Chap. 14 



THE BY-PRODUCTS OF METABOLISM EXCRETION 



253 



the cells and brought by the blood. Later the urea is released from the liver 

 cells into the blood and carried to the kidney where it becomes the basis of 

 urine in most animals. 



Explorations of the Kidney 



The knowledge of excretion and regulation has been and is still being built 

 up, especially by experiment and observations upon animals (Fig. 14.7). 



The malpighian body of the vertebrate kidneys and the malpighian tubules 

 of insects were named for Marcello Malpighi (1628-1694), an astute ob- 

 server whose admiration of perfection in miniature structures was stimulated 

 by those in the kidney. 



Although the function of the renal or Bowman's capsule was not known in 

 1842, Sir William Bowman (1816-1892) had a theory that the renal capsule 

 and glomerulus together might be a kind of filter. Proof of it came with experi- 

 ment. In 1920 and later years, Dr. Alfred N. Richards performed experiments 

 on frog's kidneys that dispelled any doubt that the capsule and specifically the 

 glomerulus does act as a filter. He obtained a sample of the filtrate as it was 

 being made in the kidneys of the living frog by inserting a fine glass pipette 

 into the renal capsule and drawing out some of the fluid (Fig. 14.7). What he 

 secured contained glucose and other constituents of plasma except the pro- 

 teins, a real filtrate, essentially a deproteinized plasma. 



Studies made by Dr. Homer W. Smith extending over several years (1916- 



Rod blocking tubule 



Glomerulus*' 



Pipette for 

 withdrawing 

 filtrate 



Bowman's 

 ^^ capsule 



~^-K Arterioles 



Fig. 14.7. Method used by A. N. Richards in obtaining a sample of glomerular 

 filtrate in the frog's kidney. He inserted a very fine pipette into the individual 

 capsules in the frog's kidney and analyzed what had passed across the mem- 

 branes. (Courtesy, Gardiner: General Biology. New York, The Macmillan Co., 

 1952.) 



