VITAL ANIMAL LIFE PROCESSES 67 



elements required for the construction of protoplasm. The necessary 

 nitrogen is obtained from protein foods. Not all the protein eaten 

 is used for the building of protoplasm — practically all animals eat more 

 protein than the bare minimum required for the construction of new 

 protoplasm. Excess protein is used for energy. Enzymes within the 

 cell break down the protein into its component amino acids. Then 

 other enzymes split the amino acids into glucose and the amino radical. 

 NH 2 . The glucose is oxidized with energy release and the NH 2 is 

 converted into nitrogenous wastes such as urea, CO(NH 2 )2- Urea 

 composes one of the primary wastes which is excreted from the human 

 body in the urine. The amount of urea in urine is an indication of the 

 amount of protein in the diet. When a person eats heavily of the 

 protein foods, his urine will be rich in urea, and vice versa when the 

 diet is low in proteins. 



The proteins which are broken down for energy, however, do not 

 seem to be the ones which were recently eaten. By using a recent de- 

 velopment of atomic research known as tagged atoms, it is possible to 

 trace the source of the nitrogen which is in the diet. These radioactive 

 isotopes of nitrogen can be traced, and the source of the nitrogen used 

 in the body can be determined. When food containing this radioactive 

 nitrogen is eaten, it goes to form a part of the protoplasm of the cells, 

 whereas some of the existing protein of the protoplasm is broken down 

 for energy release. The urea of the urine will contain little radio- 

 active nitrogen, since most of it stays in the cells. This shows that 

 animal bodies are constantly constructing new protoplasm from their 

 food and breaking down existing protoplasm for energy. This process 

 might be compared to a man living in a house furnished with wooden 

 furniture. When new wood is brought in, he uses this wood to con- 

 struct new furniture and tears down his old furniture for fuel in the 

 fireplace. Thus, no matter how long the house stands, it will always 

 have new furniture. Likewise, there seems to be a constant turnover 

 of the elements which form the vital protoplasm of animal bodies. 



Excretion, like respiration, is a comparatively simple process in the 

 small water animals where the wastes of metabolism can diffuse out 

 of the body into the surrounding water. In the larger animals, how- 

 ever, there must be special organs to remove this waste and excrete 

 it. Kidneys take care of this function in man and other higher animals. 

 The importance of the excretory process is demonstrated when a per- 

 son's kidneys do not function properly. The excretory wastes accumu- 

 late in his system and we say that uremic poisoning has developed. He 

 will die in a short time if the proper function of the kidney is not main- 

 tained. 



