PHYSIOLOGICAL CHEMISTRY 459 



quantity of lactic by the muscles, which thus passes into the blood, is, 

 therefore, a direct result of great muscular activity. 



This increased production of lactic acid serves a very useful purpose 

 in making the blood more acid, and so stimulating the respiratory 

 centre, and probably the heart also. The increase of pulse rate, blood 

 pressure and respiration produced by violent exercise are matters 

 of common observation. Although the output of carbon dioxide is much 

 increased, the alveolar carbon dioxide (see p. 197) is not high, except for 

 a short time after the start, and falls markedly below the normal level 

 for about half an hour after the period of violent exercise, so that the 

 increased respiration cannot be due to carbon dioxide alone. If, how- 

 ever, we suppose that the activity of the respiratory centre is always 

 regulated by the acidity of the blood, which depends on the amounts 

 of carbonic and other acids present, we see how lactic acid may take 

 the place of carbon dioxide in increasing the activity of the respiratory 

 centre. 



Experimentally in animals the urinary output of lactic acid may be 

 greatly increased by reducing the supply of oxygen, or by carbon 

 monoxide poisoning. The experiment has to be continued for several 

 hours with the animal in the collapse stage of asphyxia. In man 

 increased lactic acid excretion has occurred from long-continued acci- 

 dental carbon monoxide poisoning, but not as yet from voluntary 

 shortage of oxygen. Shortage of oxygen is not, therefore, as efficient 

 a cause of increased production of lactic acid as muscular activity, 

 although respiration is increased and the alveolar carbon dioxide much 

 reduced even in short experiments on man. It may be that in this 

 case the respiratory centre is the first to feel the lack of oxygen, and is 

 stimulated by lactic acid, which it itself produces, instead of by the 

 acidity of the general blood stream. 



The results of the experiments of Fletcher and Hopkins on surviving 

 frogs' muscles are in general agreement with the above results from 

 man. These observers showed that the muscles of frogs at rest contain 

 very little lactic acid, provided the muscles are killed with a minimum 

 amount of stimulation. For this purpose they cooled the hind limbs 

 with ice, then rapidly separated the muscles and ground them up with 

 ice-cold alcohol and sand. The lactic acid was isolated as anhydrous 

 zinc lactate after ether extraction. Resting fresh muscle gave 0*03 to 

 0*045 per cent, zinc lactate. When the muscle was tetanised the lactic 

 acid rose, until when irritability was lost by fatigue the yield of zinc 

 lactate was about 0-2 per cent. If the muscle was then kept in oxygen, 

 the irritability returned and the zinc lactate fell to about 0*1 per 

 cent. Oxygen inhibited, but did not entirely prevent, the onset of 



