Tuesday, March 14, 2006

Design flaw in the duodenum?

I love it when the word of the day from Wordsmith.org is related to anatomy. Here is today's:

duodenum (doo-uh-DEE-nuhm, doo-OD-n-uhm, dyoo-) noun

The first portion of the small intestine (so called because
its length is approximately twelve-finger breadth).

[From Medieval Latin, short for intestinum duodenum digitorum (intestine of twelve fingers), from Latin duodeni (twelve each), from duodecim (twelve).]

An illustration of a duodenum: http://www.infovisual.info/03/057_en.html
And a view from the inside: http://www.endoatlas.com/du_ge_01.html

So, exactly how long is twelve finger breadths? Anatomy textbooks say the length of the duodenum is about 25 cm (just under 10 inches). The width of my four fingers, side by side, is about 7 cm, which means that I would need just over fourteen fingers to reach 25 cm, not twelve. Maybe if early anatomists all had bony fingers like me, they would have dubbed the first part of the small intestine the quattuordenum.

The duodenum's major anatomical claim to fame is its major duodenal papilla. That's the little bump (papilla means "little pimple" in Latin) where two important tubes - the common bile duct and the main pancreatic duct - converge and dump their contents (see the figure below). The common bile duct carries bile, a greenish biodegradable detergent that is manufactured in the liver, concentrated and stored in the gall bladder, and released into the duodenum to digest fats. The main pancreatic duct carries pancreatic juice, which contains bicarbonate (the active ingredient in baking soda) for neutralizing acid from the stomach and at least 19 different enzymes for breaking down proteins, fats, sugars, and nucleic acids.

The flow of bile and pancreatic juice is regulated by a tiny circular muscle called the sphincter of Oddi. At mealtime the sphincter of Oddi relaxes and the juices flow. Incidentally, for years anatomists have been pushing to get rid of eponyms in favor of more descriptive terms, so the sphincter of Oddi is more properly known as the - take a deep breath - sphincter of the hepatopancreatic ampulla. Uh huh.... I think this is one case where the eponym will never die.

Because of my enduring fascination with unintelligent design, I can't help but wonder if there is any good functional reason for both bile and pancreatic juice to enter the duodenum via a single opening. Normally this arrangement isn't a problem: both secretions come in handy whenever the stomach squeezes another glob of partially digested goo into the duodenum. However, things can get ugly if you have gallstones. Although it isn't common, a stone can form that is small enough to travel down the bile duct, but too big to squeeze through the sphincter of Oddi. A stone lodged near the papilla obstructs the flow of bile and pancreatic secretions. Blocking bile flow is bad enough, causing pain and jaundice, but blocking the flow of pancreatic juice can lead to acute pancreatitis, a potentially life-threatening condition in which the pancreas literally starts to digest itself. Acute pancreatis has many causes, but the most common is a gallstone clogging the drain.

"But wait," chimes in the anatomically informed reader, "isn't there a second pancreatic duct - the accessory pancreatic duct - that is connected to main duct but drains into the duodenum at a different papilla? Couldn't that accessory duct serve as an alternate route if the main duct is obstructed?" The answer is yes, but only in about 60% of the population. The remaining 40% would be out of luck, because their accessory duct drains only into the main duct, not into the duodenum directly.

So is there any good functional reason for bile and pancreatic juice to drain at the same point? I can't think of any, and neither can a colleague here at UVM whose does research on gall bladder function for a living. It would make more sense for everyone to have an alternate drainage route for pancreatic juice, not just 60% of us. But, hey, no body is perfect. :-P


At 3/24/2006 7:36 AM, Blogger Brad said...

Thanks for the comment! I love the comparative anatomy perspective. Other vertebrates can teach us a lot about human structure and function.

At 9/13/2006 12:46 AM, Anonymous Anonymous said...

I would suggest you to go through the rotation of duodenum and development of pancreas.You would surely understand why the bile duct and pancreatic ducts fuse to open at major duoedenal papilla.
By the way when will people get rid of the term "common bile duct"?
There is common hepatic duct and a cystic duct and a bile duct.
Dr.Venugopala rao

At 1/15/2007 8:15 AM, Anonymous Anonymous said...

nice article....
dr venugopal rao?... as in the aiims director 'dr venugopal rao'?

At 1/28/2007 5:49 PM, Blogger Brad said...

Sorry for the long delay in replying - I just rediscovered these messages in my inbox. Not sure if the comment was left by that Dr. Venugopala Rao (who, according to this article, is no longer the Director of AIIMS). At any rate, I'm not sure that the development of the pancreas sheds any light on the problem. The pancreas starts out as two separate structures: a dorsal pancreatic bud and a ventral pancreatic bud. Each bud has its own duct. The duct in the ventral bud becomes the main pancreatic duct, while the duct in the dorsal bud becomes the accessory duct, although in many people (40%) it just fizzles. During development, the two buds rotate around the duodenum and eventually fuse (usually), leaving us with a single pancreas and one or two pancreatic ducts.

So here's the weird thing: the duct of the ventral bud - the main pancreatic duct - forms as a tributary of the bile duct (bad), while the duct of the dorsal bud - the accessory duct - drains directly into the duodenum (good). Why is it the "bad" one that usually wins? Wouldn't it make sense for the dorsal duct to persist and the ventral duct to fizzle, so that we never have to deal with pancreatitis as a consequence of gallstones? The dorsal bud actually forms first and remains bigger than the ventral bud. We could certainly do without the ventral bud if the dorsal bud just grew a bit bigger and always kept its duct. So the real question is, why does the ventral bud form at all? I have no idea.

At 5/04/2011 12:03 PM, Blogger Brad said...

BTW, if your user name refers to any PDE-5 inhibitor, your comment will be deleted.

At 5/25/2012 6:08 AM, Blogger Akshay S Dinesh said...

what if having two ducts attached reduces the mobility of duodenum? like, imagine a tube with one attachment, it can move wherever it wants. but a tube with two attachments would need both the attachments to move in synchronization and also the attachments will have to be in the same direction too.

I'm just cooking up an explanation. (because there ought to be an explanation how it helps survival)

like, accessory duct would be a second duct, but it would be lined up perfectly, because it comes from the same pancreas.

At 11/12/2012 9:26 PM, Anonymous Marcy said...

It seems to me that bile doesn't just help digest fats, but also alkalizes chyme to reduce it's acidity so that enzymes in the pancreatic juices can be activated, and digestion can in fact happen. It's a rhythmic secretion, kind of an assembly line, so that a perfect mixture of the two fluids, at the precise moment can happen in the duodenum, and nutrients can be made absorbable. I think of it as the magic zone.. If the fluids didn't flow at approximately the same time, to the same spot from the same tube, we would have a totally different chemical action, and well, we would need more fluids and tubes to manage that. :) So no, there's no flaw, in fact, really efficient design in my opinion.

At 3/05/2016 11:14 PM, Blogger Unknown said...

Well, for me the explanation for this has always been whatever follows:
The major Pancreatic duct and bile duct open into an the Ampulla of Vater (Hepatopacreatic Ampulla for that matter) which is guarded by the Sphincter of Oddi. So when the circular muscles forming the Sphincter are relaxed, both the secretions are dumped into the descending part of duodenum together. That means a single hormonal stimulus will carry out the pouring of both secretions at the same hours of need. Our Body just likes to keep things simple.


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