How many hearts does a Time Lord have?

A Time Lord has two hearts. This is one of the clearest physiological differences between Time Lords and humans in Doctor Who, and the Doctor’s two-heart biology has been referenced repeatedly throughout the programme.

The dual-heart system is more than a visual character detail. In different stories, the Doctor’s hearts have been shown to beat independently or at different rates, and damage to one heart does not always immediately stop the Doctor from functioning. The arrangement helps explain some of the Doctor’s unusual resistance to injury and exhaustion, although it does not make a Time Lord invulnerable.

Time Lords also possess the ability to regenerate when fatally injured or dying. Regeneration repairs the body while changing its appearance and aspects of its personality, but memories generally remain. The traditional limit was twelve regenerations, producing thirteen lives, though Doctor Who later introduced exceptions and complicated that rule.

The Doctor’s two hearts are sometimes confused with the number of regenerations a Time Lord can have. They are unrelated facts: two hearts describe Time Lord anatomy, while regeneration concerns the renewal of the body and the number of lives available.