Secretion of bile into bile canaliculus is by

Most of the bilirubin in the body is formed in the tissues by the break down of hemoglobin. The bilirubin is bound to albumin in the circulation. Some of it is tightly bound, but most of it can dissociate in the liver, and free bilirubin enters liver cells a member of the organic anion transpoing polypeptide (OATP) family and then becomes bound to cytoplasmic proteins. It is next conjugated to glucuronic acid in a reaction catalyzed by the enzyme glucuronyl transferase (UDP-glucuronosyltransferase). This enzyme is located primarily in the smooth endoplasmic reticulum. Each bilirubin molecule reacts with two uridines diphosphoglucuronic acid (UDPG) molecules to form bilirubin diglucuronide. This glucuronide, which is more water-soluble than the free bilirubin, is then transpoed against a concentration gradient most likely by an active transpoer known as multidrug resistance protein-2 (MRP-2) into the bile canaliculi. A small amount of the bilirubin glucuronide escapes into the blood, where it is boundless tightly to albumin than is free bilirubin and is excreted in the urine. Thus, the total plasma bilirubin normally includes free bilirubin plus a small amount of conjugated bilirubin. Most of the bilirubin glucuronide passes the bile ducts to the intestine.Reference: Ganong review of medical physiology 23rd edition Page no: 483