278 



TEMPERATURE AND HUMIDITY 



physiologist can supply a sufficient answer. 

 The thermal resistance between the deep 

 body region and the skin can be varied. 

 As in a heat exchanger, the transfer of heat 

 from a circulating medium (blood) to the 

 receiving surface (skin) is a function of the 

 velocity of circulation in the system and the 

 thermal resistance of the heat exchanger 

 wall. By dilation of blood vessels in the 

 skin, the skin thermal resistance may 

 be greatly decreased. Through increased 

 rapidity of circulation in the system gen- 

 erally, and particularly in the dilated skin 

 area, thermal head at constant heat produc- 

 tion may be varied by 5° or 6°C. The 

 physiological cost of this adjustment is borne 

 primarily by the heart. The details of such 

 adjustments in circulation index are de- 

 scribed by Hardy and DuBois (23) and by 

 Winslow, Herrington, and Gagge (42). 



The engineer will also note that the 

 conditions at the surface of the body as 

 reported in Table V are also different from 

 the mannikin's response. He will then ask: 

 How is it possible for the standard heat 

 production delivered from skin to air over 

 a 4°C gradient at a room temperature of 

 25° or 30°C to be passed to the air over a 

 0° gradient at a room temperature of 

 35°C? 



This question is largely rhetorical since 

 the process of evaporation or sweating is 

 generally familiar to all. The physiologist 

 adds that when the body is subjected to a 

 larger stress than can be met by its internal 

 thermal head reduction mechanism, it 

 stabilizes the skin surface at a value 1° 

 to 2°C below internal body temperature by 

 secretion and evaporation of sweat from its 

 external surface. Under these circum- 

 stances air temperatures may reach and 

 exceed skin temperature and body tempera- 

 ture. 



The Neural Integration of Human 

 Temperature Regulation 



We have observed that the body produces 

 heat consistent with different grades of work 



and eliminates this variable heat input to a 

 variety of thermal environments from a 

 rather constant internal body temperature 

 near 37 °C. In a very elementary compari- 

 son of the thermal response of a heated 

 mannikin with that of a human subject, we 

 have isolated the gross physical features of 

 this adjustment. It is important to know 

 how these adjustment resources are con- 

 trolled and integrated. 



The primary element in human tempera- 

 ture regulation is a group of cells located by 

 Ranson (34) in the hypothalamic area of the 

 brain. This temperature regulating center 

 is directly sensitive to temperature. Due 

 to its location and blood supply, it is con- 

 stantly perfused by blood whose temperature 

 is a representative sample of the thermal 

 state of the important vital tissues and 

 organs of the body. This center operates 

 as a thermostat set normally at 37°C for 

 resting levels of activity, but capable of 

 resetting itself for higher levels as total heat 

 production increases. Thus, under condi- 

 tions of strenuous exercise, Nielsen (31) 

 has found that 39°C is the approximate 

 control point. In contrast with the usual 

 mechanical thermostats, this biological 

 temperature regulator initiates positive ad- 

 justments for excesses as well as deficiencies 

 of temperature, and hence, in a gross sense, 

 resembles the dual control of an air-condi- 

 tioning system with both heater and cooler 

 units under its control. Not only are the 

 adjustments which this center may initiate 

 to temperature stress influenced by the 

 actual temperature of the cells which com- 

 prise the center, but, through its correlative 

 function, it is also influenced by temperature 

 events affecting the numerous warm and 

 cold sensory receptors of the skin surface 

 (Bazett, 3). Through such inter-connec- 

 tions the center is sensitive both to the slow 

 trend of the temperature of the internal body 

 mass and to the sudden changes which may 

 occur at the body surface. This gives the 

 regulation what would be mechanically 

 regarded as an anticipatory function. 

 Hardy and Oppel have shown that the heat 



