questioning—and exile him in "Hell Bent" before running off again. Physical characteristics Time Lords and human beings look alike, however they differ in many respects. Physiological differences from humans include two hearts which normally beat at 170 beats per minute, 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". Time Lords also seem to have an increased resilience to higher frequencies of sound, as seen in "The Christmas Invasion" and "Partners in Crime". 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. In the serial Destiny of the Daleks, Romana was able to voluntarily stop both of her hearts beating, to fool the Daleks into believing that she was dead. The Doctor also shows a greater tolerance to cold compared to humans in The Seeds of Doom and "Planet of the Ood" and even Romana in The Ribos Operation, and in "42", the Tenth Doctor states he is able to survive at absolute zero for a short period of time. In "World War Three", the Doctor is able to shake off an electrocution attempt which is fatal to a number of humans, and appears unaffected by the energy whip wielded by the Sycorax in "The Christmas Invasion". In "Smith and Jones" the Tenth Doctor says that the radiation given off by X-rays pose no real threat to Time Lords, and proceeds to absorb an amount that would be lethal to a human, which he subsequently expels through his foot. "The End of Time" shows the Tenth Doctor as being capable of surviving (for a short period) a massive burst of radiation that would have killed anything else instantly. However, the radiation burst caused enough damage to start a regeneration. Time Lords are extremely long-lived, routinely counting their ages in terms of centuries; the Second Doctor claimed in The War Games that Time Lords could live "practically forever, barring accidents." The series has suggested that Time Lords have a different concept of ageing from humans. In Pyramids of Mars, the Fourth Doctor considers an age of 750 years to be "middle-aged". In "The Stolen Earth", the Tenth Doctor