RESPIRATION 191 



made in this way by various physiologists is, for oxygen in the 

 blood 72-2 millimetres mercury pressure, for carbonic acid 

 20-5 millimetres mercury pressure. At a glance it is seen that, 

 since the tension of oxygen in the blood never exceeds 72 milli- 

 metres, whereas its tension in pulmonary air never falls beneath 

 124 millimetres, there is no difficulty in accounting for its 

 passage from air to blood. The position is somewhat otherwise 

 with regard to carbonic acid. Aeration continues in the lungs 

 until the tension of this gas in the blood returning to the heart 

 does not exceed 20-5 millimetres ; whereas the tension in pul- 

 monary air, even accepting the lowest figure obtained by ex- 

 perimental means, is as high as 19-5 millimetres. This leaves a 

 very small margin of pressure to account for the escape and it 

 is undoubtedly a rapid escape of carbonic acid from blood as 

 it circulates through the lungs. As was said regarding the 

 fixation of carbonic acid in the blood, it is somewhat doubtful 

 whether the problem has been completely solved. 



The carbonic acid exhaled contains all the carbon of the 

 digestible food, with the exception of a comparatively small 

 quantity given off in urea. It amounts to about 900 grammes 

 per diem. 



How are we to determine the quantity of air which an 

 individual requires ? We can but make the general statement 

 that it must be sufficient to dilute the carbonic acid exhaled to 

 an extent which precludes poisoning. It is impossible to fix a 

 limit. Breathing becomes embarrassed, and frontal headache 

 and other symptoms make themselves felt when 10 per cent, of 

 pure carbonic acid is mixed with air. Even in so large a pro- 

 portion as this, carbonic acid is not fatal to life. Yet an atmo- 

 sphere in which there is present a hundredth part of this amount 

 of carbonic acid, produced by respiration, is extremely injurious 

 to health under the ordinary conditions in which people live. 

 It may be asserted, therefore, that under ordinary conditions 

 0-1 per cent, is the extreme limit for wholesome living. But 

 again we are obliged to add that air contaminated to this extent 

 is not under all circumstances injurious to health. The explorers 

 on the recent Antarctic Expedition were obliged at times to 

 sleep three men in one sleeping-bag, with the aperture of the 

 bag tightly closed. The atmosphere must have been heavily 

 laden with carbonic acid. Dr. Wilson assures us that it was 



