In my world, ingesting a special fluid allows a person to perform magic.
The fluid is powerful and precious in my world, so it cannot be involuntarily excreted from the body in a way that would leave it lying around wherever the person goes. It is critical that the body hold on to the fluid it has absorbed until used.
Even though the fluid is the source of magical effects, it cannot have any impossible properties while it is being processed through the body. The magic begins only at the hands. The fluid must exist in the skin of the hand for the magic to happen.
I am investigating how that this concept could work with real human biology. I wish that this be based on real biology, or else the point is lost since the whole issue could be just waved away as magic. Unfortunately, presently I cannot think of a good way to make this happen.
Therefore, I wish to know...
Is there a credible way for an unmodified human to ingest a fluid, store it within the body, and transport it to the hand at will?