Two Hearts: Better than One?

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Mark McCullough gets to the heart(s) of the Doctor’s alien biology.

Before I start I should make you aware of two things: I have a reasonable amount of medical knowledge and I have nothing better to do. This is the result of that, applying my knowledge to the main protagonist of one of my favourite TV shows. As Tennant’s Doctor would say; “I’m sorry. I’m so sorry”

“What’s the point in two hearts…?”

matt-smith-heartsPerhaps the most fascinating thing about the Doctor is the fact that he has two hearts. As someone who studies anatomy, this raises a lot of questions: how does everything fit inside, what else does he have two of and what effect does it have on the rest of his body? Most of these are unanswerable or can be explained as like his TARDIS his body could be bigger on the inside. That said there are some reasonably firm deductions which can be made about how the Doctor’s body works.

With two hearts there are two options for how the pumps (the hearts) connect to the circuitry (the blood vessels). The first is that the two hearts are in series; that is, both are connected to the same vasculature. Basically the human design with an extra heart added in somewhere along the line. The second option is that the Doctor’s hearts each have their own closed vascular systems. This would mean that each of the Doctor’s organs receives a supply from each heart, essentially getting double the supply which the corresponding human organ would receive. Of these the latter is most likely. Using the evidence present from The Girl Who Waited I can be confident I am right. That fact that a disease only affects organisms with two hearts and not those with one suggests that the disease probably worked by causing an imbalance between the two circulatory systems.

Building on what has been established in the previous paragraph. This would also mean that the Doctor’s lungs would receive twice the amount of blood as a Human would. A quick look at the statistics for Human respiration it is apparent that blood supply is the limiting factor (the one which prevents more happening). I can guess this from looking at the proportions of expired gasses, which contains 16% Oxygen (21% atmospherically) that this is the case. Assuming the Doctor’s blood flow is twice that which we have, it is not unreasonable to assume that he will be able to extract 10% (double what we are able to) from the air.

In the body Oxygen is one of the main drivers of energy production within the body. In particular it is a major input into the process of Respiration; from with we obtain our energy held within food. With twice the oxygen levels that mean that if the Doctor consumed enough food he would be able to obtain twice the energy. What does this mean for the Doctor? Firstly he would be able to hold his breath for much longer periods of time, that’s a given. Perhaps a much more intriguing line of thought involves his ability to regenerate.

“When he’s dying his body, it repairs itself. It changes.”

time-doctor-regen-handThe human body is continually regenerating by a process known as Mitosis. This is where a cell divides into two new ones, thus replacing lost or damaged cells. Unlike in the Doctor’s case DNA needs to be preserved with each division, if not the effects can be disastrous. Much like the Doctor is limited in the number of regenerations, we cannot undergo Mitosis forever. The reason for this is believed to be that the Spindle Fibre shortens slightly each time and eventually reaches the stage where it cannot work. Mitosis is a process which requires a lot of energy, so by what has been established above, Time Lords should be better at it.

That possibly explains how he can change every cell in his body at once, and the residual energy could be effect off as the effect we see onscreen. This still begs the question as to how he is able to alter the DNA within the cell. Since regeneration tends to be a response to injury to the Doctor, I believe it to be an associated response of the immune system. Being a more sophisticated organism he is likely to have higher functioning immune cells. Even humans possess cells which are capable of altering the expression of genes (but not editing the DNA itself). It is possible for the Doctor immune system to go one step further and programme the DNA for a complete change.


It could be possible that the Doctor’s ability to regenerate stems from the fact that he has two hearts. The intention is for this article to be taken with a pinch of salt, as it just a bit of fun speculation. Still, I hope you enjoyed it.