ENERGY 507 



Clearly the last three factors are by far the most important except 

 perhaps in hot moist climates where all losses may be reduced well 

 below the lowest levels shown. For example, radiation loses its im- 

 portance when prolonged high air temperatures raise the temperature 

 of the surfaces of the environment above that of the body (37°C.). 

 Under such conditions of hot days and nights, the body may actually 

 gain a net amount of heat over at least part of the day. 



In spite of all the factors modifying heat loss, all mammals and 

 birds have temperature-control mechanisms which compensate effec- 

 tively for large environmental and physiological changes. Tempera- 

 ture regulation permits a much broader scope of action to animals 

 thus equipped, enabling them to survive and function more or less 

 normally when weather conditions are extreme. Plants, insects, and 

 cold-blooded animals must be sheltered, become dormant, or die when 

 the environmental temperature becomes exceptionally hot or cold. 



Animals normally lose heat at characteristic rates even when at 

 rest. This heat is replaced as a result of metabolism with some arising 

 from the continuous muscular work of the heart beat, peristalsis, and 

 respiration. The combined metabolic and physical processes occurring 

 in fasting animals at rest at 20 to 25°C. are called the basal metab- 

 olism and represent heat losses under the sj^ecial conditions men- 

 tioned. 



Basal Metabolism 



Measurements of heat loss can be made under conditions avoiding 

 or standardizing many of the physiological and environmental factors 

 mentioned above. Even though species and individuals still differ, 

 some of these differences can be correlated with special physiological 

 properties of both scientific and medical value. 



For study of the basal metabolism (basal metabolic rate), identical 

 conditions must be used for all individuals examined in order to ob- 

 tain comparable results. Human beings are studied in all modern 

 hospitals after a night's sleep without food and after 30 min. of com- 

 plete rest and quiet. Under these standardized conditions, heat pro- 

 duction is calculated from the rates of oxygen consumption and 

 carbon dioxide evolution. This basal metabolism depends upon vari- 

 ous factors like age, sex, body surface area, diet, levels of certain 

 hormones (especially thyroid hormones), environmental temperature, 

 and to some extent the type of daily activity. 



Body area reflects both height and weight and shows a remarkably 

 close correlation with the basal metabolic rates of the warm-blooded 



