keep it inside your body, where it belongs [antecedent of 'where']

Solution 1:

Yes, so if you want to keep it inside your body, where it belongs, you should stop jumping off trucks doing eighty on the interstate!

If you split the relative clause into a separate sentence, it would be "It belongs inside your body." So I think it's reasonable to say that the prepositional phrase "inside your body" is the antecedent of where in the quoted sentence.

Actually, your body, where it belongs, needs it.

This sentence doesn't sound very good to me (I'm not sure why).*

Let's look at another sentence with a similar structure. I found one that seems suitable in "Multilingual Writing, for Example" by Anne Tardos:

Once grown up, I moved to New York, where I live now.

Split into a separate sentence, the relative clause would be something like "I live in New York now." But the sentence with the relative clause doesn't contain in, of course. So we'd have to say that the antecedent here is just the noun phrase "New York".

Based on this, I assume you're wondering why we can't say the antecedent of "where" in the first sentence is just the noun phrase "your body". And I guess I don't have a definite answer to that question. I would say that where can have as its antecedent either an entire prepositional phrase, or a noun phrase that could occur as part of a prepositional phrase. The attachment of the relative clause would thus be ambiguous, as it is for relative clauses after noun phrases that contain other noun phrases.

It's clear I think that where can take a prepositional phrase as its antecedent because there are some prepositional phrases in English that don't contain an (independently written) noun phrase—e.g. "right inside" (which historically is from the preposition in and the noun side, but synchronically is not syntactically divisible and is written as one word)—and where clauses can be used with these in the same way that they can be used with prepositional phrases containing conventional noun phrases:

Each is lovingly cooked on our special slanted grills, gently placed at a degree just tilted enough to keep all that juicy goodness right inside, where it belongs.

(Hoffbrau Steak & Grill House)

It doesn't seem attractive to argue that the antecedent of where here is "-side".

*I can give an example of a similar structure where I have a matching intuition about using a noun phrase vs. a prepositional phrase alongside where:

• "New York is where I live now."/"Where I live now is New York." These are fine.

• "Your body is where it belongs."/"Where it belongs is your body." I think that this is not unacceptable to me, but I prefer "Inside your body is where it belongs."/"Where it belongs is inside your body."

Solution 2:

1. I believe you can remove the comma and still keep the sentence grammatically correct. However I'm not sure if that would make it a restrictive relative clause, as where it belongs doesn't give us any additional information as to the blood. (Sources: https://www.thoughtco.com/restrictive-relative-clause-1691914)

2. The antecedent in this case is inside, with your body modifying it.