258 RESPIRATION 



A second, but somewhat less urgent, need is for a continuous re- 

 moval of carbonic acid or any other acid product formed in the 

 tissues. We can probably express this generally as a need for pre- 

 venting an abnormal proportion of hydrogen ions to hydroxyl 

 ions. The effect on the central nervous system of a sudden flooding 

 with CO 2 , without deficiency of oxygen, is almost as striking, 

 though not so immediately dangerous to life, as the effect of 

 deprivation of oxygen. The results of even a slight variation in 

 arterial CO 2 pressure have often been referred to already. 



Other conditions in the blood besides the diffusion pressures of 

 oxygen and CO 2 or other acid products are just as important to 

 life. For instance there are the diffusion pressure of water (inac- 

 curately identified with osmotic pressure) and the diffusion pres- 

 sures of the ions of various inorganic salts, on the importance of 

 which the investigations of Ringer and many others have thrown 

 much light. But none of these values vary in the same rapid 

 manner as the diffusion pressures of oxygen and CO 2 do ; and of 

 ordinary nutrient substances present in blood, the tissues them- 

 selves appear to possess a store which can be drawn on if the 

 supply from the blood fails for a time. The results of perfusion 

 experiments continued with the same blood indicate that if only 

 the blood is properly aerated it continues for a very long time to 

 support life in the tissues. 



It would seem, therefore, that the regulation of circulation 

 through the tissues must in the main be determined in correlation 

 with the need for supplying oxygen and removing CO 2 . There 

 are evidently, however, cases where some other factor determines 

 the circulation rate. For instance, the skin circulation is de- 

 termined to a large extent in relation to the regulation of body 

 temperature; and the circulation through an actively secreting 

 gland is probably determined to a considerable extent in corre- 

 lation with local excess or deficiency of water or dissolved solids. 



We can form a general idea as to what changes in gaseous 

 composition determine the circulation rate through the tissues if 

 we compare the arterial blood with the mixed venous blood re- 

 turning to the lungs. As regards this point, analyses showing the 

 difference in composition have already been quoted in Chapter V, 

 and indicate that, in the animals experimented on, the blood in 

 its passage through the tissues had lost about a third of its avail- 

 able oxygen, and gained the amount of CO 2 which would cor- 

 respond to the loss of oxygen when allowance is made for the 

 existing respiratory quotient of the animal. If we applied these 



