Anatomy of the Doctor: Part 2

      Time Lords have two hearts and a binary vascular system, which enables them to survive major accidents and many physical and temporal shocks that would kill a human being. They also have a respiratory bypass system that enables them to survive without breathing for some time.

Photo taken by: BBC America

     A new version of the Doctor is created after a biological metacrisis combines genetic information from the Doctor's severed hand and from the human Donna Noble, with energy from an abortive regeneration. Although he looks like the Tenth Doctor, this half-Time Lord and half-human creation has only one heart and is unable to regenerate.