1226 PHYSIOLOGY 



the oxygenation of the blood is already insufficient, marked amelioration of the 

 symptoms may be produced by inhalation of pure oxygen. Especially is 

 this noticeable where there is failure of the heart. In these cases the heart, 

 already affected, is unable to keep up an adequate circulation and to supply itself 

 with sufficient oxygen. A vicious circle is thus established in which the heart 

 tends to get steadily worse. By administration of oxygen an adequate supply 

 of this gas to the heart-muscle is assured ; the heart-beat therefore becomes more 

 effective and the whole circulation is improved and therewith the provision of 

 oxygen to the body at large. 



If a warm-blooded animal be immersed in a chamber and sub- 

 mitted to pure oxygen at a pressure of four atmospheres, it dies as 

 rapidly as if it were in an atmosphere of pure nitrogen. At this pressure 

 the oxidative processes of the body as well as the intake of oxygen into 

 the lungs are absolutely abolished. It is interesting to note that certain 

 other oxidative phenomena, e.g. the spontaneous oxidation of phos- 

 phorus, also cease if the tension of the oxygen be sufficiently high. 

 Exposure of an animal over a considerable period of time to a pressure 

 of oxygen of two atmospheres may, as Haldane and Lorraine Smith 

 have shown, set up severe inflammation of the lungs and thereby cause 

 death indirectly. 



CHANGES IN TENSION OF OXYGEN. If a man breathe a mixture 

 of nitrogen and oxygen free from carbon dioxide, and the oxygen be 

 gradually diminished, no feeling of ' want of breath ' may be ex- 

 perienced. With percentages of oxygen as low as 12 per cent, there may 

 be no change in the breathing, even though the deficient oxygenation 

 of the blood may be shown by the blueness of the lips and face. If the 

 oxygen be reduced still lower, a certain amount of hyperpnoea may 

 occur, but in many cases the individual experimented on may not feel 

 any ill effects until he suddenly becomes unconscious from lack of 

 oxygen. If fresh oxygen be not supplied this unconsciousness may 

 be followed by convulsive movements and death. If the administra- 

 tion of low percentages of oxygen, e.g. about 10 to 12 per cent, of an 

 atmosphere, be continued for some time, the subject of the experi- 

 ment may suffer considerable discomfort. One of the signs of oxygen 

 lack is often severe headache, and this may be accompanied by vomit- 

 ing or nausea and by a feeling of discomfort in the precordial region. 

 Many experiments have been made both on animals and man by 

 submitting them to a lowered atmospheric pressure in chambers 

 specially built for the purpose. The limit to which the pressure may 

 be reduced varies in different individuals, the variations being deter- 

 mined by the type of respiratory movement of the individual in 

 question, since on the depth of respiration depends the relation 

 between the tension of oxygen in the alveoli and that in the inspired 

 air. The lowest limit at which life is possible corresponds to an oxygen 

 tension in the alveoli of 27 to 30 mm. Hg. 



