CHANGES IN THE BLOOD BY RESPIRATION. 297 



portion combined with the alkaline salts ; the blood, when artificially 

 saturated with this gas, containing about three-fifths in a state of solu- 

 tion and about two-fifths in a state of combination. We do not know, 

 however, what this proportion is in the venous blood as it exists in the 

 living body ; and the large amount of carbonic acid removable by the 

 action of a vacuum does not represent that which is capable of being 

 exhaled from the blood through the pulmonary membrane. This quan- 

 tity is very much smaller. We know that, on the average, 13 cubic 

 ceniirnetres of carbonic acid are discharged from the lungs in man with 

 each expiration ; and during this interval, judging from the capacity 

 of the left auricle and the frequency of its pulsations, there can hardly 

 be less than 400 cubic centimetres of blood passing through the pulmo- 

 nary circulation. This would give only a little over 3 per cent, as the 

 volume of carbonic acid discharged from a given quantity of blood in 

 respiration. The average results obtained by extraction with the mer- 

 curial air-pump, in the experiments of Ludwig, give this quantity as 

 the actual difference between venous and arterial blood, as follows : 



AVERAGE QUANTITY OF CARBONIC ACID REMOVABLE BY THE AIR-PUMP, FROM 



Venous blood 31.27 per cent. 



Arterial blood 27.99 " 



Difference 3.28 



All the different modes of analysis, whether by carbonic oxide, other 

 indifferent gases, or the air-pump, though differing in the quantity of 

 gas extracted, show that there is less carbonic acid in arterial than in 

 venous blood, and accordingly that this gas is exhaled from the circu- 

 lating fluid during its passage through the lungs. 



Unlike the oxygen, the carbonic acid of the blood is principally con- 

 tained in the plasma^ and not in the blood globules ; since the capacity 

 of absorption for this gas is not essentially different for the serum and 

 for the entire blood. 



Source of the Carbonic Acid of the Blood The source of the car- 

 bonic acid of the blood, as well as the destination of its oxygen, is in 

 the tissues themselves. From the experiments of various observers it 

 is found that every organized tissue, in the recent condition, has the 

 power of absorbing oxygen and exhaling carbonic acid. G. Liebig, 

 for example, showed that frogs' muscles, recently prepared and com- 

 pletely freed from blood, will continue to absorb oxygen and discharge 

 carbonic acid. Similar experiments with other tissues have led to the 

 same result. It is in the substance of the tissues, accordingly, that the 

 oxygen becomes fixed and assimilated, and that the carbonic acid takes 

 its origin. These two phenomena, however, are not immediately de- 

 pendent upon each other. This is shown by the fact that animals and 

 fresh animal tissues will continue to exhale carbonic acid in an atmo- 

 sphere of hydrogen or of nitrogen, or even when placed in a vacuum. 

 Marchand found that frogs would live for from half an hour to an hour 

 20 



