system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how can our hearts beat (practically) forever?

Our heart is a muscle just like our biceps for example, but I can’t curl 10 pounds indefinitely, my arm gets tired eventually, even with such a light weight. Yet our hearts pump for DECADES. How is this possible, and what sets the heart apart that gives it this unique ability?
gpt
Your other muscles are actually capable of long-term repetitive usage. The two main things any muscle needs to do that is a consistent fuel source and damage prevention.

Skeletal muscle has stores of fuel in side each cell.
There's extremely short term energy, in the form of two chemicals called ATP and Creatine Phosphate. Your muscles have a bunch of this floating around, but once they start working hard, those stocks will last about 10-15 seconds. 

Your muscles also store sugar by turning into a big chained molecule called Glycogen. Once you start using your ATP and Creatine up, your muscles will start unpacking the Glycogen into sugar (glucose) to use. This is a little slower to get going - hence why you need the extremely short term fuel floating around. When they use sugar for energy, they turn it into Lactic Acid. Lactic acid makes your muscles feel sore and tired in the short term, during exercise.

When your muscles are sore and tired the day after exercise, this is *not* lactic acid, but actually because of lots of little micro-tears in the muscle that need to be repaired. The process of these tears forming and healing isn't hugely understood, but it's thought to be one of the main processes behind muscle growth.

Once your muscles have used up both their ATP/creatine phosphate stores AND their glycogen stores, they'll switch to using fuel directly from your blood (sugars, ketones, and other things). This form of energy is not very good at fueling a burst in power, but is very consistent. This is the kind of energy that marathon runners are using. This is also why marathon runners have very lean, thin-looking muscles - the glycogen storage takes up a lot of space in a muscle, and they're not training to use it. 

Your heart is a special kind of muscle that avoids both lactic acid build up (short term tiredness) and micro-tears. Part of this is just management: it's especially focused on getting rid of lactic acid and repairing damage *fast*. Your skeletal muscles know you'll rest and sleep eventually, so they mostly wait until they can take their time to repair.

Part of this is prevention. Your heart has a ton more blood flowing through it than your muscles, and so it uses fuel from the blood as its primary fuel source. The cellular structure is also different, and built to spread out the strain much more, causing fewer tears.

But keep in mind, your heart *does* rest. It doesn't stop, of course, but it does slow down a lot when you sleep. Also, your skeletal muscles never entirely switch off. They may not be *flexing*, but just being alive uses energy.