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Comment by parpfish | Hacker News Reader

I was just thinking about heart transplants and how they’re treated as commonplace now, but they are nuts when you think about them.

You can hook up the nerve wires and blood pipes from one body into the heart from another body and it works? Just thinking about the simple physical connections would make me nervous. Why isn’t there blood just leaking out in your body? Why isn’t that other heart sliding out of place while you move around?

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Fun fact: the nerves between the brain and heart are never reattached. A heart already has all of the facilities to keep working all by itself.

Even weirder is that an adult kidney transplanted into a child will actually shrink to fit.

But presumably the nerves between the brain and heart exist for a reason. What functionality do you lose by not reattaching them? Do things like "heart beating faster when nervous" depend on this nerve signaling, or is it done via other chemical signaling.

Reading up on this there is some chemical signaling from the blood, but the brain is not controlling the rate of the new heart

That is actually wild and I wonder about the practical consequences as well.

So you get into a stressful situation or get a flee or flight response, and your heart is not affected by it, or at least the heart rate, but your cardiovascular system may be needed because in that case the rest of your body is, and presumably you may need much more blood to be pumped out to your organs among other things.

It is a cool food for thought.

He's asking about what the nerves do in a normal person

Yes, nerves from the brain to the heart can influence heart beat (and other features like heart conduction and blood flow to the heart itself) in response to stress and exercise. Heart transplant recipients lose these features. They make poor marathon runners :)

Heart rate muscle tissue is largely influenced by hormones. The sympathetic system releases noradrenaline to speed up the heart rate, while the parasympathetic system releases acetylcholine to slow it down. But the release of these hormones is controlled by nerves, which are largely severed. So you typically end up with a less "dynamic" heart rate -- resting rate higher and responsiveness to stimuli reduced. These nerve connections can regenerate to some degree but it's individual and rarely close to what they were before.

Another fun fact: you can remove all of the brain of a cat on threadmill except the brainstem and he'll keep walking.

…Do you have a link? I’m not sure if I want to see proof of that, but I’m at the same time curious how you’d manage to do it without disturbing the cat.

Will it grow again as the kid ages?

Yes

The part that always gets me is the lack of nerve reconnection - the heart keeps beating on its own, no nervous system hookup required. It's literally running off internal timing, like a mechanical watch made of cells

A human counting is also a mechanical watch made of cells.

It's not exactly the same as a biological heart transplant (I assume), but you might be interested in reading the surgical instruction manual for a SynCardia artificial heart: https://www.accessdata.fda.gov/cdrh_docs/pdf3/P030011c.pdf

For instance, there's a fun diagram on page 11.

There's also an operator's manual for the "driver" that powers the heart: https://www.vumc.org/cvicu/sites/default/files/2020-03/Opera... Which includes all kinds of (appropriately!) paranoid warnings such as:

> To avoid accidentally switching off the AC power to a docked Driver, do not plug the Driver into any electrical outlet controlled by a wall switch.

Blood pipes yes, nerves no. Transplanted hearts are de-enervated.

Heart surgeons are straight up deleting the equivalent of dead code produced by evolution.

Heart surgeons are superheroes.

In all viscera sympathetic and para- keep balance of muscle function. But in steady state the para- must fire more powerfully because the sympathetic intrinsically dominates a little. So if you cut off both systems, the organ has a sympathetic boost. In heart's case this will make it beat faster, I believe about 20-30 more beats per minute (after some self-compensation). This is easily treated with a beta blocker.

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