VPB 121: Veterinary Physiology - II (2+1)

BILE PIGMENTS

• The bile pigments are the waste products of the haemoglobin (Hb) breakdown give yellow/green colour to bile. Bilirubin,the brownish yellow pigment is readily oxidized into biliverdin which gives green colour to the bile of the herbivores.
• Bilirubin is produced from Hb with destruction of RBC in the reticuloendothelial system (RES) of the body (eg. spleen, liver and bone marrow).
• It is normally present in the blood in low concentration. Bilirubin is highly soluble in all the cell membranes. But high concertration of bilirubin is very toxic, needs excretion through bile,one of the very important functions of the liver.
• When the RBC have lived out their life span of 120 days they have become too fragile to exist longer in the circulatory system, their cell membrane rupture and release Hb.
• Released Hb undergo phagocytosis by tissue macrophages (RES). Haemoglobin is split into haem and globin of which globin is readily transported to protein pool of the body for reutilization.
• Haem ring gives (a) free iron that is transported in the blood by transferrin and (b) 4 pyrol nuclei , the substrate for bile pigments. The first pigment formed is biliverdin and is rapidly reduced to free bilirubin , which is gradually released into the plasma.
• Free billirubin immediately and strongly combine with the plasm albumin and transported through the blood and the interstitial fluid. Bilirubin bound with plasma protein called as “Free bilirubin” is insoluble that can’t pass through kidney.
• Within hours, this free billirubin pass through the hepatic cell membrane, where it is released from the plasma albumin, but instantaneously combines with one of the 2 protein (Y and Z proteins) inside the liver cell which traps the bilirubin.
• Soon after bilirubin is released from these proteins it conjugated with glucuronic acid, phosphate and sulphate to form bilirubin glucuronide and 10% as bilirubin phosphate and rest as billirubin sulphate respectiely.
• These conjucated forms are excreted by active transport into the bile canaliculi.

Formation and fate of urobilinogen

• In the intestine, ½ the conjugated bilirubin is converted into highly soluble urobilinogen and stercobilinogen by bacterial action.
• Some of the uro bilinogen gets absorbed from intestinal mucosa into the blood.
• Most of them re-excreted into the gut by the liver
• After exposure to air the urobilinogen is oxidized to urobilin and excreted through urine, giving yellow colur to urine.
• Stercobilinogen is converted into stercobilin. and excreted through the feces.