HUMAN PROBLEMS IN AVIATION — BRONK 403 



precipitate drop in the oxygen surrounding the cells. Rapid failure 

 of nervous function then ensues. 



To protect the cells of the brain against this danger, the human 

 organism possesses a remarkable system of reflexes. When the oxy- 

 gen concentration in the blood flowing to the brain declines, certain 

 nerve cells are stimulated to action, and that action causes an increase 

 in the rate and depth of respiration. Thus more oxygen is again 

 supplied to the blood flowing through the lungs. The heart rate is 

 also accelerated, and more blood is sent to the brain in order to main- 

 tain the necessary diffusion of oxygen out to the nerve cells. 



Even in the early days of flight, such a sequence of events was 

 observed to be a natural protection of an airman against one of the 

 dangers of high altitude. For in 1804 Gay-Lussac reported that his 

 heart rate and respiration were progressively increased during the 

 course of a balloon ascent. But as the aeronauts ascended to higher 

 altitudes, where the oxygen was less plentiful, the physiological pro- 

 tective mechanisms were ultimately inadequate. To supplement these 

 natural powers of adaptation, it was, accordingly, necessary to provide 

 some means for increasing the oxygen delivered to the lungs. To do 

 this, Paul Bert simply provided small containers of oxygen, with tubes 

 which could be inserted in the mouth. 



For 50 years this simple expedient satisfied the needs of airmen, 

 excepting the few who went to unusual heights. But now the strategy 

 and tactics of aerial war make operations above 30,000 feet a routine 

 event, and new problems have arisen. At those altitudes, the oxygen 

 in the atmosphere is so low that care must be taken to seal the nose 

 and mouth against the outside air, lest the oxygen drawn from the 

 reservoirs be too much diluted. To accomplish this, facial masks have 

 been designed which open to the air through a valve only during 

 expiration. It is thus possible to deliver pure oxygen to the flyer. 



If this be done for the 10-man crew of a bomber on a 6-hour mission, 

 hundreds of pounds of oxygen tanks are required. This, in turn, re- 

 duces the gasoline and bomb load by a corresponding amount, and the 

 airmen, although physiologically protected, accomplish less on their 

 military mission. To make the most effective compromise between 

 human necessity and strategic efficiency, it is, accordingly, desirable to 

 supply just enough oxygen to satisfy the requirements of each indi- 

 vidual under any condition of altitude or bodily activity — just that 

 and no more. 



The natural indicators of what this need is at any moment are the 

 nerve cells of the respiratory centers, which regulate the rate and 

 depth of respiration. Their action can, in turn, be made to control the 

 flow of oxygen to the aviator's mask, by placing in the supply line a 

 regulating valve which is activated by the suction created by each 



