10 ORGANISM AND ENVIRONMENT 



quantity of CO 2 brought to the lungs by the blood 

 is constantly varying in accordance with varying 

 states of bodily activity. For instance during the 

 exertion of walking at a moderate rate the quantity 

 of CO 2 brought to the lungs is three or four times 

 what it is during rest. If the breathing did not 

 increase correspondingly, the percentage of CO 2 in 

 the alveolar air would rise, and loss of consciousness 

 would result. But with the slightest rise in the alveo- 

 lar CO 2 percentage the breathing begins to increase, 

 and thus keeps down the alveolar CO 2 . When, there- 

 fore, the production of CO 2 is three times what it is 

 during rest, the breathing is also increased to nearly 

 three times what it is during rest. The alveolar CO 2 

 percentage rises, it is true; but only by 0.4 per cent. 

 This slight rise produces, as we have seen, an increase 

 of 200 per cent in the breathing, so that the increase 

 in breathing is almost proportional to the increase in 

 the production of CO 2 . Analysis of the alveolar air, 

 and determination of the CO 2 produced and volume 

 of air breathed during rest and work show that this 

 explanation works out in practice, provided that no 

 disturbing causes come in. 



As the oxygen percentage in the alveolar air runs 

 parallel with the CO 2 percentage, it is evident that 

 regulation of the oxygen percentage is involved in 

 regulation of the CO 2 percentage. The net result is 

 that both the percentage of oxygen and that of CO 2 

 in the alveolar air are very constant, in spite of great 

 changes in the amount of oxygen consumed and CO 2 

 given off by the body. 



