254 FUNCTIONS OF NUTRITION. 



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 the blood in a free form, but in combination with the alkaline 

 carbonates of the plasma ; since a watery solution of sodium bicarbon- 

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

 carbonate by being- subjected to a vacuum, or even by agitation with 

 hydrogen at the temperature of the body. Lehrnann* found 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 which a certain quantity 

 of the carbonic acid previously given off must have been united in the 

 form of bicarbonate. 



It is estimated by Bert, from the experiments of Fernet, that a por- 

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

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

 with this gas, containing about three-fifths in a state of solution, and 

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

 ever, what this proportion is in the living body; and the large 

 amount of carbonic acid removable by a vacuum does not represent 

 accurately that which is capable of exhalation through the pulmo- 

 nary membrane. This quantity is very much smaller. We know that, 

 on the average, 13 cubic centimetres of carbonic acid are discharged 

 from the lungs, in man, with each expiration ; and during this interval, 

 judging from the capacity of the heart, and its frequency of pulsation, 

 there can hardly be less than 400 cubic centimetres of blood, pass- 

 ing through the pulmonary circulation. This would give only a little 

 over three 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 mercurial air-pump, in the experiments of Lud- 

 wig, give this quantity as the actual difference between venous and 

 arterial blood, as follows : 



AVERAGE QUANTITY OF OAKBONIO 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, indif- 

 ferent gases, or the air-pump, though differing in the quantity extracted, 

 show that there is less carbonic acid in arterial than in venous blood, 

 and accordingly that this gas is exhaled from the circulating fluid during 

 its passage through the lungs. 



Unlike oxygen, the carbonic acid of the blood is principally contained 

 in the plasma, and not in the globules ; since the serum has nearly the 

 same capacity of absorption for this gas as 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 



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



