ON HEAT CONDUCTION; 98.6° F: A CONSTANT? 225 



temperature in different parts of the body may vary. Especially on the outer 

 part of the skin and in the extremities (fingers, toes), the temperature is 

 lower than 37° C — i.e., at points farthest from the glycogen storehouse, the 

 liver, and where the area to volume quotient is high. 



As we saw in Chapter 7, heat is produced by oxidation of glycogens and 

 by hydrolysis of fats and proteins. Under the reversible conditions of a 

 perfect energy-converting machine, no heat energy would be given off as 

 heat because AF is used for work and TAS is needed to establish the state 

 conditions of the products of reaction. However, the body "machine" is not 

 perfect, and in it conversions take place at efficiencies somewhat less than 

 the maximum thermodynamic efficiency. Thus, 



AH = AF' + q' + Q 



where AF' is the work extracted, Q. is the reversible, unavailable heat used 

 to bring the products to the reaction temperature, and q' is that part of 

 AF which could have been used to do work but which appears as heat be- 

 cause the "engine" could not extract the work reversibly. The degradation 

 reactions of fats and proteins are especially inefficient from this point of 

 view, and are thus good producers of "wasted" heat energy, q', which in 

 fact is not wasted but serves to maintain body heat-content or temperature 

 during cold weather. (Eskimos, for example, by design eat unprocessed 

 animal fat for its heat-producing effects.) 



Heat Loss; Fourier's Law 



Heat energy is lost from the body by several mechanisms, all of which 

 are simple physical transport processes or change-of-state processes. The 

 basic method is by conduction, for which the rate of loss, i\, is given by 



K 4 dT 



v x = K T A — 

 ax 



where A is the area exposed, T is temperature, and x is thickhess of the in- 

 sulation. If T is in degrees Fahrenheit, A in square feet, and thickness in 

 inches, the rate of heat loss is given in BTU per hour; and the proportional- 

 ity constant, K T , is given in BTU per hr per sq ft of area per ° F per in. of 

 thickness. Common values of K T for good insulating materials are: cork, 

 0.28; wood, 0.35; wool, 0.30; plaster, 0.48; fat, 0.33; skin, 0.30. Since 



BTU Cal 



hr ft 2 °F/in. hr m 2 °C/cm 



the conversion, if useful, is easy. Approximately 4 BTU = 1 Cal (or kcal). 

 Usually engineers use the units on the left side of the conversion equality, 

 and physiologists thpse on the right side. 



