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LaPlace’s Law and Toothpaste

September 13, 2010

Enabling an intrinsic understanding of human structure and function is a paradigm shift for medical education.  We are taught so many things by rote, and fundamental concepts never seem to be conveyed.

For years, I struggled with understanding how colonic diverticula develop.  It seemed that what I’d been taught didn’t fit together enough for my mind to grasp.

We’d been taught that the origin of diverticula can be understood through the principle of LaPlace’s law:

T = Pr 

meaning the tension (T) on a cylindrical structure is equal to the product of the intraluminal pressure (P) and the radius (r) of the lumen.  This was a formula I could regurgitate on tests.  But it didn’t help me understand how diverticula form. 

Then one day, I was brushing my teeth, and I noticed how hard I had to squeeze the practically empty toothpaste tube to get out enough toothpaste to do the job.  I started to realize that, if it weren’t for the tube’s presence, squeezing that hard would cause toothpaste to ooze between my fingers.

 This, in fact, is exactly how diverticula develop.  When the human colon doesn’t have enough bulk due to poor fiber intake, it has to squeeze harder and harder to push waste further down toward the rectum.  That is, there is a higher tension (T) being generated in the muscle of its wall.  This produces smaller radius (r) due to the squeeze.  But to offset it, there must be a significantly higher pressure (P).  In the case of my hands squeezing, the higher pressure forces toothpaste between my fingers.  In the case of the colon, the higher pressure forces the inner lining of the intestine – the mucosa – to bulge through the muscle layer as an outpouching known as a diverticulum.

 To be a purist, these are known as false diverticula because the outpouchings do not contain all the normal layers of the intestinal wall.  True diverticula, which do contain all three layers of the intestinal wall, are almost always congenital – that is present since birth.  But these false diverticula are the ones that usually give us the most trouble.

The paradigm shift in medicine will be to ensure that our educational processes convey true understanding rather than rote memorization.

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