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CHAP. XXIX.] CHANGES IN THE BLOOD. 411 



exhaled from the lungs of man, was about yo^ n ^ ^ ne anaount of 

 carbonic acid removed in the same time. It must also be observed, 

 that a very small proportion of nitrogen is constantly being expired 

 in the form of ammonia. 



Changes in the Blood resulting from Respiration. Dark venous blood 

 in its passage through the capillary vessels of the lungs assumes the 

 bright red colour characteristic of arterial blood. This change, as 

 is well known, depends upon the removal of carbonic acid gas and 

 the absorption of oxygen. Magnus showed that venous blood con- 

 tained 25 per cent, of its volume of carbonic acid, and 5 per cent, 

 of oxygen ; and that arterial blood, on the other hand, contained as 

 much as 10 per cent, of its volume of oxygen, and only 20 per cent. 

 of carbonic acid. The seat of this change is clearly in the red 

 blood corpuscles; but its precise nature has not yet been satisfactorily 

 determined. It has been ascribed to a chemical change taking 

 place in the coloured constituent of the blood globule (page 304), 

 but later researches render it probable, that it is, at least in some 

 measure, due to a physical alteration. Henle was the first ob- 

 server who referred the change of colour in the blood corpuscle to 

 a change in its form. 



Scherer states that the blood corpuscles of venous blood are 

 nearly spherical, and their walls thin and transparent, a condition 

 which causes them to transmit light freely. On the other hand, 

 the corpuscles of arterial blood are bi-concave, their walls thicker, 

 and they reflect light more readily, which is considered to account 

 for the brighter colour of arterial blood. Harless has even been 

 able to measure the difference in size between the blood corpuscles 

 of arterial and venous blood of the frog. 



Magnus proved that serum would dissolve twice as much car- 

 bonic acid as an equal quantity of pure water, a power which 

 Liebig attributed to the amount of phosphate of soda contained in 

 blood serum. 



Of the gases existing in the blood, a very small proportion only 

 is in chemical combination with any of the constituents of that 

 fluid, but the greater quantity is held in solution in a free and un- 

 combined state. This is readily proved by passing a current of 

 hydrogen gas through some defibrinated blood placed in a bottle to 

 which tubes are adapted. The hydrogen takes the place of the 

 gases previously held in solution, and the latter escape ; and, by 

 causing them to pass through lime-water, evidence of the presence 

 of much carbonic acid is at once obtained by the formation of a 

 precipitate of carbonate of lime. If the carbonic acid had existed 



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