384 RESPIRATION 



It is a familiar fact that, apart from the contained gases, the 

 composition of blood plasma is extremely constant. The varied 

 experiments initiated by Ringer and carried forward by many 

 other observers indicate directly the physiological importance of 

 the various salts or their ions which are present in blood plasma, 

 and render intelligifele the exactitude with which their concentra- 

 tions are regulated by the kidneys. The facts collected in the pres- 

 ent book show that also as regards hydrogen and hydroxyl ions 

 and free oxygen the composition of the blood plasma in contact 

 with any particular part of the tissues is, and must be, very con- 

 stant, and is kept so by regulation of breathing, circulation, kidney 

 excretion, and other physiological activities. Thus oxygen and 

 hydrogen and hydroxyl ions take their place in a strict quantita- 

 tive sense beside the salts, proteins, sugar, etc., which help to 

 make up Bernard's "conditions of life." 



We also now know that what is called the osmotic pressure of 

 blood plasma is so constant that the existing methods of measur- 

 ing it by depression of freezing point or vapor pressure are too 

 coarse for the detection of such differences as are constantly oc- 

 curring during life and evoking the ordinary physiological re- 

 sponses of the kidneys and other organs. Osmotic pressure de- 

 pends, however, as already mentioned (Chapter VI 11)- on the 

 difference between the diffusion pressure of a solvent in a solution 

 and in the pure solvent. It is thus in reality the diffusion pressure 

 of water in the blood that is maintained so constant. The diffusion 

 pressure of water can thus be placed in the same category as that 

 of other substances among Bernard's "conditions of life." The 

 experiments of Priestley and myself-'^ on the excretion of water 

 by the kidneys show that the regulation by the kidneys of the 

 diffusion pressure of water in the blood is comparable in its ex- 

 treme delicacy to the regulation of blood reaction. 



As a general rule salts, water, and various other substances 

 present in blood plasma are to only a very small extent used up 

 by or given off from the tissues. Hence in the case of most 

 tissues it would require only a very slow circulation to keep the 

 concentrations of these substances constant in the blood, pro- 

 vided that the temperature was constant. If, however, the circu- 

 lation were much slower than it is, and if this were rendered 

 possible by the provision in the blood of much greater capacity 

 for carrying oxygen and CO2 as easily dissociable compounds, 



* Haldane and Priestley, Journ. of Physiol., L, p. 296, 1916; Priestley, Ibid., 

 L. p. 304. 1916. 



