Dev. Team: workload versus capacity

Hi again,
something crossed my mind:
during the Sprint Planning the Dev. Team decides about their own workload for the next Sprint. The Dev. Team estimates the PBI's, but also get input from the PO about what should be done.
It may be unrealistic, but how should the Dev. Team act, if the workload for one Sprint exceeds the capacity? I know, that the workload may me renegotiated with the PO. But what's about other solutions, e.g. recruting of additional Dev. Team members? Or beginning the Sprint known that the workload definitely will be unfinished?

Thanks!