400 ANIMAL BIOCHEMISTRY 



require correspondingly less water to drink. An even greater dietary 

 effect occurs when the food is very salty, requiring a large volume of 

 urine for excretion of the excess salt at a concentration compatible 

 with normal function of the blood and kidneys. High-protein diets 

 also tend to raise the water requirement because the excretion of urea 

 is increased. 



Since general daily requirements cannot be set, water is provided 

 on a basis of free choice, and the animal allowed to suit himself when 

 possible. In the higher animals there is a thirst-control center in the 

 brain which produces the sensation of thirst or lack of it. The 

 osmotic concentrations of body fluids are believed to activate the thirst 

 response. Under rather average environmental conditions, a normal 

 himian adult has an obligatory loss of water amounting to about 

 600 ml. from the skin, 400 ml. in expired air, and 500 ml. in urine. 

 If water intake, including drinking, the water of foods, and metabolic 

 water, does not reach this total, the difference is supplied by dehydra- 

 tion of body tissues. The osmotic concentration of the body fluids 

 rises and thirst begins. 



The overall situation is less clear for carbon dioxide. Plants, of 

 course, require this compound for photosynthesis and fix it in the 

 organic materials of the plants. To a much smaller extent bacteria 

 and animal tissues also fix carbon dioxide. It is now known that cul- 

 tures of isolated animal tissues grow poorly or not at all in atmos- 

 pheres free of carbon dioxide. Apparently this material is needed 

 for important carboxylation reactions like the following: 



coo- 



I 



coo- c=o 



I I 



C— OPO3H- + CO2 + ADP -> CHo + ATP 



II I 



CHo COO- 



phosphoenolpyruvatc oxaloacetate 



In normal intact animals the metabolic formation of carbon dioxide 

 is a continuous process. A supply is always available in the tissues for 

 reactions like the above. Hence, there is probably little need for 

 carbon dioxide in the environments of most animals. However, since 

 unicellular and other small forms would lose their metabolic carbon 

 dioxide quickly by diffusion, they need an environmental supply. 



In a somewhat similar way, ammonia as ammonium ion is required 

 by animals. The synthesis of amino acids depends on the presence of 

 this substance, which is utilized according to: 



