RESPIRATION, KATABOLISM AND EXCRETION 83 



general biological sense takes place in the living cells. 

 The return blood stream comes back to the lungs laden 

 with carbon dioxide, the result of the tissue respiration, 

 and this gas, diffusing into the air passages, passes out 

 of the body in the " expired " air. Here again we 

 get the contrast between the function of the organism 

 as a whole, in this case carried out by special organs — 

 the lungs — and the function of the individual cells of 

 which it is composed. It is especially in the muscles 

 that sugar is most actively broken down in the bodies 

 of the higher animals, and accordingly it is here that 

 the greatest amount of kinetic energy is set free, as we 

 see in the vigorous contractions of our muscles. 



Plants have no " organs of respiration " comparable 

 with the lungs of the higher vertebrate animals. The 

 oxygen used in respiration by the living cells of plants 

 diffuses into the plant from the air through the system 

 of air spaces between the cells (intercellular spaces), 

 and so through the water saturating the wet cell walls 

 into the protoplasm. The oxygen actually used by the 

 living cells in respiration, both in animals and plants, 

 is always oxygen dissolved in liquid, though it is 

 ultimately derived from the air. 



(6) Katabolism and (7) Excretion. — Respiration is 

 essentially a kataboUc process, involving the breaking 

 down of an organic substance (sugar) in the Uving cell. 

 But animals have, in addition, a nitrogenous katabolism 

 involving the breaking down of proteins. This process 

 goes on largely, though not wholly, in the liver, and the 

 comparatively simple nitrogenous substances formed in 

 the liver pass into the blood and are excreted through 

 the kidneys as urea (CH^NgO) and uric acid (CjH^N^Oj), 

 and pass out of the body in the urine. In the amoeba 

 these nitrogenous excretions are probably expelled 



