He has two hearts.
Two lumps of cardiac muscle (cardiac myocyte), each weighing around 11 grams, crammed into his ribcage, which for all intents and purposes could be larger on the inside for all the organs it contains within such a small space. Each heart pumps blood at a rate of 4-6 LPM, working in unison to bring around 12 liters of blood spiraling through his binary cardiovascular system every minute.
It's very little wonder Grace said his blood doesn't look like blood. It simply doesn't. It's like human blood run through a cheese grater. Smaller, more efficient cells, maybe, but different. They don't move nutrients the same way because the hearts don't pump them the same way.
The hearts don't move emotions the same way, either. In the figurative sense, of course, as the muscle of the heart is hardly the part of the body that holds emotion. It's a silly notion derived from human romanticism. In fact, saying "heartsbreak" in Gallifreyan is really just a mockery, a play on words trivializing emotion to its most simplistic form: human.
His left heart is purely Gallifreyan. Solid, beating a single beat without any wavering. It's the heart he was born (created, Loomed, whatever that insignificant term prior to a first regeneration is) with. The heart that beat fiercely as he ran from the Tempered Schism, the heart that first swelled at the sight of a meteor shower, and the heart that first closed up and pushed someone away.
It's a heart that belongs to the Master, in many ways. A heart that was given when the Doctor was young and things seemed so simple. He never quite had the courage to ask for it back, and even now he isn't certain he would. So the heart is his alone as, after all, the Master would never share a possession. Especially one as unique as the Doctor's left heart. Even from his grave the Doctor can often feel the Master's hand curling around his heart like a tumor, threatening to slip out of remission and overtake the entire torso with the slightest change of climate.
It's a heart that ages. He aged as it beat, then as a second one grew with his regeneration, he stopped aging. It never did, though. He can still feel it, hard and crusted, beating against his ribcage during his most trying times. Beating out a staccato of what is right and Lawful at 67 beats per minute without fail.
It's a heart that doesn't truly know how to love. It's the heart Joan laid her hand on when she asked him if one of his hearts belonged to John Smith. It made telling her 'no' easier. It made breaking her all-too-human heart passable. No matter how much his other heart cried out for her.
His right heart is human. First formed (born) after his first regeneration. It sits a little too far to the left, as if worried it might pull away; lose its place in the Doctor's chest. It beats a little off, always few tics too early or too late. Not nearly bad enough for serious medical treatment (so his Time Lord doctors would tell him), but it's enough to be a bother at times. It's the heart that first fell in love, the heart that fluttered for the first time in anticipation, the heart that broke when he couldn't save someone.
It's his mother's heart. It's Susan's heart. It's John Smith's heart. It's a heart that falls in love too quickly and it's a heart that bruises too easily. It's the part of him that seems to fall out when he loses someone. It's the part of him that feels empty when he's alone in the Void of space. It's a muscle that's not strong enough to work on its own and requires its aged counterpart just to keep him going.
It's a heart that loves, deeply but quietly. Solidly but fearfully. Completely but imperfectly.
It's the heart he offers to his companions. It's the only part of him that can love. He keeps his cold, aged heart close. Keeps his alien-ness inside while showing them the side of him they might be able to understand.
And perhaps, just perhaps, he's too terrified of them breaking his heart to offer them both. So he gives the one that's already bruised, keeps the hard and cold one safe. Just in case. Because it might happen if he lets it.
But it's a self-fulfilling fear, of course. By only offering them half, he can never offer them enough.
Or perhaps he offers them enough but pulls it back just as quickly, fear of that hurt settling in. Or perhaps he's afraid of damaging what he gives to them more than he's afraid of losing them.
After all, while that right heart may never age, it can grow bitter. Fearful. Jaded.
So he only gives them one.
And when they break his heart---as they inevitably do---the other one still beats that solid, cold rhythm in his chest. Reminding him how he was right to keep himself inside. Reminding him that it's not the first time and it won't be the last.
He simply has to accept it.
Half of him does.
Half of him doesn't.
His hearts beat blood to every part of him. The parts that blush, the parts that cry. The parts that find reason and understanding, the parts that run away.
But as with the two hearts, his parts don't work together. Things don't come out perfectly. No matter how hard he tries, he still loses part of his human heart. Broken as they're ripped away, walk away to another universe, or leave him to be with a being who can offer his only heart without any fear.
But the Doctor has two hearts.
They don't beat in time with the ones he loves.
Muse: The Doctor (Ten)
Fandom: Doctor Who
Word Count: 990
Special thanks to ambitious_woman for the beta!