Chemical structure of creatinine. Creatinine is a breakdown product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass). Physiology It is mainly filtered by the kidney, though a small amount is actively secreted. There is some tubular reabsorption of creatinine, but this is compensated by a roughly equivalent degree of tubular secretion. If the filtering of the kidney is deficient, blood levels rise. This effect ...
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