288 HUMAN PHYSIOLOGY 



it. But a moderate amount of water is constantly 

 formed as a second respiratory product. The oxygen 

 which goes to form water, of course, fails to appear in 

 combination with carbon. Later on we shall find that 

 the proportion between the oxygen absorbed and the 

 carbon dioxid evolved by an animal furnishes valuable 

 information to the student of metabolism. 



If the expired air contain 16 per cent, of oxygen and 

 4 per cent, of carbon dioxid, as we have assumed, we 

 must still bear in mind that the first portion of each 

 expiration is from the dead space and hence almost 

 like fresh air. It follows that the last part of each ex- 

 piration is lower in oxygen and higher in carbon dioxid 

 than the average of the whole and that it is a fair sample 

 of the air in the terminal sacs of the lungs. Such air 

 may contain nearly 6 per cent, of carbon dioxid and 

 something like 14 per cent, of oxygen. It is the alveolar 

 air and it is this which we must consider as standing in 

 relation to the blood in the pulmonary capillaries. 



The nitrogen which makes 79 per cent, of the atmos- 

 pheric air is not believed to be drawn upon or added 

 to by the blood. This is the case, at any rate, in the 

 long run. When there is a rise of barometric pressure 

 there is doubtless some absorption of additional nitrogen 

 by the blood and when the pressure falls again there is 

 some yielding up of nitrogen. But this is an accidental 

 matter and not related to the life-processes. 



Whether anything of an organic nature passes out 

 with the expired air is a question that has been much 

 discussed and perhaps not fully decided. It used to 

 be asserted that the breath carries from the body com- 

 pounds of a volatile and poisonous character. This has 

 been difficult to establish by experiment. Of course the 

 breath may have an odor and this indicates the presence 

 of some substance but not in quantity to be measured 

 by other tests than the olfactory. Odor, when present, 

 is not likely to be due to anything coming from the lungs 



