There are some very clear illustrations in Nephropals blogs that help you understand the way Vitamin D is metabolized in the body and how it performs some of it's functions.
Nephropal on mineral-metabolism
Nephropal The leaky gut
Nephropal on Vitamin d
I should point out there are checks and balances and feedback loops that make what happens in practice somewhat more complicated but for a basic understanding this is a good place to start.