Fourth Power Law and Fluid Flow

[jeskill]

I can't find the reasoning behind the "fourth power law" in my notes, nor a simple enough version of this on the web.

 

This is what I have in my notes:

"Fluid flow in animals can occur via diffusion or bulk flow. Diffusion is distance-limited, occurs in the fine capillary networks and has low hydraulic conductivity potential because of the fourth power law"

 

I'm assuming the fourth power law has something to do with the radius of the capillary vs. the radius of a vein or artery. IOW, the reason bulk flow doesn't occur in capillaries is because they're too small. I'm just not sure why it has to be to the fourth power.

 

Can anyone explain?

[jeskill]

OK, just found a fairly good explanation here:

 

 

http://courses.washington.edu/biomechs/lect25.pdf