FOREWORD 



The way the kidney works is described clearly and ex- 

 plicidy in this book— a story of vertebrate evolution and 

 adaptation seen through kidney function. The author, 

 Dr. Smith, is an eminent kidney physiologist whose 

 broad range of interests takes him to the realm of evolu- 

 tion—without ever leaving the kidney. He describes how 

 the kidney functions on different phylogenetic levels and 

 how it facilitates the adaptation of organisms, such as the 

 estivating lungfish ("Kamongo" of an earlier book), to 

 highly speciahzed environments. 



The importance of the kidney cannot be overesti- 

 mated; it removes and excretes the waste products of 

 cell metabolism and it maintains, in constant and rela- 

 tively equivalent composition, the body fluids which 

 bathe the cells. The kidney works closely with the blood 

 system, removing the wastes from it and adjusting the 

 level and balance of sugars, salts, and water as the blood 

 courses through the fine structure of the kidney. In fact, 

 its basic unit, the nephron, is made up of a tuft of in- 

 tertwined blood capillaries (the glomerulus) closely 

 aligned with a kidney tubule. Through mechanisms of 

 filtration and reabsorption, the balance of the internal 

 environment is maintained. This constancy of environ- 

 ment is critical to vertebrate evolution and adaptation. 



In addition to his comprehensive survey, Dr. Smith 

 deals with the problem of the origin of the earliest verte- 

 brates, ancient fossil fishes called the ostracoderms. 



