RESPIRATION. 527 



believed that the passage of CO 2 from the blood to the alveoli is determined 

 simply by the laws of diffusion, but Bohr l has found in experiments in which 

 analyses of the blood and alveolar air were made simultaneously that the par- 

 tial pressure of CO 2 in the alveolar air may be less than the average tension in 

 the blood. Moreover, Bohr found in a series of experiments that even when 

 the quantity of CO 2 in the atmosphere in contact with the blood was very 

 small, but little more CO 2 diffused from the blood. Facts of this kind are 

 explicable on the hypothesis that the pulmonary membrane is, as contended by 

 Ludwig, Bohr, and others, actively engaged in the process, playing a specific 

 excretory r61e, but our knowledge is as yet too incomplete to require the 

 acceptance of such an hypothesis. Under ordinary conditions the tension of 

 CO 2 in the alveoli is less than in the blood, and the transmission of CO 2 from 

 the blood to air-cells may be explained satisfactorily by the laws of diffusion. 



The Forces Concerned in the Interchange of O and CO 2 between the 

 Blood and the Tissues. Innumerable facts show that the chief seat of the 

 chemical processes in the body is in the tissues, and that the decompositions 

 are essentially of an oxidizing character whereby CO 2 is formed as one of 

 the most important effete products; consequently the blood as it is carried 

 through the capillaries gives up O and receives CO 2 . 



Experiments show that the tissues exert a strong reducing action, and that 

 their avidity for O is so great that they will take it up at extremely low 

 pressures. Moreover, never more than mere traces of O can be obtained 

 from the tissues, because the gas upon its absorption immediately enters into 

 chemical combination. 



The tension of CO 2 in the tissues is considerably higher than in blood. 

 Strassburg, 2 in a loop of intestine into which he injected atmospheric air, found 

 that the tension was 58.52 millimeters of Hg, which is considerably greater 

 than in either arterial or venous blood. Thus we find that the tension of O in 

 the tissues is nil, owing to their greediness for this gas, while that of CO 2 is 

 very high. Comparing the tensions of these two gases in the blood and the 

 tissues, it will be observed that there are present conditions which are highly 

 favorable to the passage of O to the tissues and of CO 2 in the reverse direction : 



o. co 2 . 



Tensions in arterial blood . . 29.64 21.28 



Blood-vessel walls + 



Tensions in tissues . 0.00 



5&25 



It is manifest from the above that O should pass from the blood to the tissues, 

 and CO 2 from the tissues to the blood. 



The lymph is probably merely a passive medium in this interchange. It 

 contains, according to Hammarsten, only traces of O, from 37.5 to 47.1 vol- 

 umes per cent, of CO 2 , and from 1.1 to 1.63 volumes per cent, of N. The 

 mean percentage of CO 2 is lower than in serum, but Gaule has shown that the 

 tension is higher. Doubtless the same relations hold good for the plasma and 

 1 Loc. cit. * Loc. tit. 



