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A TEXTBOOK OF PHYSIOLOGY 



ventilation is then due in part to the lactic acid formed within the 

 muscles. This increase in ventilation is seen from the following table : 



Resting 

 Walking 

 Running 

 Swimmit 



Running up and down stairs (greatest ) 

 possible effort of a noted swimmer) / 



190 



over 60 



It will be seen that, with moderate exercise, the greater ventilation 

 is brought about by increased depth rather than increased frequency. 

 The frequency of respiration, even the sensation of depth of respira- 

 tion, is no guide to the actual amount of ventilation. Thus, when 



FIG. 157. 



Upper tracing, respiration by diaphragm slip; lower tracing blood -pressure. During 

 first period 9-6 per cent. C0 2 in air ; during second period 10 per cent. CO 2 with 

 33 per cent. O 2 . Time every two seconds. (F. H. Scott.) 



reclining on a couch after a sea-bathe, one has the sensation of deep 

 prolonged breaths, and imagines a great ventilation of the lungs is 

 taking place. When measured, such ventilation may amount to but 

 4 to 6 litres per minute. Frequent shallow breathing may, in realitj 7 , 

 put but little air into the lungs. Such breathing takes the air in and 

 out of the mouth and trachea rather than into the lung alveoli. 



It is, then, the hydrogen ion concentration of the arterial blood 

 to which the respiratory centre responds. During normal life and 

 health the centre reacts sharply to the slightest differences in the 

 hydrogen ion concentration differences so slight that they cannot 

 be demonstrated by the present methods of blood gas analysis; 

 special electrical methods are required to show them. The regulation 

 of the hydrogen ion concentration seems to depend chiefly upon the 

 kidney. The hydrogen ion concentration of the urine alters enor- 



