RESPIRATION OF ABNORMAL AIR, ETC. 359 



INTERNAL RESPIRATION. 



The arterial blood, while flowing through the capillaries of the 

 systemic circulation and supplying the tissues with nutriment, 

 undergoes changes which are called internal or tissue respiration, 

 and which may be shortly defined to be the converse of pulmo- 

 nary or external respiration. In the external respiration the blood 

 is changed from venous to arterial ; whereas in internal respira- 

 tion the blood is again rendered venous. 



There can now be no doubt that these chemical changes take 

 place in the tissues themselves, and not in the blood as it flows 

 through the vessels. The amount of oxidation that takes place 

 in the blood itself is indeed very small. The tissues, however, 

 along with the substances for their nutrition, extract a certain 

 part of the O from the blood. In the chemical changes which 

 take place in the tissues, they use up the oxygen, which rapidly 

 disappears, the tension of that gas becoming almost nil ; at the 

 same time other chemical changes are indicated by the appear- 

 ance of CO 2 . The disappearance of the O and the manufacture 

 of CCXj need not exactly correspond in amount, and they doubt- 

 less often vary in different parts and under different circum- 

 stances. Of the intermediate steps in the tissue chemistry we 

 are ignorant. We do not know the way in which the oxygen is 

 induced by the tissues to leave the haemoglobin ; we can only say 

 that the tissues have a greater affinity, for O than the haemoglobin 

 has, and they at once convert the O into more stable compounds 

 than oxyhsemoglobin, and ultimately manufacture CO 2 , which 

 exists in the tissues and fluids of the body at a higher tension 

 than even in the venous blood. 



RESPIRATION OP ABNORMAL AIR, ETC. 

 The oxygen income and carbonic acid output are the essential 

 changes brought about by respiration, therefore the presence of 

 oxygen in a certain proportion is absolutely necessary for life. 

 The 21 per cent, of O of the atmosphere suffices to saturate the 

 haemoglobin of the blood, and 14 per cent, of O has been found 

 to be capable of sustaining life without producing any marked 

 change in respiration. 



