RESPIRATION. 525 



chemical combination in the plasma is probably in part combined with glob- 

 ulin and alkali, and in part with sodium as carbonate and bicarbonate, the 

 proportion of each varying with the tension of the CO 2 . The white blood- 

 corpuscles, so far as they contain any of the CO 2 , hold it probably in combina- 

 tion with globulin and alkali and as carbonates of sodium. The great bulk of 

 the gas disengaged from the corpuscles is derived from the red cells, but in what 

 combination or combinations it exists is not positively known. The experiments 

 of Setschenow, Zuntz, Bohr, 1 and others indicate that it is associated in some 

 obscure way with haemoglobin, and probably with a third body, such as globulin 

 or alkaline phosphates ; and yet haemoglobin seems to have the power to hold 

 the CO 2 in the absence of a third body. This latter fact has been shown by 

 the experiments of Bohr, who compared the quantities of CO 2 absorbed by 

 pure water and by solutions of pure crystallized haemoglobin at constant tem- 

 perature and varied pressure. He found that the weight of CO 2 absorbed by 

 the water increased regularly with the increase of pressure, whereas the quan- 

 tity absorbed by the solution of haemoglobin was very large relatively to the 

 lower pressures and small for higher pressures, and that the increments of 

 absorption were in decreasing ratio to the rise of pressure. The absorption 

 curve is therefore steep at first, becoming less and less so with the increase of 

 pressure, and entirely different from the absorption line for pure water, which 

 is straight. Moreover, the quantity of CO 2 dissolved was considerably in 

 excess of that which physical laws could admit. The CO 2 , in whatever form 

 or forms it may exist in the red corpuscles, is in looser combination than in 

 serum. 



Strassburg's experiments show that the average tension of CO 2 in arterial 

 blood is 21.28 millimeters of Hg, or 2.8 per cent, of an atmosphere, and in 

 venous blood 41.04 millimeters, or 5.4 per cent, of an atmosphere. 



Tension of N. The quantity of nitrogen in the blood is about 1.8 volumes 

 per cent. It is in simple solution in the blood-plasma, and the quantity in 

 both venous and arterial blood is practically the same. Its presence and quan- 

 tity are not of physiological importance. 



The Interchange of O and CO 2 between the Alveoli and the Blood. 

 Let us now inquire into the factors which bring about the passage of O from 

 the alveoli to the blood and of CO 2 from the blood to the alveoli. If we have 

 two mixtures of the same gases, but in unlike proportions, and separate them 

 by means of an animal membrane, diffusion will occur through the membrane 

 until the partial pressures of the two gases are the same on the two sides of the 

 membrane. Now modify this experiment by bringing an atmosphere of air 

 in contact with water containing O, CO 2 , and N in solution or in chemical 

 combination : if the partial pressure of O in the air be greater than the tension 

 of O in the water, O will pass to the water; if the partial pressure of CO 2 in 

 the air be less than the tension of CO 2 in the water, CO 2 will pass to the air. 

 If now we interpose an animal membrane between the atmosphere and the 



1 Exper. Untersuch. u. d. Sauerstoffaufnahme d. Blutfarbstoffes, Kopenhagen, 1885 ; Beitrage zur 

 Physiologic, Festschr. f. C. Ludwig, 1887, pp. 164-172. 



