Excretion - 377 



of river estuaries, although the ancestral 

 stock originated in salt water. An early prob- 

 lem in chorda te development was, accord- 

 ingly, an adequate elimination of water 

 since water kept seeping into the salt-rich 

 body fluids from the surrounding hypotonic 

 medium. In these early chordates the glom- 

 erulus appears to have arisen as a water- 

 eliminating device. At first, however, the 

 filtrate was passed into the coelomic fluid, 

 which was drained off by the nephric tubules. 

 But later the glomerulus became surrounded 

 by the end of the tubule, forming Bowman's 

 capsule, and this arrangement became very 

 well developed in the truly fresh-water fishes 

 and in the amphibians. 



Some of the evolving fishes returned to 

 the sea, however, and now the problem was 

 to hold water in the body fluids. Among such 

 marine fishes two lines of evolution came into 

 being. One group, which led to the modern 

 sharks and other cartilaginous fish, returned 

 to the ocean very early in Silurian (p. 563) 

 times. This group developed a system of 

 maintaining a high osmotic pressure in the 

 blood and lymph by virtue of a very heavy 

 reabsorption of urea. Accordingly, the plasma 

 of these fish today contains a truly astonish- 

 ing quantity of this nitrogenous waste — up 

 to some 25 grams per liter. The other group, 

 which led to the bony fishes of contemporary 

 times, did not go back to the sea until the 

 end of the Mesozoic era (p. 565), and this 

 group adopted another procedure. They de- 

 veloped a means of eliminating excess salts 

 from the blood through an active transport, 

 or secretory, mechanism localized in the 

 membranes of the gills. The glomerulus of 

 these marine fish also came to be reduced or 

 even absent, as in the modern goose-fish and 

 allied forms. 



Land vertebrates, particularly reptiles and 

 birds, likewise faced a problem of water con- 

 servation, owing to evaporative losses suf- 

 fered by exposure to the atmosphere. In these 

 groups, however, a biochemical adjustment 

 occurred. Instead of forming urea as an end 

 product of the catabolism of proteins and 



other nitrogenous compounds, the reptiles 

 and birds produce uric acid. This compound, 

 which is only sparingly soluble in aqueous 

 media, precipitates out in the cloacal cavity 

 of the animals and does not require much 

 water for its elimination, in the form of 

 guano. Likewise, the glomeruli of reptiles 

 and birds are considerably reduced. 



Mammals arrived rather late upon the 

 evolutionary scene. Accordingly, the expedi- 

 ent evolved for the mammalian kidney is 

 different. Mainly the mammalian kidney has 

 resorted to an acceleration of its water-reab- 

 sorbing mechanisms. To this end a special 

 hormone, the antidiuretic hormone, vaso- 

 pressin, has developed (p. 412). When a di- 

 lution of the blood results from drinking 

 excess water, a more copious secretion of this 

 posterior pituitary hormone is triggered; 

 the converse occurs when insufficient water 

 is imbibed and absorbed into the blood. The 

 dehydrating effects of high alcohol consump- 

 tion appear to be related to the fact that 

 ethyl alcohol tends to suppress the produc- 

 tion or liberation of the antidiuretic hor- 

 mone. 



SUMMARY 



The function of the kidney is to maintain 

 a normal blood composition by removing 

 excesses of the various blood components 

 and passing these substances into the urine. 

 The kidneys differ from the lungs in that 

 not all the blood of the body passes through 

 the renal vessels during each circuit. How- 

 ever, the renal blood flow is very copious. In- 

 deed it may be calculated that all the body 

 fluid, including the volume of both lymph 

 and blood, is subjected to filtration and re- 

 absorption some 15 times in the course of 

 every day. 



The important units of renal function are 

 the nephrons, of which there are about a 

 million in each kidney. The nephrons form 

 the urine mainly by two processes — filtration 

 and reabsorption. Each hour an average of 

 about 6000 cc of colloid-free filtrate is forced 



