PROBLEM 5 How Does the Body Get Rid of Wastes 



Formed by Cell Activity? 



Wastes formed by living cells. Oxidation 

 goes on in all living cells. Food and oxy- 

 gen are brought to them by the blood 

 stream. When carbohydrates and fats are 

 oxidized the carbon and hydrogen in 

 them unite with the oxygen, forming 

 carbon dioxide and water. These are 

 waste substances because under normal 

 conditions they are not used again in the 

 body. 



When protein (containing carbon, 

 oxygen, hydrogen, nitrogen, sulfur, and 

 phosphorus) is oxidized, other com- 

 pounds besides water and carbon dioxide 

 are formed. The nitrogen does not unite 

 with oxygen directly; nitrogen is an in- 

 active element and does not easily enter 

 into new combinations. But, in a rounda- 

 bout way, a compound consisting of 

 carbon, oxygen, hydrogen, and nitrogen 

 is formed. This substance is called urea 

 (you-ree'a). Urea, too, is called a waste 

 because it is of no further use to the body. 

 Another nitrogenous waste produced is 

 uric acid. There are also "salts" formed 

 that are wastes. All of these are soluble 

 in water. 



Cell wastes are removed. You know 

 that the cells in an animal's body lie in a 

 liquid known as lymph. The wastes, 

 which are all soluble, diffuse out of the 

 cells into lymph and capillaries. Directly 



Fig. 233 The waste products formed when 

 sugar, fat, and protein are oxidized. Which food 

 substance yields the largest number of wastes? 

 What are the five wastes produced by the oxi- 

 dation of this compound? 



