Hemoglobinuria is the presence in the urine of hemoglobin (the oxygen-carrying pigment in the blood). Hemoglobin is mainly contained within erythrocytes (red blood cells), although a small amount is free in the blood plasma. Excessive hemolysis (breakdown of red bloods), which may be caused by heavy exercise, cold weather, falciparum malaria (blackwater fever), and hemolytic anemia, increases the concentration of free hemoglobin in the plasma. The excess of free hemoglobin in the plasma is excreted in the urine.