Are people actually able to give up on life?

Working with older people, you sometimes hear when they are getting ready to die; "Mom/Dad, its okay to let go. You don't have to fight anymore. You've done lots of things through out ur life, So n' So is waiting for you in heaven." My question is, can people who are on their death bed actually make a conscienous effort to stop their heart, to stop their breathing? Or is it just a natural progression that the heart and lungs become so tired that they just shut down? If a person can actually make the effort to actually stop living, could people who live longer on their death beds stay alive to see a certain relative, hit a certian special date, or wait for that one person who is not wanting them to go to say its alright to go? These questions make me wonder! Thanks for listening.