CHANGES IN THE BLOOD DURING RESPIRATION. 245 



and arterial blood. Magnus 1 found that the proportion of oxygen 

 to carbonic acid, by volume, in arterial blood was as 10 to 25 ; in 

 venous blood as 10 to 40. The venous blood, then, as it arrives at 

 the lungs, still retains a remnant of the oxygen which it had pre- 

 viously absorbed ; and in passing through the pulmonary capil- 

 laries it gives off only a part of the carbonic acid with which it has 

 become charged in the general circulation. 



The oxygen and carbonic acid of the blood exist in a state of 

 solution in the circulating fluid, and not in a state of intimate chemi- 

 cal combination. This is shown by the fact that both of these 

 substances may be withdrawn from the blood by simple exhaustion 

 with an air-pump, or by a stream of any other indifferent gas, such 

 as hydrogen, which possesses sufficient physical displacing power. 

 Magnus found' that freshly drawn arterial blood yielded by simple 

 agitation with carbonic acid more than 10 per cent, of its volume 

 of oxygen. The carbonic acid may also be expelled from venous 

 blood by a current of pure oxygen, or of hydrogen, or, in great 

 measure, by simple agitation with atmospheric air. There is some 

 difficulty in determining, however, whether the carbonic acid of 

 the blood be altogether in a free state, or whether it be partly in a 

 state of loose chemical combination with a base, under the form of 

 an alkaline bicarbonate. A solution of bicarbonate of soda itself 

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

 condition of a carbonate by simple exhaustion under the air-pump, 

 or by agitation with pure hydrogen at the temperature of the body. 

 Lehmann has found 3 that after the expulsion of all the carbonic 

 acid removable by the air-pump and a current of hydrogen, there 

 still remains, in ox's blood, 0.1628 per cent, of carbonate of soda ; 

 and he estimates that this quantity is sufficient to have retained all 

 the carbonic acid, previously given off, in the form of a bicarbonate. 

 It makes little or no difference, however, so far as regards the pro- 

 cess of respiration, whether the carbonic acid of the blood exist in 

 an entirely free state, or under the form of an alkaline bicarbonate ; 

 since it may be readily removed from this combination, at the tem- 

 perature of the body, by contact with an indifferent gas. 



The oxygen and carbonic acid of the blood are in solution prin-~ 

 cipally in the blood- globules, and not in the plasma. The researches 

 of Magnus have shown 4 that the blood holds in solution 2J times 



1 In Lehmann, op. cit., vol. i. p. 570. 



2 In Robin and Verdeil, op. cit., vol. ii. p. 34. 



3 Op. cit., vol i. p. 393. 



In Robin and Verdeil, op. cit., vol. ii. pp. 2832. 



