684 PHYSIOLOGY OF RESPIRATION. 



It is to be remembered that in this exchange the blood and 

 the lymph act as intermediaries. The CO 2 diffuses from lymph 

 to plasma and from tissues to lymph. The oxygen diffuses from 

 lymph to tissues, from plasma to lymph, and from oxyhemo- 

 globin to plasma. Bohr* has found experimentally that in 

 blood, when the oxygen tension is low, an increase in the CO 2 

 pressure tends to dissociate the oxy hemoglobin (Fig. 275). 

 Since these conditions prevail in the capillaries of the body, it 

 is probable that the presence of the CO 2 in increased amounts 

 facilitates the liberation of the oxygen. 



Suggested Secretory Activity in the Respiratory Exchange. The view 

 that the exchange of gases in the lungs and tissues is entirely explained by 

 the diffusion of the gases from points of high tension to points of low ten- 

 sion, and that the membranes interposed are entirely passive in the process 

 has not passed unchallenged. Certain observers (Bohr, Haldane, and Smith) f 

 claim that the tension of the oxygen in the arterial blood may be higher than 

 the pressure of oxygen in the alveolar air. Bohr, moreover, in a series of ex- 

 periments made upon dogsj determined by calculation the tension of oxygen 

 within the surface layer of the lungs. This tension was found to vary from 

 35 to 105 mms. The tension of the arterial blood, determined at the same 

 time, varied from 101 to 144 mms., being in every case distinctly higher than 

 the tension of the oxygen in the surface layer of the lungs. If these facts 

 were fully demonstrated they would show that the physical theory outlined 

 above is insufficient, and would indicate that the membranes concerned take 

 an active part in the passage of the gases, exerting possibly a secretory activity. 

 That the cells of these membranes might secrete the gases is not at all impos- 

 sible, but at present it seems to be unnecessary to make such a supposition. 

 The results obtained by the observers mentioned in this paragraph have not 

 been corroborated by the numerous other observers who have worked in the 

 same field, and it seems possible that they may be due to experimental errors. 

 A well-known set of experiments that strengthen this conclusion has been 

 reported by Wolff berg and by Nussbaum and has since been repeated upon 

 man. In these experiments one bronchus in a dog was completely blocked 

 by a specially designed lung catheter, so arranged as to occlude the bronchus 

 and yet allow the observer to draw off a specimen of the air at any time. In 

 such an occluded lung the captured air is in diffusion relations with the venous 

 blood of the pulmonary artery, and if these relations are maintained for a 

 sufficient time an equilibrium should be established on the physical theory, 

 the tension of the gases in the occluded lungs becoming the same as in the 

 venous blood. .Such was found to be the case. When at the end of the 

 experiment air was drawn off and analyzed it was found to contain 3.6 per 

 cent, of CO 2 , while the tension of the CO 2 in specimens of the venous blood 

 taken from the right heart was practically identical. If there is an active 

 secretion of CO 2 from the lungs one should have expected to obtain a higher 

 tension in the carbon dioxid of the alveolar air than in the venous blood. 



* "Skandinavisches Archiv f. Physiologic/' 16, 402, 1896. 

 t See Haldane and Smith, "Journal of Physiology," 20, 497, 1896. 

 t Bohr, in Nagel's "Handbuch der Physiologic des Menschen," 1897, vol. 

 i, part 1, p. 146. 



"Archiv f. die gesammte Physiologie," 4, 465, 1871, and 7, 296, 1873. 



