https://www.britannica.com/science/blood-biochemistry
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Blood, fluid that transports oxygen and nutrients to the cells and carries away carbon dioxide and other waste products. Technically, blood is a transport liquid pumped by the heart (or an equivalent structure) to all parts of the body, after which it is returned to the heart to repeat the process. Blood is both a tissue and a fluid. It is a tissue because it is a collection of similar specialized cells that serve particular functions. These cells are suspended in a liquid matrix (plasma), which makes the blood a fluid. If blood flow ceases, death will occur within minutes because of the effects of an unfavourable environment on highly susceptible cells.
The constancy of the composition of the blood is made possible by the circulation, which conveys blood through the organs that regulate the concentrations of its components. In the lungs, blood acquires oxygen and releases carbon dioxide transported from the tissues. The kidneys remove excess water and dissolved waste products. Nutrient substances derived from food reach the bloodstream after absorption by the gastrointestinal tract. Glands of the endocrine system release their secretions into the blood, which transports these hormones to the tissues in which they exert their effects. Many substances are recycled through the blood; for example, iron released during the destruction of old red cells is conveyed by the plasma to sites of new red cell production where it is reused. Each of the numerous components of the blood is kept within appropriate concentration limits by an effective regulatory mechanism. In many instances, feedback control systems are operative; thus, a declining level of blood sugar (glucose) leads to accelerated release of glucose into the blood so that a potentially hazardous depletion of glucose does not occur.
The cellular composition of blood varies from group to group in the animal kingdom. Most invertebrates have various large blood cells capable of amoeboid movement. Some of these aid in transporting substances; other are capable of surrounding and digesting foreign particles or debris (phagocytosis). Compared with vertebrate blood, however, that of the invertebrates has relatively few cells. Among the vertebrates, there are several classes of amoeboid cells (white blood cells, or leukocytes) and cells that help stop bleeding (platelets, or thrombocytes).
Oxygen requirements have played a major role in determining both the composition of blood and the architecture of the circulatory system. In some simple animals, including small worms and mollusks, transported oxygen is merely dissolved in the plasma. Larger and more-complex animals, which have greater oxygen needs, have pigments capable of transporting relatively large amounts of oxygen. The red pigment hemoglobin, which contains iron, is found in all vertebrates and in some invertebrates. In almost all vertebrates, including humans, hemoglobin is contained exclusively within the red cells (erythrocytes). Red cells vary markedly in size among mammals; those of the goat are much smaller than those of humans, but the goat compensates by having many more red cells per unit volume of blood. The concentration of hemoglobin inside the red cell varies little between species. Hemocyanin, a copper-containing protein chemically unlike hemoglobin, is found in some crustaceans. Hemocyanin is blue in colour when oxygenated and colourless when oxygen is removed. Some annelids have the iron-containing green pigment chlorocruorin, others the iron-containing red pigment hemerythrin. In many invertebrates the respiratory pigments are carried in solution in the plasma, but in higher animals, including all vertebrates, the pigments are enclosed in cells; if the pigments were freely in solution, the pigment concentrations required would cause the blood to be so viscous as to impede circulation.
Blood components
In humans, blood is an opaque red fluid, freely flowing but denser and more viscous than water. The characteristic colour is imparted by hemoglobin, a unique iron-containing protein. Hemoglobin brightens in colour when saturated with oxygen (oxyhemoglobin) and darkens when oxygen is removed (deoxyhemoglobin). For this reason, the partially deoxygenated blood from a vein is darker than oxygenated blood from an artery. The red blood cells (erythrocytes) constitute about 45 percent of the volume of the blood, and the remaining cells (white blood cells, or leukocytes, and platelets, or thrombocytes) less than 1 percent. The fluid portion, plasma, is a clear, slightly sticky, yellowish liquid. After a fatty meal, plasma transiently appears turbid. Within the body the blood is permanently fluid, and turbulent flow assures that cells and plasma are fairly homogeneously mixed.
The total amount of blood in humans varies with age, sex, weight, body type, and other factors, but a rough average figure for adults is about 60 millilitres per kilogram of body weight. An average young male has a plasma volume of about 35 millilitres and a red cell volume of about 30 millilitres per kilogram of body weight. There is little variation in the blood volume of a healthy person over long periods, although each component of the blood is in a continuous state of flux. In particular, water rapidly moves in and out of the bloodstream, achieving a balance with the extravascular fluids (those outside the blood vessels) within minutes. The normal volume of blood provides such an adequate reserve that appreciable blood loss is well tolerated. Withdrawal of 500 millilitres (about a pint) of blood from normal blood donors is a harmless procedure. Blood volume is rapidly replaced after blood loss; within hours, plasma volume is restored by movement of extravascular fluid into the circulation. Replacement of red cells is completed within several weeks. The vast area of capillary membrane, through which water passes freely, would permit instantaneous loss of the plasma from the circulation were it not for the plasma proteins—in particular, serum albumin. Capillary membranes are impermeable to serum albumin, the smallest in weight and highest in concentration of the plasma proteins. The osmotic effect of serum albumin retains fluid within the circulation, opposing the hydrostatic forces that tend to drive the fluid outward into the tissues.
Functions of blood
Broadly conceived, the function of the blood is to maintain the constancy of the internal environment. The circulating blood makes possible adaptability to changing conditions of life—the endurance of wide variations of climate and atmospheric pressure; the capacity to alter the amount of physical activity; the tolerance of changing diet and fluid intake; the resistance to physical injury, chemical poisons, and infectious agents. The blood has an exceedingly complex structure, and many components participate in its functional activities. Some of the regulatory mechanisms with which the blood is involved include sensors that detect alterations in temperature, in pH, in oxygen tension, and in concentrations of the constituents of the blood. Effects of these stimuli are in some instances mediated via the nervous system or by the release of hormones (chemical mediators). Some of the major functions of the blood are outlined in the paragraphs that follow.
Respiration
In terms of immediate urgency, the respiratory function of the blood is vital. A continuous supply of oxygen is required by living cells—in particular those of the brain, since deprivation is followed in minutes by unconsciousness and death. A normal male at rest uses about 250 millilitres of oxygen per minute, a requirement increased manyfold during vigorous exertion. All of this oxygen is transported by the blood, most of it bound to the hemoglobin of the red cells. The minute blood vessels of the lungs bring the blood into close apposition with the pulmonary air spaces (alveoli), where the pressure of oxygen is relatively high. Oxygen diffuses through the plasma and into the red cell, combining with hemoglobin, which is about 95 percent saturated with oxygen on leaving the lungs. One gram of hemoglobin can bind 1.35 millilitres of oxygen, and about 50 times as much oxygen is combined with hemoglobin as is dissolved in the plasma. In tissues where the oxygen tension is relatively low, hemoglobin releases the bound oxygen.
The two main regulators of oxygen uptake and delivery are the pH (a measure of the acidity or basicity) of tissues and the content of 2,3-diphosphoglycerate (2,3-DPG) in red cells. The pH of blood is kept relatively constant at the slightly alkaline level of about 7.4 (pH less than 7 indicates acidity, more than 7 alkalinity). The effect of pH on the ability of hemoglobin to bind oxygen is called the Bohr effect: when pH is low, hemoglobin binds oxygen less strongly, and when pH is high (as in the lungs), hemoglobin binds more tightly to oxygen. When the availability of oxygen to tissues is reduced, the red cell responds by synthesizing more 2,3-DPG, a process that occurs over a period of hours to days. By contrast, tissue pH mediates minute-to-minute changes in oxygen handling.
Carbon dioxide, a waste product of cellular metabolism, is found in relatively high concentration in the tissues. It diffuses into the blood and is carried to the lungs to be eliminated with the expired air. Carbon dioxide is much more soluble than oxygen and readily diffuses into red cells. It reacts with water to form carbonic acid, a weak acid that at the alkaline pH of the blood appears principally as bicarbonate.
The tension of carbon dioxide in the arterial blood is regulated with extraordinary precision through a sensing mechanism in the brain that controls the respiratory movements. Carbon dioxide is an acidic substance, and an increase in its concentration tends to lower the pH of the blood (i.e., becoming more acidic). This may be averted by the stimulus that causes increased depth and rate of breathing, a response that accelerates the loss of carbon dioxide. It is the tension of carbon dioxide, and not of oxygen, in the arterial blood that normally controls breathing. Inability to hold one’s breath for more than a minute or so is the result of the rising tension of carbon dioxide, which produces the irresistible stimulus to breathe. Respiratory movements that ventilate the lungs sufficiently to maintain a normal tension of carbon dioxide are, under normal conditions, adequate to keep the blood fully oxygenated. Control of respiration is effective, therefore, in regulating the uptake of oxygen and disposal of carbon dioxide and in maintaining the constancy of blood pH.
Nutrition
Each substance required for the nutrition of every cell in the body is transported by the blood: the precursors of carbohydrates, proteins, and fats; minerals and salts; vitamins and other accessory food factors. These substances must all pass through the plasma on the way to the tissues in which they are used. The materials may enter the bloodstream from the gastrointestinal tract, or they may be released from stores within the body or become available from the breakdown of tissue.
The concentrations of many plasma constituents, including blood sugar (glucose) and calcium, are carefully regulated, and deviations from the normal may have adverse effects. One of the regulators of glucose is insulin, a hormone released into the blood from glandular cells in the pancreas. Ingestion of carbohydrates is followed by increased production of insulin, which tends to keep the blood glucose level from rising excessively as the carbohydrates are broken down into their constituent sugar molecules. But an excess of insulin may severely reduce the level of glucose in the blood, causing a reaction that, if sufficiently severe, may include coma and even death. Glucose is transported in simple solution, but some substances require specific binding proteins (with which the substances form temporary unions) to convey them through the plasma. Iron and copper, essential minerals, have special and necessary transport proteins. Nutrient substances may be taken up selectively by the tissues that require them. Growing bones use large amounts of calcium, and bone marrow removes iron from plasma for hemoglobin synthesis.
Excretion
The blood carries the waste products of cellular metabolism to the excretory organs. The removal of carbon dioxide via the lungs has been described above. Water produced by the oxidation of foods or available from other sources in excess of needs is excreted by the kidneys as the solvent of the urine. Water derived from the blood also is lost from the body by evaporation from the skin and lungs and in small amounts from the gastrointestinal tract. The water content of the blood and of the body as a whole remains within a narrow range because of effective regulatory mechanisms, hormonal and other, that determine the urinary volume. The concentrations of physiologically important ions of the plasma, notably sodium, potassium, and chloride, are precisely controlled by their retention or selective removal as blood flows through the kidneys. Of special significance is the renal (kidney) control of acidity of the urine, a major factor in the maintenance of the normal pH of the blood. Urea, creatinine, and uric acid are nitrogen-containing products of metabolism that are transported by the blood and rapidly eliminated by the kidneys. The kidneys clear the blood of many other substances, including numerous drugs and chemicals that are taken into the body. In performing their excretory function, the kidneys have a major responsibility for maintaining the constancy of the composition of the blood. The liver is in part an excretory organ. Bilirubin (bile pigment) produced by the destruction of hemoglobin is conveyed by the plasma to the liver and is excreted through the biliary ducts into the gastrointestinal tract. Other substances, including certain drugs, also are removed from the plasma by the liver.
Immunity
Cells of the blood and constituents of the plasma interact in complex ways to confer immunity to infectious agents, to resist or destroy invading organisms, to produce the inflammatory response, and to destroy and remove foreign materials and dead cells. The white blood cells (leukocytes) have a primary role in these reactions. Granulocytes and monocytes phagocytize (ingest) bacteria and other organisms, migrate to sites of infection or inflammation and to areas containing dead tissue, and participate in the enzymatic breakdown and removal of cellular debris. Lymphocytes are concerned with the development of immunity. Acquired resistance to specific microorganisms is in part attributable to antibodies, proteins that are formed in response to the entry into the body of a foreign substance (antigen). Antibodies that have been induced by microorganisms not only participate in eliminating the microbes but also prevent reinfection by the same organisms. Cells and antibodies may cooperate in the destruction of invading bacteria; the antibodies may attach to the organisms, thereby rendering them susceptible to phagocytosis. Involved in some of these reactions is complement, a group of protein components of plasma that participates in certain immunologic reactions. When certain classes of antibodies bind to microorganisms and other cells, they trigger the attachment of components of the complement system to the outer membrane of the target cell. As they assemble on the cell membrane, the complement components acquire enzymatic properties. The activated complement system is thus able to injure the cell by digesting (lysing) portions of the cell’s protective membrane.
Temperature regulation
Heat is produced in large amounts by physiological oxidative reactions, and the blood is essential for its distributing and disposing of this heat. The circulation assures relative uniformity of temperature throughout the body and also carries the warm blood to the surface, where heat is lost to the external environment. A heat-regulating centre in the hypothalamus of the brain functions much like a thermostat. It is sensitive to changes in temperature of the blood flowing through it and, in response to the changes, gives off nerve impulses that control the diameter of the blood vessels in the skin and thus determine blood flow and skin temperature. A rise in skin temperature increases heat loss from the body surface. Heat is continuously lost by evaporation of water from the lungs and skin, but this loss can be greatly increased when more water is made available from the sweat glands. The activity of the sweat glands is controlled by the nervous system under direction of the temperature-regulating centre. Constancy of body temperature is achieved by control of the rate of heat loss by these mechanisms.
Hemostasis
The blood is contained under pressure in a vascular system that includes vast areas of thin and delicate capillary membranes. Even the bumps and knocks of everyday life are sufficient to disrupt some of these fragile vessels, and serious injury can be much more damaging. Loss of blood would be a constant threat to survival if it were not for protective mechanisms to prevent and control bleeding. The platelets contribute to the resistance of capillaries, possibly because they actually fill chinks in vessel walls. In the absence of platelets, capillaries become more fragile, permitting spontaneous loss of blood and increasing the tendency to form bruises after minor injury. Platelets immediately aggregate at the site of injury of a blood vessel, tending to seal the aperture. A blood clot, forming in the vessel around the clump of adherent platelets, further occludes the bleeding point. The coagulation mechanism involves a series of chemical reactions in which specific proteins and other constituents of the blood, including the platelets, play a part. Plasma also is provided with a mechanism for dissolving clots after they have been formed.