354 RESPIRATION. 



in the composition of the blood throughout the venous 

 system. 



The important principles which are given off by the 

 lungs are exhaled from the blood ; and the gas which disap- 

 pears from the air is absorbed by the blood, mainly by its 

 corpuscular elements. 



A proper supply of oxygen is indispensable to nutrition, 

 and even to the comparatively mechanical process of circula- 

 tion ; but it is no less necessary to the vital processes that 

 carbonic acid, which the blood acquires in the tissues, should 

 be given off. 



Respiration may be defined to be the process by which 

 the system receives oxygen, and is relieved of carbonic acid. 



As it is almost exclusively through the blood that the 

 tissues and organs are supplied with oxygen, and as the 

 blood receives and exhales most of the carbonic acid, the re- 

 spiratory process may be said to consist chiefly in the change 

 of venous into arterial blood. But experiments have demon- 

 strated that the tissues themselves, detached from the body 

 and placed in an atmosphere of oxygen, will absorb this gas 

 and exhale carbonic acid. 1 Under these circumstances, they 

 certainly respire, and it is evident, therefore, that in this 

 process the intervention of the blood is not an absolute 

 necessity. 



The tide of air in the lungs does not constitute respiration, 

 as we now understand it. These organs only serve to facili- 

 tate the introduction of air into the blood, and the exhalation 

 of carbonic acid. If the system be drained of blood, or if the 

 blood be rendered incapable of interchanging its gases with 

 the air, respiration ceases, and all the phenomena of asphyxia 

 are presented, though air be introduced into the lungs with 



1 G. Licbig demonstrated that the muscles of the frog, separated from the 

 body and carefully freed from blood, will continue to absorb oxygen and exhale 

 carbonic acid as long as they retain their irritability. (LEHMANN, Physiological 

 Ovemixtry, Philadelphia, 1855, vol. ii., pp. '247, 474.) 



