296 RESPIKATION. 



Finally, successive analyses of the blood, as it passes from the arte- 

 rial into the venous system, shows that it loses oxygen in proportion as 

 it has been subjected to the influence of the capillary circulation. Ber- 

 nard 1 found that the blood of the same dog, from different parts of the 

 circulatory system, 3delded, by the action of carbonic oxide, the follow- 

 ing quantities of oxygen : 



QUANTITY OP OXYGEN BY VOLUME IN 



Arterial blood 18.93 per cent. 



Yenous blood from right side of heart . . . 9.93 ' ; 

 Yenous blood from hepatic veins .... 2.80 " 



The average quantity of oxygen existing in venous blood generally is 

 8 per cent. ; that is, it is reduced about one-half from its proportion in 

 arterial blood. 



Thus the blood-globules serve as carriers of oxygen from the lungs 

 where it is absorbed, to the tissues where it is consumed ; and the first 

 object of respiration is to supply oxygen to the blood, in order that the 

 blood may supply it to the tissues. 



Exhalation of Carbonic Acid by the Blood. The venous blood, as it 

 returns to the right side of the heart, is already charged with carbonic 

 acid to such an extent that a portion of this gas is exhaled through the 

 pulmonary membrane, and discharged with the breath. Its absolute 

 quantity in the blood has not been determined with the same accuracy 

 as that of the oxygen. Carbonic oxide, which is so efficient for the 

 extraction of oxygen from the blood, displaces only a portion of its 

 carbonic acid ; and in the experiments of Bernard, the maximum quan- 

 tity of carbonic acid obtained from venous blood by this means was 

 only about 6.5 per cent, by volume. A much larger proportion may be 

 extracted by the mercurial air-pump, amounting on the average, in the 

 experiments of Ludwig, to about 28 per cent, for arterial blood, and 

 about 31 per cent, for venous blood. But a large part of the carbonic- 

 acid obtainable in this way does not exist in a free form in the blood, 

 but in a state of combination with the alkaline phosphates and carbon- 

 ates of the plasma; since it is known that a w r atery solution of sodium 

 bicarbonate will lose a portion of its carbonic acid, and become reduced 

 to the condition of a carbonate by being subjected to the influence of a 

 vacuum, or even by agitation with pure hydrogen at the temperature 

 of the ~body. Lehmann found 2 that after the expulsion from ox's blood 

 of all the carbonic acid removable by the air-pump and a current of 

 hydrogen, there still remained 0.1628 per cent, of sodium carbonate, 

 with w T hich a certain quantity of the carbonic acid previously given off 

 must have been united in the form of bicarbonate. 



It is estimated by Bert, according to the experiments of Fernet, that 

 a portion of the carbonic acid of the blood is in simple solution and a 



1 Liquides de 1'Organisme. Paris, 1859, tome i. p. 394. 



2 Physiological Chemistry, Cavendish edition. London, 1854, vol. i. p. 438. 



