WASTE OF TISSUE 47 



must, therefore, be regarded as a tissue former, and not 

 alone a supplier of heat, although it will serve in the latter 

 capacity on becoming broken up. 



Waste of Tissue. — Every tissue of the body has 

 a certain limited time for existence (a period which is 

 directly shortened by exercise), after which it becomes 

 broken up, and is absorbed into the blood. In order to 

 remove these effete and deleterious matters, the system 

 is provided with various excretory organs, such as the 

 lungs, kidneys, etc. During respiration, the oxygen, 

 which is absorbed from the air by the blood-vessels in 

 the air-cells of the lungs, is carried through the various 

 parts of the body, so that it may break up the effete 

 tissue by combining with the carbon to form carbonic 

 acid, which the blood, at the completion of its circuit, 

 conveys to the lungs, to be by them expelled into the 

 atmosphere. 



A small amount of carbonic acid is eliminated by the 

 skin. 



We may roughly express the oxidation of the various 

 tissues as follows : — 



1. Fat -f oxygen = carbon -|- hydrogen -f- oxygen 



-h oxygen = carbonic acid -\- water. 



2. Sugar -f oxygen = carbon -f- water -f oxygen 



= carbonic acid -\- water. 



3. Albumen + oxygen = carbon 4- hydrogen -f- 



nitrogen -f oxygen H- oxygen = carbonic 



acid -}- water -f degraded nitrogenous matters, 



such as urea, etc. 



The carbonic acid, as we have already seen, is got 



rid of by means of the lungs, which, together with the 



