THE REGULATION OF BREATHING 291 



Analyses of the alveolar air of the lungs reveal the fact that an 

 individual normally so regulates his respiration that the pressure or 

 concentration. of C0 2 in the alveolar air is kept constant, varying in 

 different individuals from about 4-5 to 6 per cent., measured at normal 

 barometric pressure. If the barometric pressure be increased as, 

 for example, by going into compressed air or down a mine the per- 

 centage of C0 2 in the lung falls inversely in proportion to the increase 

 of barometric pressure ; if it be decreased, by going up a mountain, 

 the percentage of C0 2 rises inversely in a similar proportion. The 

 partial pressure of C0 2 remains the same in the lung in each case. 

 At great altitudes, where oxygen-want comes into play, this no longer 

 holds good, for the balance of acid and base in the blood is then altered 

 by the excretory activity of the kidney. The C0 2 is reduced propor- 

 tionately by increased pulmonary ventilation, so that the total concen- 

 tration of acid in the blood remains the same. It is clear, then, that 



FIG. 156. AXJESIHETIZED DOG. 



Upper tracing, respiration; lower tracing, blood-pressure. The arrow marks the 

 injection of 30 c.c. of CO 2 and 2 saturated blood into the jugular vein. 



the partial pressure of carbon dioxide in the alveolar air and in the 

 arterial blood going to the respiratory centre normally plays an im- 

 portant part in bringing about respiration. The addition of CO 2 to 

 the air breathed immediately increases the depth of breathing 

 unconsciously when the amount is small (1 to 2 per cent.); consciously, 

 with marked hyperpncea, when the amount is larger (3 to 5 per cent.). 

 If, on the other hand, the tension of C0 in the blood be reduced as, 

 for example, by forced deep breathing this is followed by a period 

 of apncea (cessation of breathing), until the C0 2 again rises to a partial 

 pressure sufficient to stimulate the centre again. 



Normally, the breathing movements of the body are so regulated 

 that the partial pressure of C0 2 in the blood is kept almost constant. 

 When much C0. 2 is produced as, for example, in hard muscular work 

 the ventilation of the lungs is greatly increased. This increased 



