Creatinine is a nonprotein nitrogenous compound found in the blood and excreted by glomerular filtration at a constant rate (Feher 2012).
Creatinine is derived from nonenzymatic degradation of creatine phosphate which is a phosphagen for vertebrate animals. Phosphagens are high-energy phosphate compounds which supply immediate but limited energy during sudden demand for skeletal muscle and brain. Creatine phosphate is a phosphagen for vertebrate animals, whereas arginine phosphate is the phosphagen for invertebrate animals.
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