THE RESPIRATORY SYSTEM. 



269 



taken up by the blood. Thus the lactic acid, by producing hyperpnoea, 

 serves to protect the tissues from the ill effects of a lack of oxygen. 



The presence of an excess of oxygen in alveolar air has no effect on 

 the respiratory movements, and wide variations may occur in the 

 partial pressure of oxygen in the alveolar air without any appreciable 

 change taking place in the respiration ; even when pure oxygen is 

 breathed, respiration is unchanged in a healthy individual. It is 

 evident, therefore, that, provided the supply of oxygen is adequate, the 

 depth of the respiratory movements is normally regulated by the tension 

 (partial pressure) of carbonic acid in the alveolar air and in the blood. 



EFFECT OF CHANGES IN PARTIAL PRESSURE OF OXYGEN AND CARBONIC ACID. 



Further, this tension is extremely constant, though, as shown in 

 the foregoing table, the actual percentage of carbonic acid in the lungs 

 varies with the barometric pressure. 



For example, when the barometric pressure is 1260 mm. Hg, the 

 alveolar air contains 3'52 per cent, carbonic acid; this represents a 



partial pressure of 



3-52x1260 



760 



= 5'6 per cent, at 760 mm. Hg pressure. 



ASPHYXIA. 



The effects of a rise in the tension of carbonic acid and of lack of 

 oxygen in the blood are seen in their most extreme form in asphyxia, 

 and affect not only the respiratory, but also the circulatory system. 

 Asphyxia may be brought about by occlusion of the trachea, by the 

 absence of oxygen in the air breathed, and in other ways, and three 

 stages are usually described. 



(1) Stage of Hyperpnoea. During this period, which lasts from 

 J to 1 minute, the respiratory movements gradually increase in depth, 

 and soon involve not only the muscles usually employed in respiration, 

 but also the accessory muscles. The respiratory movements during 

 this stage are co-ordinate, and show an alternate inspiratory and 

 expiratory rhythm. Consciousness is lost at the end of this stage ; the 



