EXTERNAL AND INTERNAL RESPIRATION. 809 



tissues. It is nevertheless true that oxidations take place in the blood, 

 although only to a slight extent; but these oxidations depend, it seems, 

 upon the form-elements of the blood, hence it does not contradict the 

 above statement that the oxidations exclusively occur in the cells and 

 chiefly in the tissues. 



The gaseous exchange in the tissues, which has been designated 

 internal respiration, consists chiefly in that the oxygen passes from the 

 blood in the capillaries to the tissues, while the great bulk of the carbon 

 dioxide of the tissues originates therein and passes into the blood of the 

 capillaries. The exchange of gas in the lungs, which is called external 

 respiration, consists, as is seen by a comparison of the inspired and expired 

 air, in the blood taking oxygen from the air in the lungs and giving off 

 carbon dioxide. This does not exclude the fact that in the lungs, as in 

 every other tissue, an internal respiration takes place, namely, a com- 

 bustion with a consumption of oxygen and formation of carbon dioxide. 

 According to BOHR and HENRIQUES l the lungs take a variable but 

 always an important part in the total metabolism. This part, which on 

 an average is 33 per cent, but may even rise above 60 per cent of the 

 total metabolism, depends, these experimenters say, upon the fact that 

 the intermediary metabolic products formed in the tissues are burnt in 

 the lungs. It is also in part represented by secretory work of the lungs. 



What kind of processes take part in this double exchange of gas? 

 Is the gaseous exchange simply the result of an unequal tension of the 

 blood on one side and the air in the lungs or tissues on the other? Do 

 the gases pass from a place of higher pressure to one of a lower, according 

 to the laws of diffusion, or are other forces and processes active? 



These questions are closely related to that of the tension of the 

 oxygen and carbon dioxide in the blood and in the air of the lungs and 

 tissues. 



Oxygen occurs in the blood in a disproportionately large part as 

 oxyhamoglobin, and the law of the dissociation of oxyhsemoglobin is 

 of fundamental importance in the study of the tension of the oxygen 

 in the blood. 



Attempts have been made to prove this law by investigations on a 

 pure solution of haemoglobin, and HiJFNER 2 has made very careful and 

 important determinations on such solutions. Recent investigations 

 of BOHR 3 and his pupils, as well as of LOEWY and ZuNTz, 4 have shown 

 that the conditions in the blood are different from a pure hemoglobin 

 -solution, which, in part, may be due to a change in the hemoglobin 



1 Centralbl. f. Physiol., G, and Maly's Jahresber., 27. 



2 Arch. f. (Anat. u.) Physiol., 1890 and 1894. 



3 See Nagel's Handbuch, and Krogh, Skand. Arch. f. Physiol., 16. 



4 Arch. f. (Anat. u.) Physiol., 1904. 



