Leaking Particles

i m trying to build an artery in which blood cells travel. i have created a
particle system & a curved tube. i created a Udeflector in which i have
chosen the tube as the deflector. the particle source is linked to Udeflector
via "Bind to Space Warp".  but when i run this the particles leak out
of the tube.

please suggest how to rectify it. also ny other way to do the same will be