744 METABOLISM 



as the specific dynamic action of the foodstuff, and is most pronounced 

 with protein and least so with carbohydrate (page 574). Advantage 

 may be taken of this heating power of protein to produce more heat 

 when the cooling conditions are excessive ; in winter, for example, there 

 is an inclination to take more protein food than during summer, and the 

 per capita consumption of such food is much greater in peoples living in 

 temperate zones than in those living in the tropics. The ultimate amount 

 of heat produced by oxidation is greatest with fat and least with carbo- 

 hydrate. 



Heat loss in man is effected partly through the lungs, but mainly 

 through the skin. Through the latter pathway heat is lost by the physical 

 processes of heat conduction and radiation and by the evaporation of the 

 sweat. Through the lungs it is lost mainly in the vaporization of the 

 water contained in the expired air (latent heat of vapor). The amount 

 of heat lost from the skin by conduction and radiation depends on the 

 temperature of the skin, which again depends on the rate at which the 

 blood is circulating through the cutaneous vessels. Under ordinary con- 

 ditions of external temperature two or three times as much heat is lost 

 by these methods as by evaporation. The losses by evaporation, under 

 conditions of rest and average external temperature, are about equally 

 divided between the lungs and the skin. 



Under average conditions in man the main regulation of heat loss is ef- 

 fected ~by variations in the skin temperature brought about by peripheral 

 vaso-constriction and dilatation. The marked sensitivity of the cutaneous 

 blood supply to changes in the temperature of the environment has been 

 very clearly shown by observations made with the hand calorimeter of 

 Stewart described elsewhere (page 296). When the bloodflow through 

 the hand is examined in a person who has been exposed to the outside 

 air, it may be little more than half that which it attains after he has 

 been in a warm room for some time. In the outside air the vessels con- 

 strict to prevent heat loss by conduction and radiation; in the warm room 

 they dilate to facilitate this loss. The afferent impulses which reflexly 

 control the change in the cutaneous blood circulation may be set up by 

 local applications of heat or cold, as can be shown in the hand-calorim- 

 eter experiments by applying a cold pad to the skin of the correspond- 

 ing forearm, or allowing an electric fan to blow on the arm when 

 an immediate curtailment of bloodflow takes place. Or the reflex may be 

 excited from distant skin areas, as illustrated in the curtailment in blood- 

 flow observed when the opposite hand to that on which the observation 

 is being made is placed in cold water. The magnitude of the change 

 in cutaneous circulation is more or less dependent upon the extent 

 of the area of the body that is exposed to the change in temperature. 



