318 PULMONARY MUCOUS TISSUE. 



haled in 24 hours, only 1 kilo. 750 grms. remains; that is to 

 say, that 750 grms. of oxygen have been retained by the lungs 

 (2.500 1.750 = 750). We see thus that we retain a sum 

 total off of a kilo, (or 750 grms.) of oxygen in 24 hours (750 

 grms., or a volume of 500 litres). 



On the other hand, we know that the carbonic acid is 

 found represented by thousandths only in the atmospheric 

 air, the air which we breathe (^\nr or TTTOIRT)- ^he quantity 

 found in the exhaled air is very large : but yet it differs, ac- 

 cording to circumstances, though, on the average, we may be 

 gaid to exhale 850 grms. of carbonic acid in 24 hours (a volume 

 of 400 litres : in order to account for the diminution of volume 

 which we have observed between the air inhaled and that 

 which is exhaled, compare these figures with the 500 litres of 

 O. absorbed.) These are the principal facts to be noted in re- 

 gard to the air ; the other changes which take place are unim- 

 portant. Thus air contains f parts nitrogen (21 of O., 79 of N".), 

 the quantity of this gas inhaled and exhaled is supposed by 

 some persons to be equal : others maintain that it varies, and 

 that, at times, a little more than the usual quantity is given 

 off, showing that a certain quantity is excreted by the lungs : 

 indeed, traces of ammonia and various exhalations arising 

 from nitrogenous substances are frequently met with in the 

 lungs, as well as the vapors belonging to all those volatile 

 matters which sometimes find their way into the blood, as, 

 for instance, alcohol, ether, phosphorated products, and palu- 

 dal .gases. 



B. Modifications in the blood which passes through the 

 lungs. 



What is the process which goes on in the blood ? Experi- 

 ments have proved, what our previous knowledge enabled us 

 to guess, that the carbonic acid which is exhaled arises from 

 the venous blood ; the latter throwing off this excretory pro- 

 duct, imbibes oxygen in order to pass into the state of arte- 

 rial blood. We have already studied the gases of the blood, 

 and have seen that, from the point of view of respiration, the 

 blood may be looked upon as a solution of gas, the blood 

 globule being the vehicle of the oxygen, and the serum that 

 of the carbonic acid ; we have also found that the essential 

 difference between the arterial and the venous blood consists 

 in the predominance of oxygen in the former, and of carbonic 

 acid in the latter. 



