206 EXCRETION BY THE KIDNEYS AND SKIN 



stant, but the wide variety of such materials taken into the 

 alimentary canal accounts for the same wide variety in the 

 urine. The proportion of inert substances in the blood is 

 approximately constant kept so by the removal of any 

 excess by the kidneys chiefly. 



Sodium Chloride is eliminated thus to the extent of about 

 151 grains daily. The sulphates are unimportant. About 

 25 grains are excreted daily. The phosphates are more im- 

 portant, the acid sodium phosphate being mainly responsible 

 for the acid reaction of the urine. Nitrogen and carbon di- 

 oxide are the chief gases to be found. The color of urine is 

 due to a substance, urochrome, which is probably formed 

 from hemoglobin. Some mucus from the bladder is also 

 in the urine. 



Variation in Amount and Composition of Urine. "Its 

 constitution is varying with every different condition of nu- 

 trition, with exercise, bodily and mental, with sleep, age, sex, 

 diet, respiratory activity, the quantity of cutaneous exhala- 

 tion, and indeed with every condition which affects any part 

 of the system. There is no fluid in the body that presents 

 such a variety of constituents as a constant condition, but 

 in which the proportion of these constituents is so vari- 

 able" (Flint). 



Prolonged bodily exercise will increase the amount of 

 urea, but the urine is generally decreased in quantity because 

 perspiration is more active. The young child discharges rel- 

 atively much more urea and urine than the adult. The fe- 

 male discharges relatively more urine, but less urea, than the 

 male. Digestion increases the urinary flow. Climate and 

 season act chiefly though increasing or diminishing cutaneous 

 activity. Emotions of various kinds may give rise to an 

 abundant flow of pale urine. 



Discharge of Urine. On leaving the pelvis of the kidney 

 the urine enters the ureters and passes through them to the 

 bladder, whence it is discharged per urethram. 



The ureters run, one from each kidney, downward and 



