SECTION IV 



THE EFFECTS ON RESPIRATION OF CHANGES IN 

 THE AIR BREATHED 



WE have already seen that a moderate increase in the carbon dioxide per- 

 centage of the air breathed (e.g. up to 4 per cent.) causes a proportional 

 increase in the ventilation of the lungs so as to maintain the tension of this 

 gas in the alveoli at the normal level. The same effect is observed whether 

 the mixture breathed contains 18 or 50 per cent, of oxygen, showing that the 

 slight diminution in oxygen content caused by mixing the air with carbon 

 dioxide is in no way responsible for the effect. If the amount of carbon 

 dioxide be increased to 12 or 15 per cent, it becomes almost impossible to 

 continue the inhalation owing to the spasm of the glottis produced by the 

 irritant effects of the carbon dioxide. If these high percentages be ad- 

 ministered to an animal 'by a tracheal tube, violent dyspnoea is produced 

 which gradually diminishes, and the animal passes into a condition of 

 narcosis in which the respiratory movements become less and the oxygena- 

 tion of the blood is ineffectively carried out even in the presence of excess of 

 oxygen. The administration of larger percentages, such as 30 or 40 per 

 cent., causes rapid death and failure of the circulation and respiration, often 

 preceded by convulsions. Coincident with the increased respiration brought 

 about by moderate percentages of carbon dioxide there is a rise of blood- 

 pressure, determined partly by vascular constriction, partly by an 

 increased output from the heart. With high percentages of carbon dioxide 

 the curve of blood-pressure obtained resembles that produced by lack .of 

 oxygen. 



Oxygen itself exercises no excitatory effects on the respiratory move- 

 ments. At the normal atmospheric pressure the tension of oxygen in the 

 alveoli is about 107 mm. Hg, a pressure which, as we have seen, is amply 

 sufficient to saturate the haemoglobin passing through the vessels of the 

 lungs. Since the depth and frequency of respiration are determined by the 

 carbon dioxide tension in the alveoli no alteration in respiration will be 

 produced by increasing the tension of oxygen in the air breathed above its 

 normal amount. The respiratory movements in an atmosphere of pure 

 oxygen will, in the normal individual, remain unchanged. 



This statement is only true for the healthy individual. If from failure of the heart 

 and circulation, or diminished oxygen tension, or severe loss of blood, the oxygenation of 



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