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By what mechanism does an obstructed bile duct (for example gallstones) cause steatorrhoea (excess fat in the stool)?

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2 Answers 2

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The function of bile produced by the liver and concentrated in the gall bladder is to aid in fat digestion. Bile acts as an emulsifier to allow the fat to be packaged into small droplets, micelles. The breakdown of fats into small micelles greatly increases the surface area for lipases produced by the pancreas have to act on. These are then absorbed across the intestinal wall.

Breaking down of fats into micelles

Bile breaking down large fats into small micelles

If this first step of bile production and transport is lacking (due to liver dysfunction or obstruction anywhere in the biliary tree), the fats are not packaged into micelles and are consequently very hard to digest. They pass through the entire alimentary canal virtually undigested causing steatorrhoea, fatty, bulky and foul smelling stool.

The Biliary Tree

The Biliary Tree

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Gallstones 'block' the plumbing system (i.e. the biliary tree) that drains bile produced in the liver from flowing into the duodenum through the ampulla of Vater. Since bile is a dietary detergent for fats (cholesterol, triglycerides, free fatty acids), the blockage of bile impairs fat absorption in the small intestine and the fat passes into the large intestine and then into the stool. There is always a small amount of fat that passes into the feces (typically very low though), but if bile is not present to aid fat absorption then you get "fatty stool" aka steatorrhea.

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Thanks also here for your answer. This seems like common reasoning, but some references of, e.g., supporting websites are still helpful. +1. I added one web link as an example. You use plenty of jargon, which is OK, but linking some terms may greatly improve readability. –  AliceD Jun 19 at 1:14
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AliceD - what 'grade level' are you supposed to answer on? Do you personalize that to the language being used in the question? Thanks for your advice! –  Vance Albaugh Jun 19 at 1:56
    
Now that is a truly great question. We have a meta site meta.biology.stackexchange.com and I really encourage you to post this (I can do it for you if you wish). In general, personally, I scan the question and if the level of OP is obviously rock bottom I adapt my language accordingly. In general, this site is targeted for MSc students and grad students. But younger folks tend to be attracted for sure, as well as older ones like me :) Btw - this post is really accessible. –  AliceD Jun 19 at 2:05

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