160 RESPIRATION. 



" If we compare the venous blood of the right side of the heart with the arterial 

 blood of the left side, we find that the latter is richer in oxygen and poorer in carbonic 

 acid. In examining this more closely, we see that the difference in the oxygen is greater 

 than in the carbonic acid ; this being in accordance with the well-known fact that ani- 

 mals absorb more oxygen than is equivalent to the carbonic acid exhaled." 



These facts coincide with the views which are now held regarding the essential pro- 

 cesses of respiration. The blood going to the lungs contains carbonic acid and but a 

 small proportion of oxygen. In the lungs, carbonic acid is given off, appearing in the 

 expired air, and the oxygen which disappears from the air is carried away by the ar- 

 terial blood. 



Nitrogen of the Blood. As far as is known, nitrogen has no very important office in 

 the process of respiration. There is sometimes a slight exhalation of this gas by the 

 lungs, and analyses have demonstrated its existence in solution in the blood. Magnus 

 found generally a larger proportion in the arterial than in venous blood, although, in one 

 instance, there was a larger proportion in the venous blood. It is not absolutely certain 

 whether the nitrogen which exists in the blood be derived from the air or from the 

 tissues. Its almost constant exhalation in the expired air would lead to the supposition 

 that it is produced in small quantity in the system or supplied by the food. There is no 

 evidence that nitrogen enters into combination with the blood-corpuscles ; it exists sim- 

 ply in solution in the blood, which is capable of absorbing about ten times as much as 

 pure water. Nothing is known with regard to the relations of the free nitrogen of the 

 blood to the processes of nutrition. 



Condition of the Gases in the Blood. It is now pretty generally admitted that the 

 oxygen of the blood exists, not in simple solution, but in a condition of feeble combina- 

 tion with certain of the constituents of the blood-corpuscles, particularly the coloring 

 matter. In studying the composition of the corpuscles, we have seen that, when air is 

 admitted to venous blood, oxygen unites with the hseinaglobine, forming oxyhsema- 

 globine. Carbonic oxide, which has a great affinity for the corpuscles, displaces almost 

 immediately all the oxygen which the blood contains. When the corpuscles are de- 

 stroyed, as they may be readily by receiving fresh blood into a quantity of pure water, 

 the red color is instantly changed to black. 



Carbonic acid is more easily exhaled from the blood than oxygen. It was this 

 principle which was obtained by those who first succeeded in extracting gas from the 

 blood. While there is every reason to suppose that oxygen is in combination with the 

 blood-corpuscles, carbonic acid seems to be in a condition of simple solution and is con- 

 tained more especially in the plasma. What may be considered as the free carbonic acid 

 of the blood behaves in all regards like a gas simply held in solution. The view that it is 

 held in solution chiefly in the plasma is sustained by the fact that serum will absorb 

 more carbonic acid than an equal volume of defibrinated blood. 



Liebig has shown that the phosphate of soda, one of the constituents of the blood, 

 influences to a remarkable degree the quantity of carbonic acid which can be held in 

 solution by any liquid. One hundredth of a part of this salt in pure water will double 

 its capacity for dissolving carbonic acid. When blood is in contact with a certain 

 quantity of air, oxygen is consumed and carbonic acid is exhaled. The fact that car- 

 bonic oxide, which has such a remarkable affinity for the corpuscles, displaces oxygen 

 almost exclusively, is another argument in favor of the view that the carbonic acid is 

 contained mainly in the plasma. 



The carbonic acid which is formed in the tissues and is taken up by the blood in its 

 passage through the capillaries exists in this fluid in two forms : ,one, in simple solution, 

 chiefly in the plasma, and the other, in a state of such loose chemical combination in 

 the bicarbonates that it may be disengaged by displacement by another gas and is 



