EFFECTS OF CHANGES IN AIR BREATHED 1103 



to react in this way was evolved through generations of mountain climbing. 

 We are probably here making use of an adaptation which has been evolved 

 for the purpose of retrieving loss of blood by haemorrhage, such as must have 

 been of continual occurrence in the struggle of individual against individual, 

 which has resulted in the survival of the animals of to-day. 



ALTERATIONS IN THE NITROGEN TENSION. The nitrogen of the 

 atmosphere plays no part in the metabolism of the body, and must be 

 regarded as a purely inert gas. It is a matter of indifference whether under 

 normal atmospheric pressure we breathe an atmosphere of pure oxygen or 

 one containing one-fifth part of this gas diluted with four-fifths of nitrogen. 

 The very inertness of nitrogen may be of danger to the body under certain 

 conditions. If a man or an animal be exposed, as in a diving-bell, to a 

 pressure of three, four, or six atmospheres, the respiratory functions are 

 unaffected, but the amount of nitrogen dissolved in the fluids of the body 

 is increased in direct proportion to the pressure. If the pressure be now 

 suddenly released, the nitrogen, which cannot be used up by the tissues, is 

 given off from the body-fluids in the form of bubbles, just as carbonic acid 

 gas rises in bubbles from soda-water when the pressure is removed by with- 

 drawing the cork from the bottle. These bubbles occurring in all the 

 capillaries obstruct the flow of blood, and therefore, if the evolution of gas is 

 sufficiently large, the animal dies in convulsions. A similar evolution of gas 

 may occur in the spinal cord, giving rise to destruction of the cord and 

 paralysis (' divers' palsy '). In order to prevent this sudden evolution of 

 gas it is necessary that the change from the high pressure to the ordinary 

 atmospheric pressure should be carried out gradually, so as to give the 

 blood-plasma, supersaturated with nitrogen, time to get rid of its excess of 

 nitrogen without the formation of bubbles. 



OTHER GASES. Hydrogen and methane are, like nitrogen, indifferent 

 gases. They may be respired if mixed with 20 per cent, of oxygen, and 

 either of the gases may be used instead of nitrogen to dilute the oxygen that 

 we breathe, without harm or inconvenience. 



Carbon monoxide is rapidly poisonous by its action on the red corpuscles. 

 It combines with haemoglobin, forming CO-hsemoglobin, a compound which 

 is much more stable than oxyhaemoglobin. The blood is therefore deprived 

 of its oxygen carrier, and the animal dies of asphyxia. We have seen, 

 however, that the displacement of oxygen by CO is not absolute, but only 

 relative. Hence, although the avidity of CO for haemoglobin is 140 times 

 that of oxygen, we can convert the CO back into oxyhaemoglobin by in- 

 creasing the mass influence of the oxygen. This may be done by giving the 

 poisoned animal pure oxygen to breathe, or even oxygen under pressure. 

 In pure oxygen at a pressure of two atmospheres an animal can breathe and 

 live, even though the whole of its haemoglobin is converted into CO-haemo- 

 globin, the amount of oxygen which is simply dissolved by the blood-plasma 

 being sufficient at this pressure for the respiratory needs of the animal 

 (Haldane). 



Other gases which have special poisonous properties are hydrocyanic 



