332 HUMAN PHYSIOLOGY 



efforts is merely heat production through friction. Or 

 take the case of his heart: this organ impresses energy 

 upon the blood but it is all turned back to heat as the 

 resistance of the vessels is overcome. In the same way, 

 the work done by the breathing muscles is rendered to 

 the calorimeter as heat, for the weight which is lifted 

 is allowed to sink back again at every expiration and 

 there is no storage of potential energy. 



If the man in the chamber should operate a force 

 pump and permanently elevate a quantity of water to 

 a tank overhead some of his energy would actually fail 

 to appear as heat. This would illustrate one of the 

 "special conditions" referred to in the preceding para- 

 graph. The general principle is that a man in a calo- 

 rimeter will receive credit for all the energy he expends 

 in muscular work, provided that he does not produce 

 lasting changes in his environment. 



Indirect Calorimetry. If a respiration chamber has 

 not the appliances to make it a calorimeter it is still 

 possible to estimate the energy production of the captive. 

 We can calculate from his material output how much of 

 the standard fuel substances he has decomposed and 

 then we can assign to these their recognized calorific 

 values. This procedure is known as indirect calorimetry. 

 It will be worth while to give a rough idea of how this 

 is done. 



Our data are (1) the nitrogen excretion; (2) the carbon 

 loss, and (3) the oxygen consumption. We do not need 

 to know the water outgo in this case. The calculation 

 of the protein metabolism from the nitrogen is a step we 

 have already taken. We will assume, as once before, 

 that the protein decomposed amounted to 75 grams. 

 This should have furnished a trifle .more than 300 Calories, 

 The carbon loss has been mainly in the respiratory 

 carbon dioxid but our total must include the small 

 quantity in the urinary compounds. Suppose that 

 the total is 250 grams. We must deduct from it the 

 car.bon in 75 grams of protein, about 39 grams. The 



