94 THE VERTEBRATE ANIMAL: METABOLISM 



impossible to regard them as primary sources of energy. These 

 oxidations, in addition to the transformation of energy, result in 

 the formation of chemical compounds which are of such a nature 

 that they are no longer of use in the protoplasmic system. The 

 combination of oxygen with carbohydrates, fats, and lipins gives 

 rise to carbon dioxide and water, while in the case of proteins the 

 end products are carbon dioxide, water, and a variety of nitrogen- 

 containing compounds. These materials are the waste products 

 of metaboUsm and are known as excretions. 



Excretion. — The waste products of the oxidations of carbo- 

 hydrates, fats, lipins, and proteins are carbon dioxide, water, and 

 various compounds containing nitrogen. These excretions must 

 be constantly removed from the cells in order that the normal oxi- 

 dation reactions may continue. It is well known that if ashes are 

 permitted to accumulate, a fire will be put out by these waste 

 products of its burning; and the continuity of all chemical reactions 

 depends upon the removal of end products. The process of 

 removal of the waste products of metabolism is excretion, and the 

 places of removal are chiefly the lungs, skin, and kidneys. The 

 waste products are carried from cells all over the body by the blood 

 stream. In air-dwelling vertebrates, as blood passes through the 

 lung capillaries, the carbon dioxide passes into the cavity of the 

 lung and is exhaled. In aquatic forms, this waste gas is eliminated 

 into the water surrounding the gills. This excretion of carbon 

 dioxide is often included under the heading of respiration. Carbon 

 dioxide is, however, one of the metabolic wastes and it seems more 

 logical to include its discharge under the heading descriptive of the 

 removal of water and nitrogenous excretions. Exhaled air is 

 moist, because the lungs also excrete water. From the skin of 

 man, carbon dioxide, water, certain salts, and minor quantities of 

 nitrogenous substances are excreted as perspiration (Fig. 52). 

 In the frog, the amount of carbon dioxide excreted by the skin is 

 relatively large and the loss of water is considerable. 



The kidneys are usually spoken of as the organs of excretion, 

 and through them about 50 per cent of the water, traces of carbon 

 dioxide, and the principal part of the nitrogenous waste, in the 

 form of urea, are discharged as urine. Urea is not produced, as 

 such, in the cells throughout the body or in the kidney. It is com- 

 pounded from ammonia in the liver, from which it is carried to the 

 kidneys. It will be recalled that the kidney is made up of tubules 



