Which of the following undergo enterohepatic circulation?
Correct Answer: Bilirubin
Description: Substances are said to undergo an enterohepatic circulation (EHC) when they are excreted into the bile, pass into the lumen of the intestine, are reabsorbed and then return to the liver the circulation. Many endogenous and exogenous substances can undergo an EHC. Among the endogenous substances are the bile salts, the biliary lipids and biliary phospholipids, bilirubin, estrone and estriol, folic acid, vitamin B12, and urobilinogen. In contrast to bile salts, which undergo a highly efficient enterohepatic circulation, glucuronic acid conjugates of bilirubin are biliary excretory molecules that in health do not have a continuing biologic life. Intestinal absorptive cells are devoid of recapture transpoers for bilirubin conjugates, and their large size and polarity prevent absorption by passive diffusion. However, unconjugated bilirubin can be absorbed passively from any pa of the small and large intestines. This can occur only if unconjugated bilirubin is kept in solution and does not undergo rapid bacterial reduction to form urobilinoids. Biliverdin results from the breakdown of the heme moiety of hemoglobin in erythrocytes. Macrophages break down senescent erythrocytes and break the heme down into biliverdin, which normally rapidly reduces to free bilirubin. Para-aminohippuric acid (PAH), a derivative of hippuric acid, is a diagnostic agent useful in medical tests involving the kidney used in the measurement of renal plasma flow. It filters freely through the glomerular membranes. Urobilinogen is a colourless by-product of bilirubin reduction. It is formed in the intestines by bacterial action on bilirubin. About half of the urobilinogen formed is reabsorbed and taken up the poal vein to the liver, enters circulation and is excreted by the kidney. Ref: guyton and hall textbook of medical physiology 12 edition page number: 420,421,422
Category:
Physiology
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