Pyroluria is a blood disorder first and foremost. Hemoglobin is a particular protein whose job it is to keep and maintain iron in red blood cells. Because of an abnormality in the synthesis of hemoglobin, a by-product is created called kryptopyrrole, known scientifically as hydroxyhemoppyrrolin-2-one (HPL). This is the chemical that is involved in the production of heme, the substance that causes blood to be red in color. In most humans, HPL causes no problems and is simply excreted from the body via the urine.