Bilirubin is a bile pigment. It is mainly derived from breakdown of hemoglobin in red blood cells (RBCs). Hemoglobin is the molecule in RBCs that carries oxygen.
Bilirubin normally circulates in the blood attached to a molecule of the protein albumin. There, it exists in two forms: conjugated (direct) and unconjugated (indirect). Bilirubin dicluguronide, the conjugated form, is water-soluble and is excreted in bile.
In hemolytic anemia, where RBCs are being broken down in the bloodstream (for any of the possible reasons cited in this article), the resulting increase in bilirubin is unconjugated, because it has been released from the red blood cells, but the resulting hemoglobin has not yet traveled to the liver for conjugation. For that reason, if a patient (dog, cat, person, etc.) presents with pale gums, icterus (jaundice) and a blood test result of elevated bilirubin that is unconjugated, the diagnosis of intravascular hemolysis is determined and the cause for the hemolysis must be found in order for treatment to be effective.
On the other hand, patients are much more likely to become jaundiced because of biliary tract obstruction. In hemolytic anemia, RBC destruction is unlikely to exceed the ability of the liver to process and conjugate RBC breakdown products.
Bile obstruction, on the other hand, can be partial or complete. If no bile can leave the liver and be delivered to its ultimate destination in the small intestine, it backs up into the bloodstream, rapidly leading to icterus. This bile will still be conjugated by the liver until the patient is quite ill, though, so a test on the blood will show a high total level of bilirubin, with most of the bilirubin being conjugated.
Bile obstruction can occur inside the liver itself when a diseased liver swells and tiny passageways called canaliculi are no longer open enough to allow bile flow.
Bile obstruction can also happen where the canaliculi dump bile into larger tubes called bile ducts, which wind their way through the pancreas and into the duodenum, the first section of the small intestine. Near the pancreas the bile duct splits in two, and obstruction past that split can result in a reduction of flow without a full stoppage. In that case, jaundice and the resulting illness will usually be less and take a more chronic course than if total obstruction occurs.
See you tomorrow, Dr. Randolph.