The top answer is missing one part: The heart is actually "resting" 60-70% of the time, the time between contractions, and it fires in a staggered fashion. Th

The top answer is missing one part: The heart is actually "resting" 60-70% of the time, the time between contractions, and it fires in a staggered fashion. The high level of mitochondria means it can produce the ATP it needs during these short rests and is ready to go the next time the SA node zaps it.