324 HUMAN PHYSIOLOGY 



of blood and, again, may secrete less than usual when the 

 skin is flushed and burning. 



The Urine and the Metabolism. Our statements thus 

 far have been qualitative rather than quantitative. We 

 must now begin to make some use of figures and it will 

 be best to adopt metric standards. The gram will be 

 our common unit of weight and if it is an unfamiliar one, 

 the reader has only to bear in mind that about 28 grams 

 make an ounce. It is desirable at this time to show what 

 can be learned about the course of events in the body by 

 analyzing the urine. 



The datum most often sought is the quantity of nitro- 

 gen contained in the day's urine. We use this figure to 

 estimate the amount of protein which has been decom- 

 posed in twenty-four hours. This involves at least two 

 assumptions: first, that all the nitrogen excreted by the 

 body is to be found in the urine and, second, that all the 

 decomposition products of protein reach the exterior quite 

 promptly. The first assumption is not strictly justified 

 for there is an appreciable loss of nitrogen in the feces 

 and a slight one through the skin. The second suppo- 

 sition, too, may not be wholly allowable, but so long as 

 we are content with approximations we may regard the 

 nitrogen of the urine as the index of protein metabolism. 



Suppose that the urine for the day contains 12 grams 

 of nitrogen. Nitrogen constitutes about 16 per cent, of 

 an average protein. To see how much protein must have 

 been destroyed to yield 12 grams of nitrogen we divide 

 12 by 16 and multiply by 100 or, what is the same thing, 

 we multiply 12 by 6.25. Our answer is 75 grams. The 

 subject under observation has therefore lost at least 75 

 grams of protein during the day of the trial. He may 

 have lost 5 or 10 grams more than this owing to the escape 

 of minor quantities of nitrogen in the feces and the sweat. 

 These are figures of average magnitude for American 

 students. 



Nitrogen Equilibrium. We cannot analyze the food 

 which a man eats, but we can analyze some more that 



