OXYGEN UNSATURATION OF THE BLOOD 445 



forded in following the treatment of pneumonia indicates the lines along 

 which future progress is likely to take place. 



THE THERAPEUTIC VALUE OF OXYGEN 



There is no therapeutic measure that is less efficiently put into prac- 

 tice than the administration of oxygen, and as a consequence, most phy- 

 sicians have little faith in its value. There are several reasons for this 

 state of affairs : in the first place, the physiological mechanism by which 

 added oxygen could assist in the respiratory functions is not understood; 

 in the second, an insufficient amount of the gas is usually given and in 

 the third, it is usually given too late. On the other hand, when oxygen 

 is properly administered in suitable cases before the patient has become 

 moribund, much evidence has accumulated to show that very great bene- 

 fit indeed results from the treatment, and as far as can be told a fatal 

 termination is often averted. 



Theoretical Considerations. In order to understand the physiological 

 principles involved in this treatment, it is important to remember that 

 although forty to fifty times as much oxygen is combined with hemo- 

 globin as is in simple solution in the blood plasma, yet it is the latter 

 which really diffuses into the tissues. The pressure of oxygen in the 

 plasma, in other words, the diffusion pressure of oxygen, is the deter- 

 mining factor in causing it to permeate the tissues, and whenever this 

 pressure begins to fall under normal conditions, more is added to the 

 plasma by dissociation from the oxyhemoglobin of the corpuscles. The 

 plasma retails the oxygen to the tissues, and the corpuscles are the 

 wholesale warehouses from which the plasma replenishes its stock. The 

 unloading of oxygen from the oxyhemoglobin to the plasma is assisted 

 by various chemical changes that take place while the blood is in the 

 capillaries. From these considerations it follows that an efficient sup- 

 ply of oxygen to the tissues could be maintained without any hemoglobin 

 if we were to put an excess of the gas into simple solution in the plasma ; 

 that is, if enough were forced into solution in the plasma in the lungs so 

 that the tissue requirements could be met without any local addition 

 from oxyhemoglobin. Two experiments may be quoted to show that it is 

 possible to fulfil these conditions. 



1. After replacing all the blood from the blood vessels of a frog by 

 artificial plasma (Ringer's solution) the animal can be kept alive for 

 days in a vessel containing pure oxygen (i. e., five times the amount 

 present in air) and during this time the rate of 2 consumption and CO, 

 production are practically the same as normally. 



2. Animals (mice) exposed to air containing more than 0.5 per cent of 



