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process by which an organism exchanges gases with its environment. The term now refers to the overall process by which oxygen is abstracted from air and is transported to the cells for the oxidation of organic molecules while carbon dioxide (CO2) and water, the products of oxidation, are returned to the environment. In single-celled organisms, gas exchange occurs directly between cell and environment, i.e., at the cell membrane. In plants, gas exchange with the environment occurs in special organs, the stomates, found mostly in the leaves (see leafleaf,
chief food-manufacturing organ of a plant, a lateral outgrowth of the growing point of stem. The typical leaf consists of a stalk (the petiole) and a blade—the thin, flat, expanded portion (needlelike in most conifers) that is normally green in color because of the
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; transpirationtranspiration,
in botany, the loss of water by evaporation in terrestrial plants. Some evaporation occurs directly through the exposed walls of surface cells, but the greatest amount takes place through the stomates, or intercellular spaces (see leaf).
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Organisms that utilize respiration to obtain energy are aerobic, or oxygen-dependent. Some organisms can live in the absence of oxygen and obtain energy from fuel molecules solely by fermentationfermentation,
process by which the living cell is able to obtain energy through the breakdown of glucose and other simple sugar molecules without requiring oxygen. Fermentation is achieved by somewhat different chemical sequences in different species of organisms.
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 or glycolysisglycolysis
, term given to the metabolic pathway utilized by most microorganisms (yeast and bacteria) and by all "higher" animals (including humans) for the degradation of glucose. Glycolysis means, literally, the dissolution of sugar.
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; these anaerobic processes are much less efficient, since the fuel molecules are merely converted to end products such as lactic acid and ethanol, with relatively little energy-rich ATP produced during these conversions.

For individual respiratory organs, see separate articles.

Animal Respiration

In complex animals, where the cells of internal organs are distant from the external environment, respiratory systems facilitate the passage of gases to and from internal tissues. In such systems, when there is a difference in pressure of a particular gas on opposite sides of a membrane, the gas diffuses from the side of greater pressure to the side of lesser pressure, and each gas is transported independently of other gases. For example, in tissues where carbon dioxide concentration is high and oxygen concentration is low as a result of active metabolism, oxygen diffuses into the tissue and carbon dioxide diffuses out.

In lower animals, gas diffusion takes place through a moist surface membrane, as in flatworms; through the thin body wall, as in earthworms; through air ducts, or tracheae, as in insects; or through specialized tracheal gillsgills,
external respiratory organs of most aquatic animals. In fishes the gills are located in gill chambers at the rear of the mouth (pharynx). Water is taken in through the mouth, is forced through openings called gill slits, and then passes through the gill clefts, spaces
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, as in aquatic insect larvae. In the gills of fish the blood vessels are exposed directly to the external (aquatic) environment. Oxygen–carbon dioxide exchange occurs between the surrounding water and the blood within the vessels; the blood carries gases to and from tissues.

In other vertebrates, including humans, gas exchange takes place in the lungslungs,
elastic organs used for breathing in vertebrate animals, excluding most fish, which use gills, and a few amphibian species that respire through the skin. The word is sometimes applied to the respiratory apparatus of lower animals.
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. Breathing is the mechanical procedure in which air reaches the lungs. During inhalation muscular action lowers the diaphragm and raises the ribs; atmospheric pressure forces air into the enlarged chest cavity. In exhalation the muscles relax and the air is expelled. This combined rhythmic action takes place about 12–16 times per minute when the body is at rest. The rate of breathing is controlled mainly by a respiratory center in the brain stem that responds to changes in the level of hydrogen ion and carbon dioxide in the blood, as well as to other factors such as stress, temperature changes, and motor activities. Some residual air always remains in the lungs, but with each breath an additional quantity of fresh air, called tidal air, is inhaled. Artificial respirationartificial respiration,
any measure that causes air to flow in and out of a person's lungs when natural breathing is inadequate or ceases, as in respiratory paralysis, drowning, electric shock, choking, gas or smoke inhalation, or poisoning.
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 is used for respiratory failure.

In higher vertebrates, oxygen-poor, carbon dioxide–rich blood from the right side of the heart is pumped into the lungs and flows through the net of capillaries surrounding the alveoli, the cup-shaped air sacs of the lungs; oxygen diffuses across the capillary membranes into the blood, and carbon dioxide diffuses in the opposite direction. The oxygen combines with the protein hemoglobin in red blood cells as the blood returns to the left side of the heart, is pumped throughout the body, and is released into tissue cells (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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). Carbon dioxide passes in the opposite direction, from the cells of the tissues to the red blood cells. In the blood, carbon dioxide exists in three forms: as bicarbonate ion, in which form it serves as a buffer, keeping blood acidity fairly constant; combined with hemoglobin; and as the dissolved free gas. Of these, only free carbon dioxide gas is available for diffusion from the blood into the lungs.

Biochemical Respiration

In biochemistry, respiration refers to the series of biochemical oxidations in which organic molecules are converted to carbon dioxide and water while the chemical energy thus obtained is trapped in a form useful to the cell. Biochemical respiration occurs in both plant and animal cells. Carbohydrates, amino acids, and fatty acids—the organic fuel molecules of the cell—can be converted to acetyl CoA, a derivative of acetic acid and coenzymecoenzyme
, any one of a group of relatively small organic molecules required for the catalytic function of certain enzymes. A coenzyme may either be attached by covalent bonds to a particular enzyme or exist freely in solution, but in either case it participates intimately in
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Acetyl CoA then enters a series of reactions in the mitochondria, organelles in the cell's cytoplasm. The series of reactions, known as the Krebs cycleKrebs cycle,
series of chemical reactions carried out in the living cell; in most higher animals, including humans, it is essential for the oxidative metabolism of glucose and other simple sugars.
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, converts the acetic acid portion of acetyl CoA to carbon dioxide, protons, and hydride ions, the latter usually as part of the coenzyme NADH. This molecule is oxidized back to NAD when it donates the hydride ion to the series of enzymes known as the electron transport chain. In a process called oxidative phosphorylationphosphorylation,
chemical process in which a phosphate group is added to an organic molecule. In living cells phosphorylation is associated with respiration, which takes place in the cell's mitochondria, and photosynthesis, which takes place in the chloroplasts.
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, each electron transport enzyme is in turn reduced (receives the hydride ion), then oxidized (donates a hydride ion to the next enzyme in the series), and the chemical energy liberated in this series of reactions is coupled to the synthesis of adenosine triphosphateadenosine triphosphate
(ATP) , organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of
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 (ATP) from adenosine diphosphate (ADP) and phosphoric acid.

ATP, the cell's form of energy storage and supply, furnishes the chemical energy needed for muscle contraction, protein synthesis, active transport of substances across membranes, and electrical impulses. At the end of the electron transport chain, a hydride ion is donated to an atom of oxygen; this pair, together with a proton from the surrounding solution, forms a molecule of water. Thus, in the overall process of cellular respiration, the fuel molecules are converted to carbon dioxide and water while the chemical energy gained is trapped in a useful form as ATP.


The various processes associated with the biochemical transformation of the energy available in the organic substrates derived from foodstuffs, to energy usable for synthetic and transport processes, external work, and, eventually, heat. This transformation, generally identified as metabolism, most commonly requires the presence of oxygen and involves the complete oxidation of organic substrates to carbon dioxide and water (aerobic respiration). If the oxidation is incomplete, resulting in organic compounds as end products, oxygen is typically not involved, and the process is then identified as anaerobic respiration. See Metabolism

The term “external respiration” is more appropriate for describing the exchange of O2 and CO2 between the organism and its environment. In most multicellular organisms, and nearly all vertebrates (with the exception of a few salamanders lacking both lungs and gills), external respiration takes place in specialized structures termed respiratory organs, such as gills and lungs. See Lung, Respiratory system

The ultimate physical process causing movement of gases across living tissues is simple passive diffusion. Respiratory gas exchange also depends on two convective fluid movements. The first is the bulk transport of the external medium, air or water, to and across the external respiratory exchange surfaces. The second is the transport of coelomic fluid or blood across the internal surfaces of the respiratory organ. These two convective transports are referred to as ventilation and circulation (or perfusion). They are active processes, powered by ciliary or muscular pumps.

In all vertebrates and many invertebrates, the circulating internal medium (coelomic fluid, hemolymph, or blood) contains a respiratory pigment, for example, hemocyanin or hemoglobin, which binds reversibly with O2, CO2, and protons. Respiratory pigments augment respiratory gas exchange, both by increasing the capacity for bulk transport of the gases, and by influencing gas partial pressure (concentration) gradients across tissue exchange surfaces. See Blood, Hemoglobin, Respiratory pigments (invertebrate)

The physiological adjustment of organisms to variations in their need for aerobic energy production involves regulated changes in the exchange and transport of respiratory gases. The adjustments are effected by rapid alterations in the ventilatory and circulatory pumps and by longer-term modifications in the respiratory properties of blood.



the totality of processes that ensure the entrance of oxygen into the organism and the discharge from it of carbon dioxide gas (external respiration); also, the use of oxygen by the cells and tissues to oxidize organic substances and release the energy contained in them, which is necessary for life processes (tissue respiration, cellular respiration). Anaerobic means of releasing energy are characteristic only of a small group of organisms—the so-called anaerobes. In the course of evolution respiration became the principal means of releasing energy in the overwhelming majority of organisms, and anaerobic reactions were maintained primarily as intermediate stages of metabolism.

Animals and humans. In protozoans, sponges, coelenterates and a few other organisms, oxygen (O2) diffuses directly through the surface of the body. More complex, larger animals have special respiratory organs and a circulatory system that contains a fluid-—bloodor hemolymph, with substances capable of binding and transporting O2 and carbon dioxide (CO2). In insects, O2 enters the tissues from a system of air-carrying tubules—tracheae. In aquatic animals, which use O2 dissolved in water, the respiratory organs are gills, which are equipped with a rich network of blood vessels. Oxygen dissolved in water diffuses into the blood that circulates in the blood vessels of the gill slits. In many fish, intestinal respiration plays an important role. Air is swallowed and O2 enters the blood vessels of the intestine. The swim bladder also plays some role in fish respiration. In many aquatic animals exchange of gases (mainly CO2) also occurs through the skin.

In land animals external respiration is ensured primarily by the lungs. Amphibians and many other animals also respire through the skin. Birds have air sacs that are connected with the lungs, change in volume during flying, and facilitate respiration during flight. In amphibians and reptiles the air is forced into the lungs by movements of the muscles of the floor of the mouth. In birds, mammals, and humans external respiration is ensured by the rhythmic functioning of the respiratory muscles (chiefly the diaphragm and the intercostal muscles), which are coordinated by the nervous system. When these muscles contract, the volume of the thorax is increased, and the lungs (located in the thorax) expand. This causes a difference between the atmospheric pressure and the intrapulmonary pressure, and air enters the lungs (inspiration). Expiration may be passive—that is, a result of the collapse of the thorax and subsequently, of the lungs, which had been expanded during inspiration. Active expiration is caused by the contraction of certain groups of muscles. The quantity of air entering the lungs in one inspiration is called the respiratory volume.

During respiration the respiratory musculature overcomes the elastic resistance that is due to the resilience of the thorax, the draw of the lungs, and the surface tension of the alveoli. The latter, however, is significantly decreased by a substance that is active on the alveolar surface and that is secreted by the cells of the alveolar epithelium. Because of this substance the alveoli do not collapse upon expiration, and they expand easily upon inspiration. The greater the elastic resistance, the more difficult is the expansion of the thorax and lungs. During deep respiration the energy that the respiratory musculature must expend to overcome the resistance is greatly increased.

Nonelastic resistance to respiration is caused mainly by friction as the air moves through the nasal passages, throat, trachea, and bronchi. It is a function of the quality of the air current and its velocity during respiration. During tranquil breathing the current is similar to a laminar (linear) flow in the straight sections of the air passages and similar to a turbulent (whirling) flow in places of branching or narrowing. With an increase in the velocity of the current (during forced respiration), turbulence increases. A greater pressure difference is required for passage of the air, and consequently, there is an increase in work for the respiratory muscles. Unequal distribution of resistance to air movement along the respiratory passages leads to unequal entry of air into various groups of pulmonary alveoli. This difference in ventilation is especially significant in lung diseases.

The amount of air ventilating the lungs in one minute is called the minute respiration volume (MRV). The MRV is equal to the product of the respiratory volume and the frequency of respiration (the number of respiratory movements per minute—in humans, approximately 15-18). In an adult human at rest the MRV is 5-8 liters per minute. The part of the MRV (approximately 70 percent) that participates in the exchange of gases between the inspired and the alveolar air is the volume of alveolar ventilation. The rest of the MRV is used to flush the dead space of the respiratory tract, which, at the beginning of expiration, retains some of the air from outside with which the space had been filled at the end of the preceding inspiration. (The volume of dead space is approximately 160 milliliters [ml].) Ventilation of the alveoli ensures the constant composition of alveolar air. The partial pressures of O2 (pO2) and CO2 (pCO2) in alveolar air fluctuate within very narrow limits and total approximately 13 kilonewtons (kN) per sq m (100 mm mercury [Hg]) for O2 and approximately 5.4 kN/m2 (40 mm Hg) for CO2.

Exchange of gases between alveolar air and the venous blood that enters the capillaries of the lungs occurs through the alveolar capillary membrane, whose total surface is very large (in humans, approximately 90 sq m). Diffusion of O2 into the blood is ensured by the difference in the partial pressures of O2 in the alveolar air and in the venous blood (8-9 kN/m2, or 60-70 mm Hg). Bound carbon dioxide (bicarbonates, carbonates, and carbohemoglobin) that has been transported by the blood from the tissues is released in the capillaries of the lung with the participation of the enzyme carbonic anhydrase and diffuses from the blood into the alveoli. The difference in pCO2 between the venous blood and the alveolar air is approximately 7 mm Hg. The capacity of the alveolar wall to pass O2 and CO2—the so-called pulmonary diffusing capacity—is very great. At rest it is approximately 30 ml O2 per 1 mm of difference in pCO2 between alveolar air and blood in one minute (for CO2 the diffusing capacity is many times greater). Therefore, the partial pressure of the gases in the arterial blood leaving the lungs is able to approach the pressure of the gases in the alveolar air. The passage of O2 into the tissues and the removal from them of CO2 also occur by means of diffusion, since the pO2 in the tissue fluid is 2.7-5.4 kN/m2 (20-40 mm Hg), while in the cells it is still lower. In the cells the pCO2 may reach 60 mm mercury.

The requirement of cells and tissues for O2 and their formation of CO2, which is the essence of tissue, or cellular, respiration, is one of the principal forms of dissimilation and is, in principle, accomplished in the same way in plants and animals. A high O2 requirement is characteristic of tissues of the kidneys, the cortex of the cerebral hemispheres, and the heart. As a consequence of the oxidation-reduction reactions of tissue respiration, energy is released that is expendable for all phenomena of life. Oxidation-reduction processes occur in the mitochondria and arise from the dehydrogenation of the substrates of respiration—carbohydrates and the products of their decomposition, fats and fatty acids, and amino acids and the products of their deamination. The substrates of respiration absorb O2 and serve as a source of CO2. (The ratio between CO2 and O2 is called the respiratory quotient.) The energy released during the oxidation of organic substances is not immediately used by the tissues. Approximately 70 percent of it is expended on the formation of ATP, one of the adenosine phosphoric acids, whose subsequent enzymic decomposition supplies the energy requirements of the tissues, organs, and the body as a whole. Thus, from a biochemical point of view, respiration is the conversion of the energy of carbohydrates and other substances into the energy of macroergic phosphate bonds.

The constancy of the alveolar and arterial pO2 and pCO2 can be maintained only on condition that alveolar ventilation corresponds to the body’s requirement for O2 and the formation of CO2—that is, to the level of metabolism. This condition is met by means of the perfect regulatory mechanisms of respiration. Reflexes control the frequency and depth of respiration. Thus, an increase in the pCO2 and a decrease in the pO2 in the alveolar air and in the arterial blood excites the chemoreceptors of the carotid sinus and cardiac aorta, resulting in the stimulation of the respiratory center and an increase in the MRV. According to classic concepts, an increase in the pCO2 in the arterial blood that bathes the respiratory center excites the respiratory center and produces an increase in the MRV. Thus, regulation of respiration according to the changes in the arterial pO2 andpCO2 is effected on the feedback principle, ensuring an optimal MRV. However, in a number of cases (for example, during muscular work) the MRV increases until the onset of metabolic shifts, which lead to changes in the gas composition of the blood. Increased ventilation is caused by signals entering the respiratory center from receptors of the motor apparatus and the motor zone of the cortex of the cerebral hemispheres, as well as by conditioned reflexes to various signals associated with habitual work and work conditions. Thus, control of respiration is effected by a complex, self-instructing system, according to the principle of regulation according to changes in the partial pressures of O2 and CO2 and according to signals that prevent possible deviations.

The succession of inspiration and expiration is ensured by a system of complementary mechanisms. During inspiration impulses from stretch receptors in the lungs travel along the fibers of the.vagus nerves to the respiratory center. When the lungs attain a certain volume, these impulses inhibit the cells of the respiratory center, whose excitation causes inspiration. If the nerve paths that ensure entry of impulses into the respiratory center are blocked, the rhythm of respiration is maintained by the automatism of the respiratory center. However, the rhythm is markedly different from the normal one. When there are disturbances of respiration and its regulatory mechanisms, the gas composition of the blood changes.

Methods of investigating respiration are varied. In the physiology of work and athletics and in clinical medicine, widely used techniques include recording the depth and frequency of respiratory movements, measurement of the gas composition of expired air and arterial blood, and measurement of pleural and alveolar pressure.


Sechenov, I. M. Izbrannye trudy. Moscow, 1935.
Holden, J., and J. Priestley. Dykhanie. Moscow-Leningrad, 1937. (Translated from English.)
Marshak, M. E. Reguliatsiia dykhaniia u cheloveka. Moscow, 1961.
Fiziologiia cheloveka. Moscow, 1966.
Comroe, J. H. Physiology of Respiration. Chicago, 1966.
Dejours, P. Respiration. Oxford, 1966.
Plants. Respiration is characteristic of all plant organs, tissues, and cells. The intensity of respiration may be judged by measuring either the quantity of CO2 eliminated by the tissue or the quantity of O2 absorbed. Young, rapidly growing plant organs and tissues have higher rates of respiration than older organs and tissues. The highest rate of respiration occurs in the reproductive organs. The leaves are second in rate of respiration, and the rate of respiration of stalks and roots is lower than that of the leaves. Plants that endure shade have a lower rate of respiration than those that require light. A higher rate of respiration is characteristic of high-altitude plants, which have adapted to a decreased partial pressure of O2 Fungi and bacteria have very high rates of respiration. With increases in temperature, the rate of respiration is roughly doubled or tripled for each 10°C (this phenomenon ceases at 45°-50°C). In the tissues of dormant plant organs (buds of deciduous trees and needles of conifers) respiration continues at sharply reduced rates, even during heavy frosts.
Respiration is stimulated by mechanical and chemical irritants (for example, wounds, certain toxins, and narcotics). During the development of the plant and its organs, respiration varies with lawlike regularity. Dry (dormant) seeds have a very low rate of respiration. With the swelling and subsequent sprouting of seeds, the rate of respiration increases hundreds and thousands of times. At the end of the plant’s period of active growth, the rate of respiration of the tissues decreases as a result of the aging of the protoplasm. During the ripening of seeds and fruits the rate of respiration decreases.
According to the theory of the Soviet biochemist, A. N. Bakh, the process of respiration (the oxidation of carbohydrates, fats, and proteins) occurs by means of the oxidation system of the cells in two stages. First, the oxygen in the air is activated by means of its addition to unsaturated compounds (oxygenases), which are capable of being spontaneously oxidized to form peroxides. Subsequently, the peroxides are activated, releasing atomic oxygen, which is capable of oxidizing organic substances that are not readily oxidized.
According to the theory of dehydrogenation of the Russian botanist V. I. Palladin, the most important link in respiration is the activation of the hydrogen of the substrate, which is accomplished by dehydrogenases. A necessary participant in the complex chain of respiratory processes is water, whose hydrogen is used in addition to the hydrogen in the substrate to reduce the self-oxidizing compounds—the so-called respiratory pigments. During respiration carbon dioxide is formed anaerobically—that is, without the participation of O2 from the air. Oxygen from the air is used to oxidize respiratory chromogens, which are converted into respiratory pigments.
The theory of plant respiration was further developed through the research of the Soviet botanist S. P. Kostychev, who asserted that the first stages of aerobic respiration are analogous to the respiratory processes that are characteristic of anaerobes. The transformations of the intermediate products formed in the early stages of aerobic respiration may proceed, according to Kostychev, with the participation of O2, which is characteristic of aerobes. In anaerobes, however, the transformation of intermediate products of respiration proceeds without the participation of molecular O2.
According to present-day concepts, the process of oxidation, which is the chemical basis of respiration, involves the loss of an electron by a substance. The capacity to take on or give up electrons is a function of the oxidation potential of the compound. Oxygen has the highest oxidation potential and therefore, the maximum capacity to take on electrons. However, the oxidation potential of O2 differs sharply from that of the respiratory substrate. For this reason, specific compounds play the role of intermediate carriers of electrons from the respiratory substrate to the oxygen. Alternately oxidized and reduced, the carriers make up the system of electron transfer. In taking on an electron from a less oxidized component, a carrier is reduced, and in giving up the electron to the next with a higher potential, the carrier is oxidized. Thus, an electron is transferred from one link in the respiratory chain to another. The final stage of respiration is the transfer of the electron to oxygen.
All these processes (activation of oxygen and hydrogen and electron transfer along the respiratory chain to oxygen) occur primarily in the mitochondria, as a result of the activity of a ramified system -of oxidation-reduction enzymes (cytochromes). Along the chain to oxygen, the electrons, which are mobilized primarily from molecules of organic substances, gradually release the energy contained in them, which is stored by the cells in the form of chemical compounds, chiefly ATP.
Because of the perfect mechanisms of energy storage and use, the processes of energy exchange in the cell proceed at a very high efficiency, as yet unattained in technology. The biological role of respiration is not exhausted with the use of the energy contained in the oxidized organic molecule. During the oxidative conversions of organic substances, active intermediate compounds are formed—metabolites, which the living cell uses to synthesize components of its protoplasm and to form enzymes. These essential processes give respiration its central role in the complex of metabolic processes of the living cell. In respiration the processes of the metabolism of proteins, nucleic acids, carbohydrates, fats, and other components of protoplasm intersect and are interconnected.


Kostychev, S. P. Fiziologiia rastenii, 3rd ed., vol. 1. Moscow-Leningrad, 1937.
Bakh, A. N. Sobr. trudovpo khimii khimii i hiokhimii. Moscow, 1950.
Tauson, V. O. Osnovnye polozheniia rastitel’noi bioenergetiki. Moscow-Leningrad, 1950.
James, W. O. Dykhanie rastenii. Moscow, 1956. (Translated from English.)
Palladin, V. I. hbrannye trudy. Moscow, 1960.
Mikhlin, D. M. Biokhimiia kletochnogo dykhaniia. Moscow, 1960.
Szent-Gyorgyi, A. Bioenergetika. Moscow, 1960. (Translated from English.)
Rubin, B. A., and M. E. Ladygina. Enzimologiia i biologiia dykhaniia rastenii. Moscow, 1966.
Racker, E. Bioenergeticheskie mekhanizmy. Moscow, 1967. (Translated from English.)
Rubin, B. A. Kurs fiziologii rastenii, 3rd ed. Moscow, 1971.
Kretovich, V. L. Osnovy biokhimii rastenii. Moscow, 1971.



The processes by which tissues and organisms exchange gases with their environment.
The act of breathing with the lungs, consisting of inspiration and expiration.


1. the process in living organisms of taking in oxygen from the surroundings and giving out carbon dioxide (external respiration). In terrestrial animals this is effected by breathing air
2. the chemical breakdown of complex organic substances, such as carbohydrates and fats, that takes place in the cells and tissues of animals and plants, during which energy is released and carbon dioxide produced (internal respiration)