230 HUMAN BIOLOGY 



freedom from changes in the external environment. How 

 is this valuable independence achieved? 



To understand regulation of temperature we must reahze 

 first that heat is continuously being produced in the body 

 by every variety of activity that occurs. All the energy 

 of the heart is turned to heat inside us, about three-fourths 

 of the energy of our muscles appears necessarily as heat, 

 the processes in the hver are accompanied by large heat 

 production. As the circulating blood passes through the 

 specially active regions, heat flows from the warmed cells 

 to the cooler blood and is thus distributed to other regions. 

 An important service which the circulation performs, 

 therefore, is that of equalizing the temperature in different 

 parts of the body. It also plays an essential role in the 

 management of heat loss through the skin. 



Let us suppose that there is a tendency for the body 

 temperature to rise because a large amount of heat has been 

 produced by muscular work. The heat will go to the colder 

 outer air by radiation and conduction if it is brought to 

 the skin, which is in contact with the air. Under these 

 circumstances the nerves governing the size of the surface 

 arterioles relax their grip, the vessels dilate, and the blood 

 flows through them and through the capiflaries to which 

 they contribute, in much greater abundance. In consequence 

 the skin becomes red. Thus the extra heat is deHvered to the 

 surroundings and a rise of body temperature is prevented. 

 If the outer air is too warm to permit the heat to pass to it, 

 however, another process is resorted to; heat is lost by 

 evaporation. When water evaporates, as much heat is 

 taken from the surroundings as would be required to cause 

 the water to evaporate. The greater deHvery of warm blood 

 to the skin can be combined with pouring of sweat on the 

 skin surface. As the sweat evaporates the surface is cooled 

 and Hkewise the flowing blood. If the air is dry, large amounts 

 of heat may be lost in this way, and thereby high external 

 temperatures may be withstood (e.g., by stokers and 

 foundry workers). Occasionally the sweat glands fail to 

 develop or they degenerate. Persons thus afflicted may have to 

 wet their garments in order to endure hot weather, or they 

 may pant, as a dog does (having inefficient swxat glands). 



