Syag (and gamanit)
the heart as an internal pacemaker as long as it has a steady supply of oxygen, glucose and the correct balance of electrolytes it can pump indefinitely (in theory) without a brain. Thus evn if the brain is not connected (as in the sheep experiment) or not functioning (like in the poor case of this girl) as long as oxygen is supplied via a ventilator the heart can continue pumping.
In contrast however breathing requires a steady signal from the brainstem for each breath.