CHAPTER XLIV 

 RESPIRATION BEYOND THE LUNGS 



Up to the present our studies in respiration have concerned the various 

 mechanisms involved in bringing about a constant change in the com- 

 position of the alveolar air. We must now consider the nature of the 

 means by which the oxygen is conveyed to the tissues and the carbon di- 

 oxide removed from them. 



In the first place, it is important to note that it is not for purposes 

 of oxidation in the blood itself that the 2 is required. In its respiratory 

 function this fluid serves as a transporting agency between the lungs 

 and the tissues, in which reside the furnaces of the body that con- 

 sume the 2 and produce the C0 2 . This does not imply that there is no 

 oxidation in the blood itself; indeed, we should expect a certain degree 

 of oxidation because of the fact that the blood contains some living 

 cells the leucocytes. It is scarcely necessary nowadays to offer evi- 

 dence for the foregoing conclusion. One well-known experimental proof 

 consists in replacing the blood in a frog with physiological saline solution 

 and then subjecting the frog with the saline in its blood vessels to an 

 atmosphere of pure 2 , when it will be found that the animal continues 

 to absorb the normal amount of 2 and exhale the normal amount of 

 C0 2 . It respires normally without any blood in the blood vessels. 



In order that this transportation of gases between the lungs and the 

 tissues may be efficiently performed, the blood must be provided with 

 means for carrying adequate amounts of gases to supply the requirements 

 of the tissues, both during rest and during their varying degrees of 

 activity. Not only, therefore, must the 2 and C0 2 capacity of the 

 blood be very considerable, but it must be capable of very rapid adjust- 

 ment from time to time. 



Our problem naturally resolves itself into three parts: (1) the call 

 of the tissues for oxygen (Barcroft) ; or, as it is stylecl, tissue or internal 

 respiration; (2) the mechanism by which the blood transports the proper 

 amounts of gases to meet the requirements of the tissues; and (3) the 

 mechanism by which the blood gases are exchanged in the lungs ex- 

 ternal respiration. For convenience, however, we shall change this nat- 

 ural order and consider the transportation of the gases first. 



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