454 THE HUMAN BODY 



quired for it is included as part of the Body's energy liberation, 

 this energy is probably furnished in part, at least, by the tissue- 

 repair protein itself. 



The Liberation of Energy in the Body. We have seen that all 

 the energy liberated by the Body can be expressed in terms of heat- 

 units, but it is not to be concluded, therefore, that heat energy is 

 the only form manifested by the Body. As a matter of fact the 

 Body undoubtedly converts the potential energy of the food into 

 at least three forms of kinetic energy ; chemical, the carrying on of 

 the digestive and other chemical processes of the Body; mechanical, 

 the working of the skeletal muscles, as well as of the heart, the 

 muscles of respiration, and the muscles of the viscera; and thermal, 

 the direct production of heat by oxidation processes. This latter 

 form of energy, although far exceeding in amount both the others 

 together, may be looked upon as in large degree a by-product of 

 the mechanical work of the Body, and arising through the ineffi- 

 ciency of the body machinery. We know that most of the heat of 

 the Body is produced in the muscles, and that though these are 

 producing some heat even when at rest, they produce enormously 

 more when they are active. A characteristic of all machines is 

 that they work more or less wastefully ; not all the energy imparted 

 to them appears again as useful work; the part that is lost, more- 

 over, appears always as heat. In the Body there is this same in- 

 ability to convert food energy into mechanical energy without 

 there being at the same time a large heat production. This is 

 brought out strikingly in calorimetric experiments with resting 

 and working men. It is found in such experiments that a man 

 making only ordinary movements gives out in a day energy 

 equivalent to 2,400 Calories, of which at least 2,000 represent 

 probably direct heat production by oxidation within the Body. 

 If the same man does in a day additional muscular work equivalent 

 to 600 Calories, which by the way is about all a man can do in one 

 day, being equivalent to 250,000 kilogrammcters (1,825,000 foot- 

 pounds), his direct heat production jumps from 2,000 to 4,800 

 Calories. In other words, the machine in converting the energy 

 of 600 Calories of food into mechanical work is obliged to oxidize 

 nearly 3,000 Calories additional. 



We shall see in the chapter on Heat Regulation (Chap. XXXII) 

 that the Body makes very good use of this by-product of 



