THE METABOLIC MACHINERY OF ANIMALS 



323 



to keep the ingredients of the plasma of the blood standardized, thus 

 regulating the salt content of the blood by altering the ratio of salt to 

 water produced in the urine, depending upon the amount taken into 

 the body. The normal healthy person eliminates the following 

 amounts of waste per day, through the kidneys : 30 grams of urea 

 (converted ammonia) ; 15 grams of urea salt ; 10 grams of other 

 soluble urea substances. The remainder, 96 per cent by weight, is 

 water, making a total of one to one and a half liters that is eliminated. 

 A sagittal section through the kidney reveals the expanded upper 

 end of the ureter on the median side draining the basinlike pelvis 

 of the kidney. The outer portion is a compact region called the 

 cortex, while the inner striated portion ending in the irregular margin 

 of the pelvis is known as the medullary substance of the kidneys. 



-Cortex 



proximal 

 diistal tubule. 



.glomerulus'] sift 



descending limt A i 

 ascending limb ,^ f 



Hanles loop 



papillary diccts 

 :r!:^papUla 



Diagram of the human excretory system. 



blocCdCar- 



. wr-athra 

 How do urea, water, and inorganic 



salts reach the pelvis i* 



The inner margin of the medullary substance forms renal pyramids 

 the tips of which are projections, or papillae, that lie in closely invest- 

 ing cuplike depressions of the pelvis, called calyces. The tip of each 

 papilla is dotted with the openings of the collecting ducts, which in 

 turn are formed from the union of several renal or uriniferous 

 tubules. 



