Major nitrogenous excretory substance in frog, rabbit and human is urea, i.e. these are ureotelic animals. The excretory physiology in these animals may be considered under two phases, viz urea synthesis and formation and excretion of urine.
Synthesis of urea in liver : Urea is formed in liver by two processes.
(1) Deamination (2) Ornithine cycle
(1) Deamination : The amino acid is oxidised using oxygen. This result in removal of the amino group \[(N{{H}_{2}})\] and leaves pyruvic acid. the pyruvic acid can enter the Krebs cycle and be used as a source of energy in cell respiration. The amino group is converted to ammonia \[(N{{H}_{3}})\] during deamination. Deamination is also known as oxidative deamination.
\[\underset{\left( \text{Amino}\,\,\text{acid} \right)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}{\mathop{\begin{array}{*{35}{l}} C{{H}_{3}} \\ | \\ CH-N{{H}_{2}}+\frac{1}{2}{{O}_{2}} \\ | \\ COOH \\\end{array}\xrightarrow{{}}}}\,\underset{\left( \text{Pyruvic}\,\text{acid} \right)}{\mathop{\begin{array}{*{35}{l}}
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