MINERAL METABOLISM 399 



for metlical and industrial purposes is a mauer ol real (oncern. Even- 

 tually reliable safety precautions, exposure levels, and preventive and 

 therapeutic treatments will jnobably be worked out. 



OTHER ESSENTIAL INORGANIC MATERIALS 



Besides the elements and ions discussed above and the various 

 organic compounds essential to animals, certain other inorganic mate- 

 rials are required. Some of these are available from both the environ- 

 ment and the metabolic reactions. Either source can serve. In at 

 least one other case, namely oxygen, the animal does not develop 

 useful quantities of the substance and depends almost solely on its 

 environment. 



For the higher animals oxygen is the major terminal acceptor for 

 electrons and protons and must be regularly available in quantity. 

 Several tissues may temporarily function with a deficiency of oxygen. 

 Muscle can accumulate rather large c}uantities of reduced compounds 

 like lactate, but a limit is soon reached and these intermediates must 

 be metabolized or tissue function ceases. Some tissues like brain are 

 especially sensitive to oxygen lack, and investigators feel that each 

 instance of oxygen deprivation causes permanent brain damage related 

 in extent to the level of the deficiency. There is good evidence that an 

 acute shortage of oxygen during certain critical stages of pregnancy 

 can cause serious deformities of the olispring, human or animal. 



Requirements for water are rather obvious but vary widely with 

 species, activity, atmospheric temperature and humidity, and diet. 

 Animals with extensive systems of sweat glands perspire even in cool 

 weather. Evaporation of water is employed in hot weather to keep 

 body temperatures near that normal for warm-blooded animals. Pro- 

 fuse sweating and panting lead to rapid loss of water and correspond- 

 ingly raise the intake levels. Physical activity increases the rate of 

 respiration and thus increases the total loss of water in the expired 

 breath. On the other hand, loss of water is inversely related to the 

 relative humidity, boili lungs and skin surface being affected in ani- 

 mals with sweat glands. 



In general, the larger the animal the more water required, with spe- 

 cial adaptations in some species indigenous to arid regions. Many 

 small desert animals are known to subsist for very long times without 

 drinking. They have an efficient conservation system and depend on 

 the moisture in their diets and the water formed by metabolic reac- 

 tions. Individuals or species eating exceptionally moist foods should 



