 Time Lords and human beings look alike,<ref></ref> however they differ in many respects.Physiological differences from humans include two hearts which normally beat at 170 beats per minute,<ref></ref> three brain stems [S10E06 Extremis], an internal body temperature of 15 degrees Celsius (59 degrees Fahrenheit) and a "respiratory bypass system" that allows them to survive strangulation.The Twelfth Doctor was able to survive direct exposure to the vacuum of space in Oxygen.<ref></ref> Time Lords also seem to have an increased resilience to higher frequencies of sound, as seen in "The Christmas Invasion"<ref name="Christmas Invasion"></ref> and "Partners in Crime".<ref></ref> If severely injured, Time Lords can go into a healing coma which lowers their body temperature to below freezing which the Doctor did in "Planet of the Daleks".