These words, as well as doch, denn, mal etc. are called Modalpartikel, because they just give an emphasis to the sentence, in this case to emphasize that 'ich' had really said it. Often they are used to make a sentence sound more polite, e,g. Kommen Sie bitte her! sounds pretty rude, while Kommen Sie doch bitte mal her! is more acceptable.

You could replace this "ja" with "doch". However I really don't know how to explain the meaning in English as such a word does not exist in the English language. Maybe someone else has an idea.