SECTION IV 



THE EFFECTS ON RESPIRATION OF CHANGES 

 IN THE AIR BREATHED 



WE have already seen that a moderate increase in the carbon 

 dioxide percentage 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 

 administered 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 oxygenation of the blood is ineffectively carried out even in 

 the presence of excess of oxygen. The administration of larger per- 

 centages, such as 30 or 40 per cent., causes rapid death and failure of 

 the circulation and respiration, often preceded by convulsions. Co- 

 incident with the increased respiration brought about by moderate 

 percentages of carbon dioxide there is a rise of blood pressure, deter- 

 mined 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 

 movements. At the normal atmospheric pressure the tension of oxy- 

 gen 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 move- 

 ments 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, 



1225 



