226 STUDIES IN ADVANCED PHYSIOLOGY. 



The oxygen pressure in the plasma is very low, less than a 

 half pound. For this reason all of the oxyhsemoglobin has 

 been disassociated and the haemoglobin only is left. Thus 

 it will be seen that the reason why the blood does not give 

 off any of its oxygen until it reaches the tissues is, that all 

 through the arteries the pressure of the oxygen in the 

 plasma remains constant, but sinks below the critical half 

 pound limit in the capillaries only. In the chapter on Blood 

 it was stated that by far the larger amount of oxygen was 

 carried by the corpuscles and a relatively small amount 

 only by the plasma. The proportion is given by some physi- 

 ologists as ten to one, but while this is true, it will be 

 noticed from the preceding that the plasma and the oxygen 

 dissolved in it, play a most important role in the process of 

 respiration. 



2. The Elimination of the Carbon Dioxide. But the 

 preceding is only half of the story. In the process of res- 

 piration not only is oxygen taken up in the lungs and 

 carried to the tissues, but carbon dioxide is picked up in the 

 tissues and eliminated from the lung. There now remains 

 a more detailed description of the actual manner in which 

 this carbon dioxide is picked up in the capillaries and finally 

 thrown out in the lungs. 



Carbon dioxide is produced when any tissue in the body 

 is in action. It is the result of activity of the brain no less, 

 probably, than that of the muscle, but on account of the 

 difficulty of observation, the finer details as to the sources 

 of the carbon dioxide have been worked out for the muscles. 

 In the chapter on the nutrition of the muscle it was 

 pointed out that the food brought by the plasma and the 

 oxygen brought by the blood were built up into living 

 muscle. It is important, therefore, to bear in mind that as 

 far as we now know the oxygen carried to the muscles is 

 not used in burning them, as is so frequently stated, but is 

 used in building them up. Consequently the carbon dioxide 

 does not arise as the immediate product of combustion due 

 to the arrival of the oxygen in the muscles. In the case 



