CHANGES OF THE BLOOD IN RESPIRATION. 155 



the difference in the oxygen is greater than in the carbon dioxide ; this being 

 in accordance with the well known fact that animals absorb more oxygen 

 than is eouivalent to the carbon dioxide exhaled." 



These facts coincide with the views which are now held regarding the 

 essential processes of respiration. The blood going to the lungs contains 

 carbon dioxide and but a small proportion of oxygen. In the lungs carbon 

 dioxide is given off, appearing in the expired air, and the oxygen which dis- 

 appears from the air is carried away by the arterial blood. 



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

 office connected with 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 large 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 is supplied by the food. 

 There is no evidence that nitrogen enters into combination with the blood- 

 corpuscles. It exists simply in solution in the blood, which is capable of ab- 

 sorbing about ten times as much as can be absorbed by 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 generally admitted that 

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

 association with the haemaglobine of the blood-corpuscles. In studying the 

 composition of the corpuscles, it has been seen that when air is admitted to 

 venous blood, oxygen unites with the haemaglobine, forming oxyhaemaglobine. 

 Carbon monoxide, which has a great affinity for the corpuscles, displaces 

 almost immediately all the oxygen which the blood contains. When the 

 corpuscles are destroyed, as they may be readily by receiving fresh blood into 

 a quantity of pure water, the red color is instantly changed to black. 



The condition under which carbon dioxide exists in the blood has already 

 been considered in connection with the mechanism of its passage from the 

 venous blood into the air-cells. This gas is contained chiefly in the plasma ; 

 a small quantity, however, probably exists in the red blood-corpuscles. The 

 greatest part of the carbon dioxide of the plasma is either in solution or in 

 association with other matters, a condition in which the union does not take 

 place in definite proportions as in true chemical combination. It has been 

 ascertained that the blood-serum will absorb much more carbon dioxide than 

 is absorbed under similar conditions by pure water. It has been shown, also, 

 that neutral sodium phosphate increases to a remarkable degree the quantity 

 of carbon dioxide that can be absorbed by any liquid. It is probable that a 

 small part of the carbon dioxide of the plasma, which passes into the expired 

 air, is in combination with sodium in the form of sodium bicarbonate. 



General Differences in the Composition of Arterial and Venous Blood. 

 All observers agree that there are certain marked differences in the composi- 



