422 TEXT-BOOK OF PHYSIOLOGY 



arterial blood 21.8 mm. Hg., or 2.8 per cent. Very different results have 

 been obtained by Fredericq with the aerotonometer devised by him and by 

 the employment of a method different from that of Strassburger. Thus he 

 states that the oxygen tension in the pulmonic alveoli is 136 mm. Hg., or 

 1 8 per cent, of an atmosphere while in the arterial blood it is 106 mm. Hg., 

 or 14. per cent.; while the carbon-dioxid tension in the tissues varies from 

 45 to 68 mm. Hg., or from 6 to 9 per cent, of an atmosphere; while in the 

 venous blood it varies from 30 to 41 mm. Hg., or from 3.8 to 5.4 per cent, 

 and in the pulmonic alveoli it is about 21 mm. or 2.8 per cent. 



The more recent investigations of other experimenters, Haldane, 

 Priestly, have furnished different results which no doubt represent more 

 closely the true values and which are presented in the diagram on page 423. 



CHANGES IN THE COMPOSITION OF THE TISSUES AND LYMPH 



From previous statements the inferences can be drawn that the oxygen 

 leaves the blood as the latter flows through the capillaries; that it passes 

 through the capillary wall into the surrounding lymph and so to the tissue- 

 cells; that it oxidizes food materials in the tissue-cells whereby the potential 

 energy of the former is liberated as kinetic energy; that the carbon dioxid 

 so evolved passes into the lymph and through the wall of the capillary into 

 the blood. 



While this is doubtless the case, the presence of free oxygen in the tissues 

 cannot be demonstrated by the usual methods of gas analysis. Only in the 

 saliva and in the blood of the placental umbilical vein can it be shown 

 that oxygen has directly passed through the capillary wall. For this reason 

 it has been claimed by a few investigators that oxygen does not leave the 

 blood, but that the field of its activity as an oxidizing agent is limited to the 

 blood-current, where it meets with and oxidizes easily reducible substances 

 entering from the tissues. On this view the potential energy of the food 

 would be liberated by mere decomposition or cleavage in consequence of 

 cell activity. 



Nevertheless many facts from the fields of comparative physiology and 

 physiologic chemistry combine to support the view that oxygen is absolutely 

 necessary to the maintenance of the life of all tissue cells. Though they 

 will continue to manifest their characteristic activities e.g., contraction on 

 the part of a muscle, secretion by a gland, the conduction of a nerve impulse 

 by the nerve, etc. for a variable length of time after oxygen is prevented 

 from gaining access to them, nevertheless they will in due time die. 



The necessity for oxygen on the part of the tissues and the avidity with 

 which they absorb it, is shown by their power of reducing pigments such as 

 alizarine blue. If this pigment be injected into the blood-vessels of an 

 animal and the animal killed in about ten minutes, it will be found that while 

 the blood exhibits a deep blue color the tissues present their usual colors. 

 But after exposure to the air or to free oxygen the latter also acquire the 

 characteristic blue color. The explanation offered for this fact is that the 

 tissues in their need for oxygen absolutely extract it from the pigment, 

 reducing it to a colorless compound, which, however, on exposure recom- 

 bines with oxygen and regains the original color. 



Though free oxygen cannot be shown to be present in the tissues, there 



