THE CHEMISTRY OF RESPIRATION 507 



The Condition of Nitrogen in the Blood. — By far the greatest 

 amount of the nitrogen present in circulating blood, is held in solu- 

 tion and is therefore subject to the law of Henry. The same state- 

 ment may be made regarding blood kept outside the body, if it is 

 saturated with atmospheric air. It is true, however, that blood al- 

 ways absorbs a larger amount of nitrogen than is taken up by an equal 

 volume of air when subjected to the same conditions. This fact 

 tends to prove that a small portion of this gas is held in combination. 

 Moreover, the presence of this extra amount cannot be dependent 

 upon a special activity of certain tissues for the obvious reason that 

 blood experimented with outside the body, behaves in precisely the 

 same manner. The separate determinations of the nitrogen absorp- 

 tion of the plasma and corpuscles have shown that the nitrogen con- 

 tent of the former is proportional to the tension of this gas, whereas that 

 of the latter is not. Hence, it may be concluded that the corpuscles 

 are the element most directly concerned in this absorption. Besides, 

 it has been proved by Bohr^ that this union takes place solely in the 

 presence of oxygen and that the factor primarily responsible for it is 

 the hemoglobin. This investigator surmises that the nitrogen is held 

 here in the form of an unstable oxid, the functional significance of 

 which has not been established. 



Internal or Tissue Respiration. — The freshly aerated blood tra- 

 versing the pulmonary veins, left side of the heart and systemic 

 arteries is in a state of almost complete saturation with oxygen which 

 is held here tmder a pressure of at least 100 mm. Hg. It has been 

 shown above that its saturation amounts to about 90 per cent., and 

 that this degree of saturation can be obtained with an oxygen tension 

 of little more than 30 mm. Hg. Thus, it will be noted that the oxygen- 

 carrying capacity of the blood is amply safeguarded, at least as far 

 as pressure is concerned. This is also shown by the fact that this 

 type of blood may be shaken with atmospheric air at the tempera^ 

 ture of the body without absorbing more than about 2 volume per 

 cent, of oxygen in addition to that just stated. Venous blood, on 

 the other hand, requires 8 to 10 volume per cent, of oxygen for its 

 saturation. 



The blood traversing the capillaries of the different tissues is 

 brought into diffusion relation with the cells through the intervention 

 of the lymph. It is a well-known fact that the cells acquire oxygen 

 constantly and give off carbon dioxid. It is evident, therefore, that 

 the oxygen tension is higher in the blood than in the tissues, whereas 

 that of the carbon dioxid is higher in the tissues than in the blood. 

 Thus, the physical conditions are such that the oxygen must flow from 

 the blood into the cells, while the carbon dioxid must pass from the 

 cells into the blood, as follows : 



^ Compt. rend., cxxiv, 1897, 414. 



