10 ORGANISM AND ENVIRONMENT 
quantity of CO, 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, 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, in 
the alveolar air would rise, and loss of consciousness 
would result. But with the slightest rise in the alveo- 
lar CO, percentage the breathing begins to increase, 
and thus keeps down the alveolar CO,. When, there- 
fore, the production of CO, 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, 
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,. Analysis of the alveolar air, 
and determination of the CO, 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, percentage, it is evident that 
regulation of the oxygen percentage is involved in 
regulation of the CO, percentage. The net result is 
that both the percentage of oxygen and that of CO, 
in the alveolar air are very constant, in spite of great 
changes in the amount of oxygen consumed and CO, 
given off by the body. 
