Water ec 



filled micropipette and analyzed each fluid sample. In both froK and 

 Necturus the glomerular fluid has essentially the same sugar and chloride 

 concentration as the blood plasma; it is free of the normal blood proteins, 

 although some small protein molecules can pass."^''- -•"*• --••' These findings 

 support the view that the glomerular fluid is an ultrafiltrate from the plasma, 

 i.e., contains the blood solutes except for protein, and that the filtration j)res- 

 sure is the difterence between the blood pressure and the colloid (protein) 

 osmotic pressure of the blood. The analyses by Richards and his asscK-iates of 

 fluid collected at different regions of the tubules showed that the chloride 

 content decreases along the distal tubule, hence chloride reabsorptitjn must 

 occur in that region. Glucose diminishes in concentration early in the proximal 

 tubule. After injection of the drug phlorizin the glucose is not reabsorbed and 

 actually increases in concentration so that the reducing power of fluid at the 

 distal end of the proximal tubule is 25 per cent above that of the j)lasma in 

 NecUirus and 40 per cent abo\e that of frog plasma, while at the distal end 

 of the distal tubule reducing power of the fluid is 2'/2 times that of the plasma 

 in Necturus and 3 times that of frog plasma. -•*"• -^"' -^^ Water may be 

 reabsorbed all along the tubule, about twice as much in the distal as in the 

 proximal tubule. Acidification of the urine occurs in the distal tubules. The 

 water reabsorption in Necturus is suflicient to account for the doubling of the 

 concentration of urea. In the frog, however, the water reabsorption is insuf- 

 ficient, and some urea is probably secreted by tubule cells.^''"'' -^^ Urea 

 excretion by frogs with renal arteries ligated is due to filtration from collateral 

 circulation. ^-^ 



The mechanisms of osmoregulation in aquatic Amphibia can be summar- 

 ized as follows. Water enters osmotically but at a retarded rate, owing to low 

 permeability of the normal skin. The water which does enter is excreted as a 

 copious dilute urine. Salt is lost in the urine, and to a lesser degree by the 

 skin. This salt loss is made good by food and by active absorption from pond 

 or tap water. Also salt loss is minimized by active reabsorption in the distal 

 tubule of the chloride which filters through the glomeruli. There is also some 

 water reabsorption, which accounts for increased urine concentration of waste 

 products that are not reabsorbed; under some conditions tubular secretion of 

 specific substances may occur. Among these mechanisms at least four require 

 expenditure of energy: maintenance of low skin permeability, active salt 

 absorption by the skin, tubular reabsorption against a concentration gradient, 

 and tubular secretion of specific wastes. 



ADAPTATIONS TO LAND, WITH SOME DIVERSIONS TO 

 FRESH AND SALT WATER 



In two large groups of animals, certain land arthropods and the amniotes, 

 adaptation away from an aquatic medium is complete or nearly so; their prob- 

 lem is to retain water rather than to exclude it. and to a\oid desiccation. 



Terrestrial Arthropods. Of the land arthropods such as insects, spiders, 

 scorpions, onychophorans, and isojiods, the insects are the animals in which 

 the mechanisms of water retention and responses to humidity ha\e been best 

 studied.^-^' ^■"'"' ^'^- -''*^ Insects \ary considerably in water content (50 to 90 

 per cent). -"'*' The osmotic concentration of their body fluids may Ix? higher 

 than that in any other group of animals. Osmotic concentrations vary with the 



