CHANGES IN THE BLOOD DURING RESPIRATION. 243 



The proportion of oxygen which disappears in the interior of the 

 body, over and above that which is returned in the breath under 

 the form of carbonic acid, varies in different kinds of animals. . In 

 the herbivora, it is about 10 per cent, of the whole amount of oxy- 

 gen inspired ; in the carnivora, 20 or 25 per cent, and even more. 

 It is a very remarkable fact, also, and an important one, as regards 

 the theory of respiration, that, in the same animal, the proportion of 

 oxj^gen absorbed, to that of carbonic acid exhaled, varies according 

 to the quality of the food. In dogs, for instance, while fed on ani- 

 mal food, according to the experiments of Eegnault and Reiset, 25 

 per cent, of the inspired oxygen disappeared in the body of the 

 animal ; but when fed on starchy substances, all but 8 per cent, 

 reappeared in the expired carbonic acid. It is already evident, 

 therefore, from these facts, that the oxygen of the inspired air is 

 not altogether employed in the formation of carbonic acid. 



CHANGES IN THE BLOOD DURING RESPIRATION. If we pass from 

 the consideration of the changes produced in the air by respiration 

 to those which take place in the blood during the same process, we 

 find, as might have been expected, that the latter correspond 

 inversely with the former. The blood, in passing through the 

 lungs, suffers the following alterations : 



1st. Its color is changed from venous to arterial. 



2d. It absorbs oxygen. And 



3d. It exhales carbonic acid and the vapor of water. 



The interchange of gases, which takes place during respiration 

 between the air and the blood, is a simple phenomenon of absorp- 

 tion and exhalation. The inspired oxygen does not, as Lavoisier 

 once supposed, immediately combine with carbon in the lungs, and 

 return to the atmosphere under the form of carbonic acid. On the 

 contrary, almost the first fact of importance which has been estab- 

 lished by the examination of the blood in this respect is the fol- 

 lowing, viz : that carbonic acid exists ready formed in the venous blood 

 before its entrance into the lungs ; and, on the other hand, that the 

 oxygen which is absorbed during respiration passes off in a free state 

 with the arterial blood. The real process, as it takes place in the 

 lung, is, therefore, for the most part, as follows : The blood comes to 

 the lungs already charged with carbonic acid. In passing through 

 the pulmonary capillaries, it is exposed to the influence of the air 

 in the cavity of the pulmonary cells, and a transudation of gases 

 takes place through the moist animal membranes of the lung. 



