EESPIKATION. 595 



If, now, such a solution of reduced haemoglobin, or the crystals of 

 reduced haemoglobin, be exposed to the atmosphere, they then again 

 absorb oxygen and now change to a reddish color, the amount of oxygen 

 absorbed being again precisely similar to that which was given off to the 

 vacuum. 



The above phenomena serve to explain what occurs in the process 

 of respiration. As the venous blood leaves the right ventricle, its haemo- 

 globin has already largely lost its oxygen by passing through the sys- 

 temic capillaries, and is now mainly reduced haemoglobin. It absorbs 

 oxygen from the atmosphere through the walls of the pulmonary vesicles 

 while passing through the pulmonary capillaries, and now becomes oxy- 

 hemoglobin and is now arterial blood. 



As the arterial blood passes through the systemic capillaries, the 

 oxygen of the oxyhemoglobin is largely yielded up and haemoglobin 

 becomes reduced. 



The mode in which the carbon dioxide is held in the blood does not 

 admit of as ready demonstration as is the case with the oxygen. It is, 

 however, clear that almost the total amount of carbon dioxide which may 

 be extracted by the vacuum pump from the blood is held, in some way or 

 other, in the blood-serum, for almost quite as much of this gas may be 

 extracted from the serum as from the blood itself. The carbon dioxide 

 is not, however, simply dissolved, for here also the degree of absorption 

 of this gas by the blood is not governed by the same laws as would apply 

 to its absorption by water. 



When blood-serum is exposed to a vacuum, about 25 volumes per 

 cent, of this gas is extracted, and this amount is spoken of as the loose 

 carbon dioxide. If, now, an acid be added to the serum, about 5 volumes 

 per cent, more are released, and it may therefore be concluded that this 

 latter amount exists, in all probability, in the serum in the form of car- 

 bonates, and probably united with sodium. 



The corpuscles also contain about 5 volumes per cent, of carbon 

 dioxide ; for, if the corpuscles separated from the serum be exposed to a 

 vacuum, they also will yield this amount of carbon dioxide. 



If, now, corpuscles and serum, from both of which gas has been 

 separately removed by pumping, be together exposed to a vacuum, 5 per 

 cent, again of carbon dioxide is given up, from which it would appear 

 that the corpuscles in this experiment play the r61e of an acid ; and it 

 may be assumed that perhaps the haemoglobin in its decomposition 

 liberates an acid, which again sets free the combined carbon dioxide from 

 the serum. 



It is not by any means established as to in what manner the free 

 carbon dioxide is held in the blood-serum. A certain portion of it is, 

 without doubt, held in solution under the simple laws of absorption of 



