Why must animals excrete nitrogenous wastes?

Metabolism of proteins and nucleic acids produces ammonia, urea and uric acid, which are toxic and must be removed to keep body fluids balanced.

The three main nitrogenous wastes & their toxicity/water need

Ammonia (most toxic, needs most water), urea (less toxic, moderate water), uric acid (least toxic, least water).

Excreting ammonia (mostly by diffusion across the body surface or gills). Seen in many bony fishes, aquatic amphibians and aquatic insects.

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