Question: The Doctor in Doctor who has 2 hearts, an binary vascular system (whatever that is) and a body temperature of slightly over 15 degrees. Considering he runs at roughly the same speed as his human companions, how does the strength of his muscles compare to humans?

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  1. Haha this is a great question, if a little bit out there! I’ll have a go at it from a theoretical perspective…

    First, the two hearts issue – I am not quite sure how this would work for him, unless the right and left sides of the heart were split into separate organs or the doctor has two distinct circulatory systems (blood vessels and all). No humans have two hearts except occasionally conjoined twins. Octopuses, however, have three hearts – the first pumps the oxygenated blood to the organs and then two other hearts push the blood through the gills to oxygenate it and send it back to the systemic heart.

    If his body temperature is around 15 degrees then his enzymes (the proteins that facilitate the chemical reactions in our cells) must have adapted to work optimally at this temperature. There is no good example of a mammal-like creature on earth that would fit this model. Humans and other mammals are homeotherms – so maintain a constant core body temperature, and endotherms – because they create their own heat. Homeothems on Earth typically have a core temperature between 35-40 degrees Celsius. The opposite of homeotherms are poikilothems – creatures whose temperature changes with the environmental temperature, and the opposite of endotherms are ectotherms who rely on external sources of heat such as the sun (what we would typically call ‘cold-blooded’).

    So the Doctor is theoretically a homeothermic endotherm who just maintains a much cooler core body temperature than other mammals and humans. Which makes me think, why does he need to wear clothes?! Surely to maintain a body temperature of 15 degrees he would get very very warm on Earth when he is inside buildings/running around – 20 degrees C for the Doctor would feel like 40 degrees C for humans!

    Similarly, you would expect the optimum temperature for his muscles to be lower than that of humans (based on his core temperature) so he might be better at doing explosive exercises in very cold conditions compared to humans. However, strength alone isn’t really related to core temperature so that is another question!

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  2. I was thinking about this one for ages – Helen’s answer is great! I’d add that when a normal human body temperature moves above 40 degrees Celsius, the body fatigues very quickly. The body can go above that even when the outside weather temperature is in the high teens. When the Doctor exercises at normal British temperatures, his temperature must go up very quickly, so he must have a very good thermoregulatory response (e.g. sweat a lot) to ensure his body can stay at its optimum working temperature and he doesn’t fatigue too quickly.

    Can he keep running for as long as a normal human though? This rapid development of fatigue would have a big effect on his ability to keep exercising for a long time, so if he is able to keep exercising for as long as a normal human then he must have other physiological adaptations to ensure this. How strong he is isn’t necessarily related to how fast he can run (e.g. a weightlifter is very strong, but it doesn’t mean they can run fast), but it might be down to his muscles working at a better temperature for his body, or maybe he is able to get them to work differently? As we often see him running and jumping, maybe it’s a combination of these two things? As we often see him doing these tasks on TV, it suggests his strength is about the same as a normal human.

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  3. Wow. what an amazing question. I am a huge Dr Who fan so love this. The two hearts would be highly beneficial, assuming both are healthy and prefuse the circulatory system we can now estimate that he has a pumping capacity of around 60 litres per minute under maximal conditions compared to an average human of 25 litres per minute. At rest there would be no difference in the output as the muscular demand would be the same. The lower body temperature is though a problem. Metabolic processes operate best at a neutral temperature of ~37 degrees C. So with a body temperature of just 15 degrees C these would be slowed down. The result of this would be that to do the same amount of work the energy producing processes would demand more oxygen for work. This wold mean that the enhanced heart would now have to eject more blood per beat of the heart to achieve this.
    SO given all of this would the Dr’s muscle be any stronger? Not so much strong but more efficient. They would have to produce the same amount of force at lower temperatures and increased O2 delivery. So overtime the muscles would adapt to cope with this and so you end up with a Dr that is perhaps not as efficient as the average human but not far off.

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