Hemoglobinuria: The presence of free hemoglobin in the urine, an abnormal finding, that may make the urine look dark. Hemoglobin is the protein in the red blood cells which carries oxygen from the lungs to the tissues of the body and returns carbon dioxide from the tissues to the lungs. The iron contained in hemoglobin gives red blood cells their characteristic color.
Red blood cells are normally taken out of circulation after approximately 4 months; they are trapped and disassembled in the spleen, bone marrow, and liver. If, however, red cells hemolyze (break down) within the vascular system, the components are set free in the blood stream. Free hemoglobin is bound by haptoglobin (another protein) and reprocessed. But if the level of hemoglobin in the blood rises above the ability of haptoglobin to reclaim it, hemoglobin begins to appear in the urine -- there is hemoglobinuria.
Normally, there is no hemoglobin in the urine. Hemoglobinuria is a sign of a number of conditions including: