506 ANIMAL BIOCHEMISTRY 



TABLE 21-5. Oxygen Uptake and Heat Content of Nutrients 



Carbohydrate Fat Protein 



Heat content, kcal./g. digested 

 Oxygen uptake, liters/g. digested 

 Ratio, kcal. /liter of oxygen 



olism. Using average values, the relationships of Table 21-5 show a 

 degree of constancy for the heat produced per unit of oxygen when 

 the different classes of nutrients are completely oxidized. This depend- 

 ence permits the estimation of heat of reaction from measurement of 

 the oxygen uptake. 



Energy Balance 



Growing animals ingest food sufficient to supply energy for the syn- 

 thetic purposes of growth, for mechanical activity, and for replace- 

 ment of heat lost. Adults still need energy for the synthesis of needed 

 repair and replacement materials but do not need energy for growth. 

 Most of the energy requirements of mature animals probably serve to 

 satisfy activity and heat losses. Of these two factors, the first depends 

 upon the size and movement of the animal and is considered in the 

 later section on muscular activity. Loss of heat depends upon in- 

 dividual and species variations in surface area, sweat glands and their 

 activity, blood capillaries in the skin, rates of respiration and heart 

 beat, nervous state, excretion of waste products, physical activity, and 

 insulating materials like hair, fur, and feathers. Heat loss also de- 

 pends on environmental factors like temperature, humidity, air move- 

 ment, contact with heat conductors, and artificial insulators like cloth- 

 ing. 



With such complex variations the heat losses of individuals of 

 normal size fluctuate widely for any given species. Human beings lose 

 heat to their environments by the means and in the amounts shown. 



Heat Loss, 

 Process kcal. /day 



Warming expired air 40-60 



Excretion of urine and feces at body temperature 70-90 



Evaporation of water from lungs and skin 300-1,200 



Convection from body surface 500-1,500 



Radiation from body surface 500-1,500 



