414 



RESPIRATION. 



[CHAP. xxix. 



Some very accurate experiments upon this subject have been 

 made by M. Barral upon himself during summer and winter. The 

 great difference noticed in the quantity of carbon exhaled at dif- 

 ferent periods of the year may, to some extent, explain the dis- 

 crepancies in the results of various observers 



Weight of body. Carbon in food. 

 Ibs. grs. 



In Summer 104'5 5654-1 



In Winter .. 



4090-0 



Carbon excreted. 



In fsecea . In Urine. Exhaled. 



grs. grs. grs. 



236-2 234-6 5183'3 



137-4 211-5 3741-1 



About 10*8 oz. troy of carbon, therefore, passed off daily in the 

 form of carbonic acid in winter, and 7'8 oz. in summer. The 

 amount of carbon eliminated by respiration will also be influenced 

 by the quantity and nature of the food, and by other circumstances, 

 which have been previously adverted to. 



Theory of Respiration. By the interchange of gases in the lungs, 

 the venous blood becomes of the bright scarlet colour characteristic 

 of arterial blood. The oxygen and carbonic acid both perrneal 

 the delicate, moist, limitary membrane of the air-cells at the sam( 

 time, but in opposite directions. 



The oxygen, having passed through the capillary walls, is held 

 in solution in the blood; a small part entering into chemical 

 combination with the contents of the blood corpuscle, and to a less 

 degree with some of the constituents of the serum. 



A portion of the oxygen not improbably acts directly upon cer- 

 tain substances contained in the circulating blood, and contributes 

 to the formation of carbonic acid. In this way some of the ele- 

 ments of the food lately introduced into the blood may become 

 decomposed, and their carbon removed in the form of carbonic acid. 

 The greater portion of the oxygen is no doubt carried to the 

 capillaries, and much of it then leaving the blood, and passing 

 through the capillary walls, becomes dissolved by the intercelluh 

 fluid, in obedience to the same physical laws by which it w* 

 absorbed. At the same time, the carbonic acid formed in the 

 interstices of the tissues, and dissolved by the fluid which moisten 

 their ultimate elements, leaves this latter to enter the blood, froi 

 which the oxygen has just been removed, and causes it to assum< 

 the colour of venous blood. 



Here, then, are two sources of carbonic acid one resulting from 

 the action of the oxygen upon certain elements recently introduced 

 in the food, giving rise to the production of a certain amount of 



