. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 188 Comparative Animal Physiology ammonia values have been reported as ml. in Maja,, mg. in Sepia,-"^ mg. in Homarns, mg. in snails,'^- and mg. in the earthworm Pheretima. -^ Ammonia increases on standing in blood of Hydrophihis and Dytiscits, but not in blood of crayfish, lobster, or snail. â ^'* The blood of Anodonta is low in ammonia ( ml.).-^"' Because of its toxicity ammonia must be excreted rapidly as formed or con- verted into a less tox
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 188 Comparative Animal Physiology ammonia values have been reported as ml. in Maja,, mg. in Sepia,-"^ mg. in Homarns, mg. in snails,'^- and mg. in the earthworm Pheretima. -^ Ammonia increases on standing in blood of Hydrophihis and Dytiscits, but not in blood of crayfish, lobster, or snail. â ^'* The blood of Anodonta is low in ammonia ( 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 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 Hâ0. NHg (CH2)3 COOH NH 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,. Please note that these images are extracted from scann
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