Creatinine is a break-down product of creatine phosphate in muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

, and is usually produced at a fairly constant rate by the body (depending on muscle mass). Creatinine starts to decompose above 295 °C.

In chemical terms, creatinine is a spontaneously formed cyclic derivative of creatine.
Creatinine is chiefly filtered out of the blood by the kidneys (glomerular filtration and proximal tubular secretion).