Is the D language completely dependant upon the D runtime?

Question

Lately, I've been studying on the D language. I've always been kind of confused about the runtime.

From the information I can gather about it, (which isn't a whole lot) I understand that it's sort of a, well, runtime that helps with some of D's features. Like garbage collection, it runs along with your own programs. But since D is compiled to machine code, does it really need features such as garbage collection, if our program doesn't need it?

What really confuses me is statements such as:

"You can write an operating system in D."

I know that you can't really do that because there's more to an operating system than any compiled language can give without using some assembly. But if you had a kernel that called D code, would the D runtime prevent D from running in such a bare-bones environment? Or is the D runtime simpler than that? Can it be thought of as simply an "automatic" inclusion of sourcefile/libraries, that when compiled with your application make no more of a difference than writing that code yourself?

Maybe I'm just looking at it all wrong. But I'm sure some information on the subject could do a lot of people good.