Except here, there really is no bank the way there is for corporations or private individuals: Governments owe debt to investors who are looking for a safe risk, and as long as confidence is high, the amount of debt the government can take on is practically unlimited.

Debt as a percentage of GDP is a way to quantify what the confidence is likely to be, but that's a convenient estimate, not reality. Confidence is paramount, and that's difficult to quantify.

The essential point is that nobody seriously expects America to default on its loans.

And, no, the rantings and hysteria aside, China can't "call in" loans. That's not how it works. Bonds have a maturation date, decided in advance.