In Doctor Who, season three episode one, the episode titled "Smith and Jones", the doctor lets the plasmavore suck out his blood in order for it to be captured, and passes out from the blood loss. Martha comes to his rescue and performs cpr, but in the middle of it she somehow realized that he had two hears, and starts pumping his chest in two locations.
I was confused as to how she knew he had two hearts. How did she know?