246 RESPIRATION. 



as much oxygen as pure water could dissolve at the same tempera- 

 ture ; and that while the serum of the blood, separated from the 

 globules, exerts no more solvent power on oxygen than pure water, 

 defibrinated blood, that is, the serum and globules mixed, dissolves 

 quite as much oxygen as the fresh blood itself. The same thing is 

 true with regard to the carbonic acid. It is therefore the semi- 

 fluid blood-globules which retain these two gases in solution ; and 

 since the color of the blood depends entirely upon that of the glo- 

 bules, it is easy to understand why the blood should alter its hue 

 from purple to scarlet in passing through the lungs, where the 

 globules give up carbonic acid, and absorb a fresh quantity of 

 oxygen. The above change may readily be produced outside the 

 body. If freshly drawn venous blood be shaken in a bottle with 

 pure oxygen, its color changes at once from purple to red ; and the 

 same change will take place, though more slowly, if the blood be 

 simply agitated with atmospheric air. It is for this reason that the 

 surface of defibrinated venous blood, and the external parts of a 

 dark-colored clot, exposed to the atmosphere, become rapidly red- 

 dened, while the internal portions retain their original color. 



The process of respiration, so far as we have considered it, con- 

 sists in an alternate interchange of carbonic acid and oxygen in the 

 blood of the general and pulmonary circulations. In the pulmonary 

 circulation, carbonic acid is given off and oxygen absorbed ; while 

 in the general circulation the oxygen gradually disappears, and is 

 replaced, in the venous blood, by carbonic acid. The oxygen which 

 thus disappears from the blood in the general circulation does not, 

 for the most part, enter into direct combination in the blood itself. 

 On the contrary, it exists there, as we have already stated, in the 

 form of a simple solution. It is absorbed, however, from the blood 

 of the capillary vessels, and becomes fixed in the substance of the 

 vascular tissues. The blood may be regarded, therefore, in this 

 respect, as a circulating fluid, destined to transport oxygen from the 

 lungs to the tissues ; for it is the tissues themselves which finally 

 appropriate the oxygen, and fix it in their substance. 



The next important question which presents itself in the study 

 of the respiratory process relates to the origin of the carbonic acid in 

 the venous blood. It was formerly supposed, when Lavoisier first 

 discovered the changes produced in the air by respiration, that the 

 production of the carbonic acid could be accounted for in a very 

 simple manner. It was thought to be produced in the lungs by a 



