SECRETION OF CARBON DIOXIDE. 817 



The conditions as to the elimination of carbon dioxide in the lungs 

 are not quite clear, and from the above we see that in regard to the gas 

 exchange in the lungs we have two opposing theories. According to the 

 older view suggested by the PFLUGER school the exchange of gas follows 

 simple physical laws and is on the whole a diffusion process. Accord- 

 ing to BOHR'S opinion diffusion does take place, but the lung is a gland 

 which has the powei of secreting gases, and the gas exchange in the lungs is 

 essentially a secretory process. 



The theory that a secretion of carbon dioxide takes place in the lungs 

 has been further supported by recent investigations by BoHR. 1 



One group of experiments includes those carried out, essentially, 

 according to an earlier principle, but using the microtonometer of KROGH. 

 The other group includes those experiments where each lung breathed 

 by itself, the one atmospheric air and the other a gas mixture containing 

 a considerable quantity of carbon dioxide, namely, about 8 per cent. 

 The first group of experiments gave new support for the secretion 

 theory, but the second group were of still greater interest. In these 

 experiments in which the alveolar air of both lungs had a different 

 quantity of carbon dioxide, for example in one about 3 per cent CO2 

 and the other about 9 per cent, while the circulating blood was the same 

 in the tw r o sepaiated respiring lungs (right heart blood) the carbon 

 dioxide tension in the venous blood was always lower than in the air 

 of the CO2 respiring lung, and nevertheless CC>2 was eliminated in this 

 lung. This cannot be explained by the diffusion theory; and in the 

 opinion of HAMMARSTEN it cannot be disputed that the investigations 

 thus far reported seem to support BOHR'S theory strongly, and this is 

 -also corroborated by the secretion ol gases detected in certain animals. 



That a true secretion of gases occurs in animals follows from the composition 

 and behavior of the gases in the swimming-bladder of fishes. These gases con- 

 sist of oxygen and nitrogen with only small quantities of carbon dioxide. In 

 fishes which do not live at any great depth the quantity of oxygen is ordinarily 

 as high as in the atmosphere, while fishes which live at great depths may, accord- 

 ing to BIOT and others, contain considerably more oxygen and even above 80 per 

 cent. MOREAU has also found that after emptying the swimming-bladder by 

 means of a trocar, new air collected after a time, arid this air was richer in oxygen 

 than the atmospheric air, and contained even 85 per cent oxygen. BOHR, who has 

 proven and confirmed these statements, also found that this accumulation is under 

 the influence of the nervous system, because on the section of certain branches 

 of the pneumogastric nerve it is discontinued. It is beyond dispute that there is 

 here a secretion and riot a diffusion of oxygen. Recently JAEGER 2 has given a 

 further explanation as to the secretory activity of the swimming-bladder. 



1 Centralbl. f. Physiol., 21, 367. 



- Biot, see Hermann's Handbuch d. Physiol., 4, Thl. 2, 151 ; Moreau, Compt. rend., 

 57; Bohr, Journ. of Physiol.. 15. See also Hiifner, Arch. f. (Anat. u.) Physiol., 1892; 

 Jaeger, Pfluger's Arch., 1)4. 



