Stem cells play a crucial part in repairing the human body by repairing damaged organs. These stemcells cannot repair tissue or organs when they cannot reach them in sufficient quantity.

The brains & eyes are a few examples of these hard to reach locations.

Basically there are 3 variables at work:

1. Free amount of stemcells in the blood.
2. The flow rate of the blood
3. How hard is it to reach the target location (eg; the organ we want to repair).