RESPIRATION 529 



is saturated to the extent of about 80 per cent., and a further 

 rise causes only a slight increase in the saturation. 



(2) The giving off of 0.-> from the haemoglobin depends 

 upon the hydroc^en ion concentration of the blood which is 

 mainly determined by the tension of CO^. With a pressure 

 of SO mm. Hg of C0„ 75 per cent, of the oxygen is given off 

 at an oxygen pressure of 20 mm. ; with 5 mm. Hg pressure only 

 about 30 per cent, is given at the same pressure (fig. 202). 

 With a low C0„ tension the blood may pass through the tissues 

 and the respiratory centre and remain of a bright red colour 

 because it has not parted with its oxygen, and the tissues and 

 the centre may thus have an inadequate supply. 



A due proportion of COo in the blood is therefore of 

 importance in securing an adequate transference of Oo to the 

 centre. 



When oxygen deficiency is sufficiently marked, the re- 

 spiratory centre not only becomes more sensitive to the stimu- 

 lating action of C0._,, but may be stimulated quite apart 

 from any action of C0„. This is seen in the oxygen-want 

 experienced in high aviation and in re-breathing air from which 

 the COo is removed by passing over soda lime. The rate 

 rather than the depth of breathing is increased. 



This increase may lead to an increased intake of oxygen 

 although, on account of the rapid shallow breathing, it does not 

 necessarily do so (p. 526). It may thus fail to relieve the 

 condition, because at the same time the COo is driven out 

 of the blood and thus the giving off of oxygen to the respiratory 

 centre may be decreased. 



The result is that, under the continued want of oxygen, the 

 respiratory centre may fail, the breathing stop, and death 

 supervene, as is seen in death from asphyxia (p. 548). 



Fortunately, the decreased supply of Oo to the muscles 

 leads to a failure to oxidise the sarcolactic acid, which may 

 accumulate in the blood and by causing an acidosis — an increase 

 in the Ch of the blood — may stimulate the respiratory centre, 

 and at the same time facilitate the giving off of Oo to the 

 tissues. 



The primary increase of breathing with oxygen-want 

 appears to be due to a direct action on the centre, and not 

 as was formerly supposed to the development of an acidosis. It 

 34 



