43 6 TEXT-BOOK OF PHYSIOLOGY 



than normal the respiratory movements will be increased in frequency and 

 depth, while a corresponding diminution in the percentage of oxygen will 

 be without effect. 



It has been shown by Haldane and Priestley that when an individual was 

 breathing normal air and the rate of the respiratory movement, 14 per minute, 

 the average depth was 637 c.c. and the total ventilation was 8.918 liters per 

 minute. On raising the percentage of the CO 2 in the inspired air from 

 0.04 per cent, to 0.79 per cent, the average depth increased to 739 c.c. and 

 the total ventilation to 10.346 liters per minute, the rate remaining the same. 

 When the percentage of the CO 2 was raised to 2 per cent, the average depth 

 increased to 864 c.c., the rate to 15, and the total ventilation to 12.960 liters 

 per minute; and when the CO 2 in the inspired air was raised to 6 per cent, 

 the average depth was increased to 2104 c.c., the rate to 27 per minute, and 

 the total ventilation to 56.808 liters. The results of these experiments 

 indicate that an increase in the percentage of the CO 2 in the inspired air 

 leads to an increase in the percentage and pressure of the CO 2 in the ar- 

 terial blood and hence in the inspiratory center, as a result of which the 

 center becomes more irritable and discharges its energy more frequently 

 and to a greater degree as shown by the increase in the rate and the depth 

 of the inspiratory movement. 



The same observers have also shown that when an individual is caused 

 to inhale air the percentage of the oxygen of which had been reduced from 

 20 to 13 and therefore to about 8 per cent, in the alveolar air instead of about 

 15 per cent, no particular change in either the frequency or the depth of 

 the inspiratory movements was noticed, but when the percentage of the 

 oxygen was lowered below this amount the inspiratory center became more 

 irritable as shown by an increase in the rate and depth of the inspiratory 

 movement. As a rule the oxygen percentage in the alveolar air must be 

 reduced fully one-half and thereby the percentage and pressure of the oxygen 

 in the arterial blood fully one-third before the respiratory center is stimulated 

 to increased activity. A reason assigned for this result is the presence in 

 the blood of some non-oxidized metabolic product, probably lactic acid, that 

 is acting as the stimulus. All recent experimental work confirms the view 

 that the specific stimulus to the inspiratory center is the normal pressure of 

 the CO 2 in the blood and so responsive is it to this agent that an increase 

 in even 0.2 per cent, in the alveolar air is sufficient to almost double the 

 respiratory ventilation. 



The Establishment of Respiration after Birth. Previous to birth 

 the exchange of the gases which constitutes the respiratory activity in the 

 mammalian fetus, takes place in the placenta. The venous blood is carried 

 by the umbilical artery to this organ in which the blood of the fetus comes 

 into close relation with the blood of the mother, the two fluids being separated 

 only by an extremely thin partition. The venous blood while passing 

 through the placenta yields up its carbon dioxid to and receives oxygen 

 from the maternal blood; after this exchange of gases the now-arterialized 

 blood is returned to the fetus by the umbilical veins. 



Immediately after birth and the detachment of the placenta, this method 

 of gaseous exchange is abolished and if the life of the child is to be main- 

 tained the respiratory movement must be established. The cause of the 

 first inspiration, therefore, must be associated with an increase in the per- 



