A Time Lord's body makes a huge amount of energy when regenerating. There is so much of this "regeneration energy" that a Time Lord can regrow a body part that gets removed, as long as it happens soon enough after regenerating. When the Doctor regenerated into his 10th incarnation, his hand was cut off in a sword fight. Because it had been less than 15 hours since he had regenerated into this incarnation, he was able to quickly grow a new hand.