 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 AIDS 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.
..... Click the link for more information. . 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 tuberculosis (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,
..... Click the link for more information. , malaria malaria, 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.
..... Click the link for more information. , and typhoid typhoid 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.
..... Click the link for more information. ) 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 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.
..... Click the link for more information. . 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 albumin (ălby
`mən) [Lat.
..... Click the link for more information. , globulins globulin, 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.
..... Click the link for more information. , and fibrinogen), and metabolic wastes (such as urea urea (y
..... Click the link for more information. ) 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.
Bibliography
See D. Starr, Blood (1998).
blood
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. See also ABO blood-group system; blood analysis; blood bank; blood pressure; blood transfusion; blood typing; Rh blood-group system.
blood
blood [bləd]
Blood
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.
)
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
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.
Coagulation
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
How to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit webmaster's page for free fun content.
| ?Page tools | ||
|---|---|---|
|