RESPIRATION. 275 



this substance cannot be always detected, and from its minute amount 

 when present, that the whole of it may be derived from decompos- 

 ing particles of food left in the mouth, or from carious teeth or the 

 like; and that it is, therefore, only an accidental constituent of expired 

 air. 



7. The quantity of organic matter in the breath is increased. It was 

 formerly supposed that this organic matter was injurious and gave rise 

 to the unpleasant symptoms which come on in badly ventilated rooms. 

 But this has been proved erroneous. 



Method of Experiment. The experiments are conducted in such a manner that 

 comparative analyses may be made between the air inspired and that expired. 

 Generally an animal is placed in a chamber, called the respiratory chamber, having 

 but two openings one for the entrance of the inspired air, the other for the escape 

 of expired air. Some form of pump is used for renewing the air in the chamber. 

 Both the inspired and expired air is made to pass through agents which will absorb 

 the contained carbon dioxide, such as baryta water or soda lime, and in turn through 

 agents which will absorb the watery vapor. When the experiment is completed 

 the differences between the two are determined. The difference in oxygen has to 

 be calculated, and is open to error. The famous respiratory chamber of Petten- 

 kofer is large enough to perform such experiments on man, and is of very elaborate 

 construction. 



How the Changes in the Air are effected. The method by which 

 fresh air is inhaled and expelled from the lungs has been explained. 

 It remains to consider how it is that ,the blood absorbs oxygen from, 

 and gives up carbonic acid to, the air of the alveoli. In the first place, 

 it must be remembered that the tidal air only amounts to about 25 30 

 cubic inches (about 500 ccm.) at each inspiration, and that this is of 

 course insufficient to fill the lungs, but it mixes with the stationary air 

 by diffusion, and so supplies to it new oxygen. The amount of oxygen 

 in expired air, which may be taken as the average composition of the 

 mixed air in the lungs, is about 16 to 17 per cent; in the pulmonary 

 alveoli it may be rather less than this. From this air the venous blood 

 has to take up oxygen in the proportion of 8 to 12 vols. per cent of 

 blood, as the difference between the amount of oxygen in arterial and 

 venous blood is no less. It seems therefore somewhat difficult to under- 

 stand how this can be accomplished at the low partial pressure of oxygen 

 in the pulmonary air. But as was pointed out in a previous Chapter 

 (V.), the oxygen is not simply dissolved in the blood, but is to a great 

 extent chemically combined with the haemoglobin of the red corpuscles; 

 and when a fluid contains a body which enters into loose chemical com- 

 bination in this way with a gas, the tension of the gas in the fluid is 

 not directly proportional to the total quantity of the gas taken up by 

 the fluid, but to the excess above the total quantity which the substance 

 dissolved in the fluid is capable of taking up (a known quantity in the 

 case of haemoglobin, viz., 1.59 cm. for 1 grm. haemoglobin). On the 



