Gallifreyan physiology

Externally, Gallifreyans are identical to humans.

Natural born Gallifreyans born before the time of Rassilon and Pythia, who cursed Gallifrey with sterility, had two hearts. (NA: Cat's Cradle: Time's Crucible, NA: Lungbarrow) Loom-born Gallifreyans (be they Time Lord or not) are loomed/born with a single heart and gain their second heart after their first regeneration. (MA: The Man in the Velvet Mask)