68 ORGANISM AND ENVIRONMENT 



nite results on this point. In any case we should 

 hardly expect to meet with active secretion of C0 2 , 

 considering that the breathing is regulated by the C0 2 

 pressure in the arterial blood, and that the oxygen 

 supply to the lungs is dependent on this regulation. 

 During very excessive muscular work it seems to be 

 the oxygen supply to the body that first begins to fail. 

 This is indicated by the fact that in very hard work the 

 alveolar C0 2 percentage begins to fall, and may even 

 become lower than during rest. 



The delicate and exactly regulated organization by 

 which C0 2 is removed from the blood in the lungs, 

 and oxygen supplied, would quite clearly be of little 

 service to the body if there were not also a regulation 

 of the circulation of blood so as to keep the removal of 

 C0 2 from the body tissues and their supply of oxygen 

 correspondingly steady. We must now, therefore, 

 consider what is known as to the circulatory regula- 

 tion. Knowledge on this subject is unfortunately still 

 very fragmentary, mainly because physiologists have 

 failed to appreciate the delicacy of organic regulation, 

 or have even lost sight of it altogether when investi- 

 gating various matters of detail. 



The blood brings to the tissues the various sub- 

 stances required for their normal life, and removes 

 from them substances which are then carried to other 

 tissues or to secretory organs. It is also a carrier of 

 heat. The carriage of oxygen and C0 2 is thus only 

 one of its many functions. Hence we must not expect 

 that the circulation will be solely regulated with refer- 

 ence to the carriage of these gases. Bernard noticed 



