THE SOURCES OF ENERGY 



71 



loses its available energy and must be removed. The steam is 

 passed out from the condenser, and is returned to the boiler to be 

 reheated, and the cylinder gases in the gas engine are blown out 

 into the atmosphere. So also in the animate engine the working 

 substance loses its available energy: protoplasmic substances 

 disintegrate into simpler ones, which are then oxidised to form 

 carbonic acid and water. These latter compounds, with the 

 urea, which is the end product of the disintegration of the nitro- 

 genous part of the protoplasmic substance, must be removed 

 from the animal body. 



So we must say a word or two about excretion. The water 

 which appears in the muscles as the result of the disintegration 



~Glandu/a.r 

 pa.rtoF kidney 



,- Cavity 



FIG. 18. THE KIDNEY OF A MAMMAL. 



A, The organ with its bloodvessels and duct; B, the kidney cut 

 through longitudinally. 



of the protoplasm, or of the oxidation of the products of dis- 

 integration of the latter, simply oozes through into the blood- 

 stream, and the carbonic acid similarly produced is also taken 

 up by the blood. The effect of work done by the body is there- 

 fore to add water and carbonic acid to the blood, and so the latter 

 becomes changed, " impure," or venous. As it streams through 

 the capillaries in the lungs the carbonic acid is excreted, passing 

 out into the expired air, while, at the same time, some of its 

 excess of water also passes out in the same way. 



But most of the water, and also the urea, is excreted by the 

 kidneys. Consider first of all the urea. The proteid substance 

 of the muscles is, as we have seen, continually disintegrating into 

 simpler substances, giving up energy as it thus breaks down. 

 These products of metabolism are not yet urea, but they are 



