182 ANIMAL BIOLOGY 



and deliver a true secretion — a lubricant for hair and skin, and a 

 conserver of body heat. (Figs. 96, 127.) 



C. Liver 



The liver, in addition to its various other functions, aids in no 

 small way in excretion. On the one hand, the liver removes delete- 

 rious compounds of ammonia from the blood and converts them 

 into urea. Then it secretes the urea into the blood from which it 

 is later removed by the kidneys. On the other hand, the liver col- 

 lects other waste products, etc., from the blood, which form the 

 bile. This passes to the gall bladder for temporary storage, or 

 directly to the intestine. (Fig. 112.) 



D. Kidneys 



The kidneys, in cooperation with the liver, are the chief excre- 

 tory organs, and any serious interference with their activity leads 

 to a poisoning of the body with waste products. However, excretion 

 is but one function of the kidneys — they act as a blood-regulator 

 to maintain a proper balance of the chemical constituents of the 

 blood plasma. A large amount of water and various salts, urea, etc., 

 pass from the blood through the glomeruli into the tubules. 

 Here such materials as are of value in the economy of the organism 

 are absorbed by the tubules and returned to the blood, while the 

 rest passes to the pelvis of the kidney and eventually out of the 

 body. In Man about 90 per cent of the nitrogenous waste is elim- 

 inated as urea. (Figs. 130-132.) 



1. Urine 



The total excretion of the kidneys, known as urine, passes from 

 the kidneys through the ureters to the urinary bladder where 

 it is stored temporarily until passed to the exterior through the 

 urethra. Since urine is the medium for the elimination not only 

 of nearly all the normal products of katabolism, but also of the 

 majority of abnormal substances that may enter or be formed by 

 the organism, there is no better indication of the general metabolic 

 condition of the human body than that afforded by a chemical 

 and microscopical analysis of the urine. Thus in Bright's disease 

 protein appears; in diabetes, glucose (grape sugar) and other ab- 

 normal substances; while in gout, uric acid is present in abnormal 

 quantity in the urine. 



