Figure 13-9.

Mechanisms of Color Abnormalities of Plasma, Urine, and Feces during Hemolysis.

Intravascular hemolysis: Several initiating processes can cause intravascular hemolysis; formation of the complement membrane attack complex is pictured. With intravascular hemolysis, free hemoglobin is release directly into the plasma, where it is scavenged by haptoglobin and hemopexin. When haptoglobin and hemopexin are saturated, the cell-free hemoglobin causes red discoloration of the plasma (hemolysis) and is excreted in the urine (hemoglobinuria; dark red urine). The liver clears haptoglobin-hemoglobin and hemopexin-methemoglobin complexes from plasma and converts hemoglobin to unconjugated bilirubin and then conjugated bilirubin. Conjugated bilirubin is normally excreted in the bile and then converted to urobilinogen (yellow) and subsequently stercobilinogen (dark brown). However, excessive bilirubin will spill over into the plasma, resulting in hyperbilirubinemia, icteric plasma (if severe enough), and urinary excretion of bilirubin (bilirubinuria; icteric urine). Extravascular hemolysis: During extravascular hemolysis, erythrocytes are phagocytized by macrophages, which digest erythrocytes, and convert hemoglobin to unconjugated bilirubin. Excessive bilirubin in plasma causes hyperbilirubinemia with or without icteric plasma. Unconjugated bilirubin is processed and excreted by the liver (as previously described) and in dogs, the kidney. C-bilirubin, Conjugated bilirubin; Hgb, hemoglobin; Hpt, haptoglobin; Hpx, hemopexin; MACs, membrane attack complexes; MetHgb, methemoglobin; U-bilirubin, unconjugated bilirubin.

(Courtesy Dr. K.M. Boes, College of Veterinary Medicine, Virginia Polytechnic Institute and State University; and Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)