Where to start understanding chemical principles of human body?
If you want to start from the chemistry upwards I'd recommend you to either look into a biochemistry textbook (buy or borrow one/get a used one, since they are quite expensive) or look for some (online) courses in that field.

The textbooks I know from university studies are:


Biochemistry (Stryer, Berg, Tymoczko) [possibly downlaodable from here]

Biochemistry (Voet &amp; Voet)
Lehninger Principles of Biochemistry (Cox, Nelson)


The MIT offers quite a few (free!) online courses and there seem to be quite a number of biochemistry ones (I haven't looked into any of them, but I reckon the MIT courses aren't too bad).