When is it ethical to declare ones life over to save another?

I am referring to the ethics of organ and skin donation. When does life support lose its meaning? When is it ethical to declare ones life over to save another? And if death is defined as the cessation of the circulatory and the respiratory systems, what do you call a person on a ventilator system? So if you could take the time to answer these questions and share your opinions it would be greatly appreciated. thanx.