THE UROGENITAL SYSTEM — EXCRETION AND REPRODUCTION 555 



pressure increase. This raises the glomerular filtration pressure and 

 more filtrate is produced. An increase in the amount of water in the 

 tissue fluid inhibits the release of an antidiuretic hormone produced 

 by the posterior lobe of the pituitary (p. 622). Since this hormone is 

 necessary for the active reabsorption of water, a reduction of the 

 amount present in the blood decreases the amount of water taken back 

 from the tubule. Increased production of filtrate and decreased reab- 

 sorption of water rapidly bring the volume of body fluids down to 

 normal. If the volume of body fluids falls below normal, as in a severe 

 hemorrhage, these factors work in the opposite direction: Less glo- 

 merular filtrate is produced, more water is reabsorbed, and the volume 

 of body fluid is soon raised to normal. The osmotic pressure of the 

 tubular contents also affects the amount of water removed. If a large 

 amount of salts or sugars is being eliminated, the osmotic pressure of 

 the tubular contents is increased and less water can be reabsorbed. The 

 urine volume is greater when there is a large amount of osmotically 

 active substances in the urine, as after a large intake of salt, or in 

 diabetes mellitus. 



Nephron Evolution. The nephrons of other vertebrates are essen- 

 tially similar in structure and function to these mammalian nephrons, 

 although there are differences in detail. In addition to being associated 

 with the glomerulus, some of the nephrons of primitive vertebrates are 

 connected with the coelom via a nephrostome, and can remove ma- 

 terials from the coelomic fluid. This is analogous to the nephridia of 

 the earthworm (p. 239). It may have been the primitive condition in 

 vertebrates, for in the tubules of still more primitive vertebrates the 

 glomerulus protrudes into the coelom, instead of into the beginning 

 of the tubule, and the glomerular filtrate is discharged into the coelom. 

 The size of the renal corpuscles and the presence or absence of 

 water-reabsorbing segments vary with the environment in which the 

 animal lives. Primitive fresh-water fishes have large renal corpuscles, 

 which produce copious amounts of filtrate, and do not have special 

 water-reabsorbing segments. The concentration of salts within their 

 bodies is greater than that in the surrounding medium and water 

 moves by osmosis into their bodies. Their problem is to pump out the 

 excess water, yet retain the needed salts. The type of tubule found in 

 fresh-water fishes is well adapted for this, and the primitive function 

 of the tubule may have been water regulation. Nitrogenous wastes can 

 be eliminated through the gills, and their removal by the kidneys may 

 have been secondary, but when vertebrates became terrestrial and lost 

 their gills, the kidneys became the main organs for removing these 

 wastes. Amphibians retain the primitive fresh-water type of tubule, 

 and have little control over the loss of water. Frogs can lose in the urine 

 an amount of water, equivalent to one third of their body weight each 

 day. The need to soak up water and to keep the skin moist for gas 

 exchange is a factor that compels frogs to stay near water. Water is 

 conserved in reptiles by the small size of their glomeruli. Less water 

 is removed from the blood by these glomeruli than by the large ones of 

 primitive fishes and amphibians. Birds and mammals have glomeruli 



