320 HUMAN PHYSIOLOGY 



sodium chlorid (common salt), and it is well to emphasize 

 the essential difference between the history of the phos- 

 phates and sulphates on the one hand and the chlorids 

 on the other. The former are oxidized products of 

 proteins; the chlorids are like most of the water of the 

 excreta merely matter which was previously received 

 in the same state. When no chlorids are fed the elimina- 

 tion is soon reduced to a very low level. 



The quantity of the urine is influenced by many factors, 

 but most radically by the amount of water taken and the 

 varying activity of the sweat glands. A hot day is 

 likely to mean a contracted urine, but some people drink 

 enough extra water in warm weather to provide for a 

 good volume in spite of the large quantity passing out 

 through the skin. A sudden cooling of the body with a 

 check on the perspiration can be depended on to increase 

 kidney activity. This is most striking when one leaves 

 the hot land and goes to sea on a summer day. 



We must not assume to judge of the actual work done 

 by the kidneys by observing how much urine they secrete. 

 It is altogether probable that these organs are most 

 severely taxed when 'they have to remove from the blood 

 a maximum of dissolved solids in a minimum of water. 

 In other words, concentration rather than volume must 

 be our criterion. Average urine is two or three times as 

 concentrated as the blood from which it is derived. 

 Students of physical chemistry tell us that the separation 

 of two liquids of unequal concentration requires the 

 application of energy in perfectly definite and large 

 amounts. The implication is that we shall favor the 

 kidneys by diluting the urine so that it shall not so 

 markedly surpass the concentration of the blood. 



The precepts of renal hygiene are few and plain. Drink 

 plenty of water. Do not eat protein foods to excess. 

 Do not eat a great deal of salt. It is not so easy to 

 apply these directions for there is no agreement as to the 

 protein standard or how much salt is "a great deal." 

 The protein question will be given further attention. 



