[TR] TR4A Piston Upgrade

[Randall]

> But the bottom line is, very small improvements in 
> artery diameter can result in very meaningful improvements in 
> perfusion.

Arnold was kind enough to send me the key phrase "Poiseuille's law".  It's
apparently pretty standard fluif dynamics.  The key point of the derivation
is that the fluid is viscous and sticks to the walls of the pipe (blood
vessel), meaning the layer immediately next to the wall remains stationary
and the adjoining layer drags against it.

Thanks to all, 
Randall