THERMAL INTERCHANGE WITH ENVIRONMENT 



279 



receptors in the skin are sensitive to changes 

 as small as .002 °C and the cold receptors to 

 .004°C. There is no direct evidence as to 

 the sensitivity of the heat regulator in the 

 hj^pothalamus in man, but animal experi- 

 ments of Ranson and Magoun indicate that 

 this sensitivity is not great; that is, the 

 regulator responds to changes of several 

 degrees in blood temperature. 



The adjustments which the center has 

 under control are of two general classes. 

 The first includes the moment-to-moment 

 emergency adjustments which comprise 

 conspicuous features of temperature control. 

 These are dilation or constriction of 

 peripheral blood vessels, alteration of 

 peripheral blood flow, stimulation of sweat 

 secretion, and stimulation of shivering. 

 These actions are not under voluntary 

 control. Actions that require a voluntary 

 element, but are basically regulative, are 

 impulses to alter posture, to increase or 

 decrease food intake, and to alter levels of 

 physical activity or clothing. 



In addition to these immediate reactions, 

 the center is involved through complex 

 nervous inter-connections with longer-term 

 acclimatization responses. In this category 

 may fall adjustments in blood volume, in 

 blood chemistry, and in endocrine activity. 

 In animals there are changes in thickness 

 of coat in response to temperature, and, 

 probably in both animals and man, fat 

 deposition is influenced by these longer-term 

 effects. 



The Physical Processes of Thermal 



Interchange with the 



Environment 



In preceding sections the range of human 

 heat production has been indicated and the 

 general effectiveness of temperature regula- 

 tion has been related to simple processes 

 under the control of a central regulating 

 center. In the thermal control of environ- 

 ments the objective is generally to reduce 

 these adjustments to a minimum. This 

 means that it is necessary to relate quantita- 

 tive measures of the total heat of the 



environment to measures of the adjustments 

 which the conditions provoke in the in- 

 dividual. In order to accomplish this a 

 clear understanding is needed of the basic 

 physical processes by which the human body 

 exchanges heat with the environment. 



In thermal exchanges between the body 

 and its surroundings, there are four major 

 factors in the picture. The details of this 

 heat interchange as studied by calorimetric 

 methods have been reported by Hardy and 

 DuBois (18, 22, 23) and Winslow, Hening- 

 ton, and Gagge (40, 42, 43). The human 

 body in a state of equilibrium with its 

 thermal environment produces heat by 

 metabohsm. It loses heat by evaporation. 

 It loses or gains heat by conduction, convec- 

 tion, and radiation, depending on the en- 

 vironmental conditions. The whole closed 

 system, when equilibrium exists, may be 

 expressed by a simple formula which com- 

 bines conduction and convection under the 

 latter term. 



(1) M (metabolism ) — E (evaporation) 

 ± C (convection) ± R 

 (radiation) = 0. 



States of disequilibrium or temporarily 

 imperfect adjustment often exist and, in 

 some respects, may be of greater assistance 

 in understanding the problem than the 

 condition of full thermal adaptation. In 

 such states the right side of the equation is 

 not zero, but has a positive or negative 

 value, representing actual chilhng or heating 

 of body tissues. This subject will be dis- 

 cussed later. 



The nature of the physical processes in- 

 volved in this interchange should be re- 

 viewed, and a condensation of a report by 

 one of the authors (Hardy (24)) will be given 

 here. 



Heat Transfer by Radiation 



By radiation is meant the exchange of 

 thermal energy between objects through a 

 process which depends only upon the 

 temperature and the nature of the surfaces 

 of the radiating objects. Therefore, the 



