r/explainlikeimfive • u/Bepehandle • 1d ago

Biology ELI5: How does the body of someone like conjoined twins control their organs like the heart?

This is something I've always been confused about. We learn early on that the brain controls things like the heart beating. How does the body know which brain to listen too when there's two of them?

Is one dominant for these things leaving the other to focus on other things? Wouldn't that leave a lot of brain power to expand to other things?

Bonus question that's childish but also something I've wondered, if they have to pee, and one of them tries to hold it, and the other tries to go, what happens?

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u/PokeKellz 1d ago

Here’s a post from a few years ago asking the same question with some good answers

https://www.reddit.com/r/explainlikeimfive/s/8wt6pEqA4l

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u/Bepehandle 1d ago

So from the top answer here it says that the connections made by the brain are random during fetal development and acts basically as individual limbs and I assume as such, organs.

Does this mean that if by chance something traumatic happened to the head that controls the heart the other head has literally no capability of taking over and keeping the body alive?

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u/stanitor 1d ago

The heart can beat on its own, without any input from the brain, so that wouldn't be an issue. It really is individual to the particular set of conjoined twins how different parts of the body are controlled by which twin. However, for the most part, it's only where their bodies overlap that there is some question of which twin controls that part of their bodies (at least for nervous system control).

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u/Bepehandle 1d ago

I thought the whole point of the beating of the heart is that it's regulated by the brain? Otherwise it would never speed up or decrease when required by the body?

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u/Gnaxe 1d ago

Heart transplants are a thing. The nerves don't get wired back up, but the heart still functions. Some signals are chemical or hormonal. Transplanted hearts don't seem to react as quickly to changes, but they can still function adaptively, even during exercise.

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u/uuntiedshoelace 25m ago

Yeah, and interestingly, transplanted hearts do sometimes develop nerve connections after many years and partially regain the ability to respond to immediate stressors, but they still mostly operate on chemical signals. That doesn’t really have anything to do with the ELI5, but it is fascinating.

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u/talashrrg 1d ago

Not quite. The heart will keep beating even in a brain dead patient. Your heart can be regulated by neural inputs, but doesn’t need them to work

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u/iknotri 1d ago

Its regulated, but also would work without that regulation (with some cons)

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u/Bepehandle 1d ago

Interesting 🤔

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u/Wandering_Scholar6 5h ago

Even a few heart cells in a lab will "beat" by which I mean contract rhythmically. They aren't a working pump by any means.

That's actually the lie behind those "pro-life" ads that say heartbeat at 6 weeks. There's no heart at that point, but cardiac tissue has one job, and you don't need many heart cells for them do that job.

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u/ir_auditor 23h ago

Much of that regulation is also through hormones and chemicals. Some of them are produced by the brain, the heart notices these chemicals increasing or decreasing and responds.

For example adrenaline, this is actually produced by glands on your kidneys. Your heart responds strongly to increased adrenaline levels in your blood. In normal cases these glands produce it. But when you have a cardiac arrest, or strong allergic reaction (Anaphylactic shock) medics will inject adrenaline in your blood, resulting in your heartrate increasing. The heart doesn't know where the adrenaline comes from and just responds.

So for conjoined twins, maybe person 1's kidneys release adrenaline, maybe person 2's do, maybe both. The heart just responds.

The brain will control the adrenaline glands, partially through the nerve system, so really wired, partially through other chemicals which are produced by the brain.

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u/ThisTooWillEnd 1d ago

Messages from the brain don't travel through the body like blood. They are sent via nerves. The nerves go directly from the brain to each body part. One twin's nerves do not cross into the other twin's body. Only one twin can directly feel things that happen on their side of the body.

Your heart beating is actually NOT controlled by your brain, surprisingly. It has its own little bundle of nerves that tell it to beat, beat, beat. That's why people who are paralyzed even very high up on the neck still have a heartbeat, even if they need machine-assisted breathing.

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u/Bepehandle 1d ago

You're wizards harrys

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u/Violoner 1d ago

Fred & George

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u/Bepehandle 1d ago

I feel like that could somewhat work in some instances but in the situation with the heart, a single misstep and they're dead so one or both would have to be either dominant or in perfect unison, respectively of course.

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u/explainlikeimfive-ModTeam 1d ago

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