Mandatory Overtime

Does anyone know what the law states about being forced to stay due to your relief not showing up? I am an ER nurse and due to short staff on nights have been forced several times to stay over because of lack of staff on nights. I even had to pull a 24 hour shift because no relief showed up and I was told by administration that I couldn't leave because I would be abandoning my patients. This looks like it's going to happen again this week and no contingency plan has been enforced and I want to know my legal rights. I've spoken to my risk manager and tried to talk to legal, but am not getting anywhere. They keep telling me that the laws for others don't necessarily apply to nurses because "people's lives are at stake and will be put in jeopardy if I leave". I don't want to abandon my patients, but I also don't want to be forced to work the hours if I'm not legally mandated to.