528 



VETERINARY PHYSIOLOGY 



With any increase in the amount, the respiratory centre is 

 stimulated and the respirations increased Qiypevpnoea), till the 

 tension of COo in the blood becomes normal. The power of ad- 

 justment is extraordinarily efficient, so efficient that the tension 

 of C0„ in the alveolar air is maintained at a constant level of 

 about 40 mm. Hg under wide variations of atmospheric pressure. 

 It has been found that an increase of 0*2 per cent, in the COo 

 of the alveolar air, i.e. a rise of tension from 40 to 41-6 mm. Hg 

 is sufficient to double the ventilation of the lungs. 



By forced breathing the tension of CO 2 in the alveolar 

 air may be so lowered that the tension in the blood is 

 markedly reduced, and the stimulus to the respiratory centre 

 so decreased, that a long period without breathing, an ajmcea, 

 may occur. In fact, in some cases in man, the face may become 

 livid from want of oxygen before breathing is re-established. 



/ 

 45 



Alveolar Air 



COo tension 

 40 mm. Hg. 



40 



Tenision of CO., in Bl 



Hyperpnoea 



in mm. Hg. 



Normal Breathing 



\ 



35 



ood 



Apnoea 



B. Oxygen. 



The influence of the oxygen content of the blood upon the 

 respiratory centre is not so manifest. 



The amount and tension of 0„ in the blood, and its transit 

 to the tissues and the respiratory centre, are in some respects 

 more complex than in the case of CO^. 



As already explained (p. 493)— (1) the amount in the blood is 

 not directly proportional to the partial pressure of the gas to 

 which it is exposed. As the latter rises from zero the amount in 

 the blood rapidly increases, till at 50 mm. Hg the haemoglobin 



