I just got to say I like the saturated red at the beginning more - gave more of a feeling of luminance. Looked like the body might look if it were somehow lit up - which is what it would have to be in order to see it - if you were transported into a blood vessell without light it would just be black. If you shine a flashlight through your hand i the dark you will get a light that looks like your original saturated image. Get a light and try it
Once I saw the blood cells with the transparency they were a lot more interesting. I also think it would be nice to have the current visible but how... more particles? I guess that would be render heavy. Hm.
Was there movement in the vessell wall like a heart beat or was that my imagination?
My thought as to the length is twice the distance and 1.5 times the speed - making the animation a little longer but the beginning a touch quicker so that the viewer is plunged into a current of rushing blood cells, is carried with them past the walls and then is out of the current in a calm spot as the cells continue rushing by. That is what I saw in the current (now pun intended) animation but I thought if you accentuate what's there it could be stronger.
Check out the subsurfacescattering on my tongue...