.716 METABOLISM 



similation itself a certain amount of heat is generated; this is known 

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

 with protein and least so with carbohydrate (page 538). 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 lest 

 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. 



From all these facts, it is evident that heat loss occurs mainly by the 

 skin and only to a small degree by the lungs. This means that under 

 average conditions in man the main regulation of heat loss is effected by 

 variations in the skin temperature brought about by peripheral vaso-con- 

 striction 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 281). 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, when an immediate curtailment of bloodflow takes place. 

 Or the reflex may be excited from distant skin areas, as illustrated in 

 the curtailment in bloodflow observed when the opposite hand to that 

 on which the observation is being made is placed in cold water. The 



