188 



Comparative Animal Physiology 



ammonia values have been reported as 0.4-2.4 mg./lOO ml. in Maja,, 2.8-4.8 

 mg. in Sepia,-"^ 1.6-1.8 mg. in Homarns, 0.7-2 mg. in snails,'^- and 2.3-3.0 

 mg. in the earthworm Pheretima. -^ Ammonia increases on standing in blood 

 of Hydrophihis and Dytiscits, but not in blood of crayfish, lobster, or snail. ■^'* 

 The blood of Anodonta is low in ammonia (0.051 mg./lOO ml.).-^"' 



Because of its toxicity ammonia must be excreted rapidly as formed or con- 

 verted into a less toxic substance. Ammonia is highly soluble in water, is dif- 

 fusible, and is excreted as ammonia by many animals which have ample water 

 for carrying away excretory products. 



Urea. This excretory product is less toxic and slightly more soluble in water 

 than ammonia. Human blood normally contains 18 to 38 mg. urea per 100 ml; 

 values above about 40 mg./lOO ml. indicate uremia, although much higher 

 concentrations are tolerated. The livers of many animals contain an enzyme, 

 arginase, which catalyzes the formation of urea from the amino acid arginine. 

 There is evidence that one method of conversion of NH^ to urea is by the 

 Krebs cycle (Fig. 32). When this cycle operates, urea production by tissue 



H„0 



NHg 

 (CH2)3 

 CH.NHe 

 COOH 



NH 



CH.NHj 

 COOH 



CITRULLINE 

 Fig. 32. The ornithine cycle. From Baldwin.* 



extracts is increased on addition of ornithine and citrulline; urea is formed from 

 dietary arginine by the action of arginase alone. In some animals urea is 

 apparently formed by unknown reactions other than those of the ornithine 

 cycle. 



Uric Acid. Some animals, particularly birds, terrestrial reptiles, and some 

 snails and insects, convert their ammonia to uric acid. This substance has a 

 low toxicity and is relatively insoluble; it can be stored or excreted in crystalline 

 form. The mechanism of formation of uric acid from ammonia is unknown, 



