SECTION VI. 

 PHYSIOLOGY OF RESPIRATION. 



Historical. — The term respiration as usually employed in 

 physiology refers to the process of gaseous exchange between an 

 organism and its environment. This exchange consists essentially 

 in the absorption of oxygen by the living matter and the elimination 

 of carbon dioxid. It is one of the generalizations of physiology that 

 all living things, with the exception perhaps of the anaerobic 

 organisms, require oxygen for their vital processes — that is, for 

 the normal course of those chemical changes in the tissue cells 

 which we group under the general term of metabolism. On the 

 olher hand, one of the universal end-products of this metabolism 

 is carbon dioxid. Hence, respiration in some form is one 

 great characteristic of living things. In the simplest animals 

 and plants, the unicellular organisms, the exchange between 

 the air (or water) and the organism takes place directly, but 

 in the more complex animals some form of respiratory appara- 

 tus is developed whose function consists either in bringing 

 the air or oxygen-laden water to the constituent cells, as in the air 

 tubes of the insects, or in bringing the circulating blood into contact 

 with the air or water, as in the case of animals provided with lungs 

 or gills. In man and the air-breathing vertebrates the latter device 

 is employed and one may distinguish in such animals between 

 internal and external respiration. By the latter term is meant the 

 gaseous exchange, absorption of oxygen and elimination of carbon 

 dioxid, that takes place in the lungs between the blood in the pul- 

 monary capillaries and the air in the alveoli. By internal respira- 

 tion is meant the similar exchange that takes place in the systemic 

 capillaries between the blood and the tissue elements. All of this 

 exchange is, so to speak, secondary, since the essential process 

 consists in the history of the oxygen after it is absorbed into the 

 tissues, — that is, the part taken by the oxygen in the metabolism of 

 living matter. This process, however, is a part of the subject of 

 nutrition. The food absorbed from the digestive organs and the 

 oxygen taken from the blood have a common history, or at least 

 their reactions are indissolubly connected after they come within 

 the field of influence of the living molecules. This side of the func- 

 tion of the oxygen may be considered, therefore, more appropriately 

 in the section on nutrition. In the present section attention will be 

 directed to the beautiful means that have been adapted to tlie pur- 



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