RESPONSIVENESS IN VERTEBRATES 



lished. These bring about inhibition of the muscular movements of inha- 

 lation, allowing the lungs to collapse. Ventilation of the lungs lowers the 

 carbon dioxide tension in the blood. When it builds up again, the respira- 

 tory centers are stimulated again. This basic rhythm of quiet breathing is 

 modified during exercise, which increases the amount of carbon dioxide in the 

 blood and so accelerates breathing. The respiratory center can also be stimu- 

 lated by emotional reactions such as fear. And man can voluntarily increase 

 the rate of breathing or hold his breath for limited periods. 



The body temperature of warm-blooded animals is not really constant, or 

 uniform in all regions. In man, for example, the temperature of the mouth 

 cavitv normally ranges from 97.7°F. to 99.5°F. with an average of 98.6°F. 

 The temperature of the rectum averages about 1°F. higher and that of the 

 skin of the axilla about 1°F. lower. Temperatures on exposed portions of the 

 skin average 4° to 9°F. lower than internal temperatures. The temperature 

 of the internal organs is, however, regulated within narrow limits. The rate 

 of chemical reactions is very sensitive to temperature changes, and the smooth 

 flow of metabolic processes depends on the maintenance of an optimal temper- 

 ature. Catabolic reactions of metabolism, especially in the liver, and muscle 

 contractions are constantly tending to increase the body temperature, since 

 during the transformation of energy much is diverted to heat. Blood in the 

 internal capillaries has its temperature raised, and distribution of heat is 

 one of the functions of the circulatory system. The maintenance of a constant 

 temperature under these circumstances requires that heat be lost under 

 optimal environmental conditions. 



Heat is lost by conduction from the blood in the capillaries of the skin 

 to the surface of the body, from which it moves to the surrounding medium, 

 whether air or water. Cooling also occurs as a result of evaporation of water 

 lost through sweat glands and the lungs. The temperature of the surrounding 

 medium influences the rate of heat loss; the cooler the environment, the more 

 rapid the loss of body heat. 



If the internal blood temperature rises, the hypothalamic centers for heat 

 control are activated. Impulses flowing over sympathetic nerves bring about 

 dilatation of the cutaneous blood vessels and an increase in cardiac output — 

 more blood passes through the cooling area. The release of secretion by the 

 sweat glands is also stimulated by way of the autonomic nerves. Evaporation 

 of sweat cools the surface of the skin and increases the loss of heat from the 

 blood. Animals which lack sweat glands exhibit increased ventilation of the 

 lungs or panting; this increases the rate of evaporation from the air passages. 



When the environmental temperature is elevated, conscious controls con- 

 tribute to heat loss. Animals seek out cooler parts of their environment, 

 expose as much body surface as possible, and become relatively inactive, 

 decreasing heat production. Man reduces his clothing, moves the air about 

 him more rapidly by means of fans in order to increase the rate of evapora- 

 tion, eats less, and works as little as possible when summer temperatures 

 are extreme. 



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