Production of Heat and Work. 81 



and tender animals, and those with thin coats need more shelter 

 and a higher stable temperature than mature heavily-coated animals 

 or those laying- on fat. Over large portions of America, particu- 

 larly in the West, where there is much sunshine and but scant pre- 

 cipitation during winter, mature and fattening animals thrive in 

 the open if protected from the wind. Animals exposed to cold rains 

 or snow not only suffer therefrom but require more food, because 

 the cold water which falls on them must be warmed and evaporated 

 from their bodies by heat generated thru the burning of food. 



104. Heat production. — Heat is produced by all the decomposi- 

 tions or oxidations occurring in the body, whether in the muscular 

 tissues, the alimentary tract, or the glands. Air breathed into the 

 lungs brings oxygen to the blood. Floating in the blood are myr- 

 iads of microscopic bodies called red corpuscles. These contain 

 liemoglobin in which there is iron, the latter giving to blood its red 

 color. The hemoglobin absorbs the inspired oxygen and holds it 

 loosely. The oxygen-laden blood, as it permeates the capillary sys- 

 tem, gives up its oxygen to the cells of the tissues, where it is used 

 for the combustion of a portion of the body nutrients with the re- 

 sult that heat is formed. Unlike the burning of fuel in a stove, the 

 oxidations in the body take place at a low temperature. In the 

 case of combustion in the body where before there were glucoses, 

 fats, and proteins in the tissues, there now remain carbonic acid gas, 

 Avater, and urea, the latter substance representing the principal 

 nitrogenous waste of the i^rotein nutrients. 



So long as there is a normal supply of oxygen the rate of burn- 

 ing of the food nutrients is independent of the supply of air, but is 

 under the control of the nervous system. The muscular work which 

 necessarily accompanies enforced breathing leads to some increase 

 in oxidation and consequently to increased heat production. An in- 

 creased supply of oxygen does not of itself, however, lead to in- 

 creased oxidation. In this respect body oxidations differ radically 

 from ordinary fuel combustion, where the rate of burning is almost 

 proportional to the oxygen supply. All the energy expended in the 

 various form of internal work of the body appear as heat, a consid- 

 erable amount being thus evolved. Most of the heat generated 

 within the body is produced in the muscular tissues, four-fifths of 

 the heat produced daily in the human body being there generated. 

 The muscles are not always actively contracting, yet heat is always 

 being produced in them. This production of heat in the muscles is 

 under the control of the nervous system. The heat produced in the 



