Gas Exchange


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Gas Exchange

 

(gaseous interchange), exchange of gases between an organism and the environment. Oxygen, which is required by all cells, organs, and tissues, enters the organism from the environment; the organism releases carbon dioxide, which it produces, and a small quantity of other gaseous metabolic products. Gaseous interchange is necessary for almost all organisms; without it normal metabolism, energy transfer, and consequently life itself, would be impossible.

The oxygen that enters the tissues is used in the oxidation of the products formed as the result of the long chain of chemical conversion of carbohydrates, fats, and proteins. Carbon dioxide, water, and nitrogenous compounds are formed during this process, and energy is freed to be used in the maintenance of body temperature and in the expenditure of work. The quantity of CO2 formed in the organism and finally released by it depends not only on the quantity of O2 consumed but also on the predominant substance being oxidized (carbohydrates, fats, or proteins). The relationship between the CO2 released by the organism and the O2 absorbed for the same amount of time is called the respiratory coefficient and is equal to approximately 0.7 for the oxidation of fats, 0.8 for proteins, and 1.0 for carbohydrates. The amount of energy liberated per liter of O2 consumed (the caloric equivalent of oxygen) is 20.9 kilojoules (kJ), or 5 kilocalories (kcal), for the oxidation of carbohydrates and 19.7 kJ (4.7 kcal) for the oxidation of fats. Thus, it is possible to calculate the amount of energy liberated in the organism by the consumption of O2 per unit of time and by means of the respiratory coefficient.

Gaseous interchange (and energy expenditure) in poikilothermic (cold-blooded) animals decreases with a decrease in body temperature. The same temperature dependence is observed in homoiothermic (warm-blooded) animals when heat regulation is shut off (under conditions of natural or artificial hypothermia); gaseous interchange increases with an increase in body temperature (upon overheating and in the case of various illnesses).

As the temperature of the surroundings drops, gaseous interchange in warm-blooded animals (especially in small animals) increases as a result of increased heat production. It also increases after the intake of food that is particularly protein-rich (the so-called specifically dynamic effect of food); it attains its greatest magnitude during muscular activity. In humans under moderate exertion, gaseous interchange increases and, three to six minutes after the commencement of activity, reaches a specific level, which is then maintained throughout the period of activity. When performing work requiring great exertion, gaseous interchange increases continuously; soon after a given individual attains his maximum level (maximum aerobic work), he must stop working, since the body’s need for O2 exceeds this level. Increased O2 consumption continues after the end of exertion and is used to cover the oxygen debt—that is, to oxidize the metabolic products that formed during the work. The consumption of O2 can increase from 200-300 milliliters per minute (ml/min) at rest to 2,000-3,000 ml/min during work and, for athletes in good condition, to 5,000 ml/min. At the same time, the liberation of CO2 and the expenditure of energy increase, and changes in the respiratory coefficient associated with changes in metabolism, acid-base equilibrium, and pulmonary ventilation take place.

The calculation of the total daily expenditure of energy among people of different professions and ways of life is based on determinations of gaseous interchange and is important for fixing dietary levels. Studies of changes in gaseous interchange during standard physical work are used in studying the physiology of work and sports and under clinical conditions for the evaluation of the functional state of systems that participate in gaseous interchange.

The comparative constancy of gaseous interchange with considerable changes in the partial pressure of O2 in the surroundings, disruptions of the function of the respiratory organs, and so on is ensured by compensatory reactions of the systems that participate in the interchange and are regulated by the nervous system.

Gaseous interchange in humans and animals is usually studied under conditions of complete rest, on an empty stomach, and at a comfortable temperature (18°-22° C). The quantity of O2 consumed and energy released under these conditions characterizes the basal metabolism. Methods based on the principle of an open or closed system are used in the study of gaseous interchange. In the first instance, the amount and composition of the air expired are determined by means of chemical or physical gas analyzers, which makes it possible to calculate the quantities of O2 consumed and CO2 liberated. In the second instance, respiration takes place in a closed system (a hermetically sealed chamber or from a spirograph connected to the respiratory tracts), in which the CO2 liberated is absorbed and the quantity of O2 consumed from the system is determined either by measuring the quantity of O2 entering the system automatically, which is equal to the quantity of O2 consumed, or by decreasing the volume of the system.

REFERENCES

Ginetsinskii, A. G., and A. V. Lebedinskii. Kurs normal’ noi fiziologii. Moscow, 1956.
Ginetsinskii, A. G., and A. V. Lebedinskii. Fiziologiia cheloveka. Moscow, 1966. Pages 134-56.
Berkovich, E. M. Energeticheskii obmen v norme i patologii. Moscow, 1964. (Contains a bibliography.)
Prosser, L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. Pages 186-237. (Translated from English.)

L. L. SHIK

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