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