THE CONTROL OF RESPIRATION 89 



(ii) Heat loss 



Loss of heat is normally accomplished by three physical pro- 

 cesses, radiation, conduction, and the vaporisation of water. At 

 normal environmental temperatures most heat seems to be lost 

 from mammals by radiation and conduction from the body 

 surface, and relatively small amounts are lost by evaporation. 

 Radiation accounts for 60 % to 65 % of the total loss from man 

 in environments of 70°-80° F. It involves the transfer of heat 

 from the surface to every relatively cool object in the vicinity — 

 furnishings, walls, the ground, etc. Conduction is less impor- 

 tant, and includes heat transferred from surfaces in contact 

 with the air — the air covering the skin, clothing, and the tidal 

 air warmed in the respiratory passages, in addition to that trans- 

 ferred to physical objects with which the body comes into con- 

 tact from time to time. Evaporation removes heat because of 

 the latent heat which requires about 0-6 kcal. for the evapor- 

 ation of each gram of water. 



The proportionate loss by these three processes changes with 

 temperature and mainly in the relative amounts lost by radia- 

 tion and evaporation. Above 31 ° C. the amount lost by evapora- 

 tion exceeds that due to radiation and below this temperature 

 radiation is the predominant factor. In cool air man loses one 

 litre of water per day by evaporation and during hard work in 

 the desert it may be as great as 1-6 litres per hour. Output of 

 sweat increases by 20 grams/hour for each 1° C. rise in air 

 temperature. Many mammals lose heat by panting, e.g., dogs and 

 cattle, whereas others such as the cat and the marsupial. Opossum, 

 salivate and lick their fur when heated. Dogs rely a great deal on 

 panting and the frequency of their respiratory movements may 

 be increased from between 15 and 30 breaths/minute to 300/ 

 minute at higher temperatures. Most mammals are less efficient 

 than man at increasing their loss of heat by evaporation at high 

 temperatures and their body temperature rises accordingly. 

 This adaptive feature of man's physiology may be related to his 

 origin under tropical conditions. 



Among birds the very large surface area of the air sacs leads 



