I am going to hijack this top comment.
> If the heart is a muscle, why doesn't it get tired?
Because it's illogical to assume muscles fatigue. They physically don't get to the point where they can no longer operate -- under normal everyday use and exercise.
What does happen, that may seem like fatigue, is a gradual shifting of fuel sources by muscle cells depending on exertion levels.
Suffice to say, it goes like this:
- First: Creatine Phosphate is the first energy source in your muscles, the most powerful one, and the least plentiful. It's the first to go during exercise and is partly why you focus on 1-5 reps for strength-focused lifting (that's roughly how long it takes to get used up)
- Second: Glycogen in your muscles and liver. This is fairly plentiful, fairly powerful, and rapidly mobilizeable. After your creatine phosphate stores are emptied, glycogen takes over. This is still a very powerful fuel source, but its metabolism creates a negative feedback loop on itself. You can only sustain moderate exertion (see: sprinting or 8-15 reps moderate weight) for around 60-90 seconds, before glycogen is no longer easily accessible (note: accessible, not depleted. It's almost impossible to deplete in a single workout)
- Finally: Fat. Once you've exhausted glycogen, your body turns to fatty acid oxidation. This is the least powerful but most plentiful. In normal exercise you use a mix of fat and glycogen depending on how hard you exert yourself. Harder: more glycogen. Easier: more fat. If you want to know how it feels like to run on only fat: do a marathon -- then hit the wall. What you experience is literally the complete depletion of glycogen, and a transition into "low-power" mode as your body starts running on the only fuel source it has left: fat. You have months of this fuel source on your body, you won't run out. You can keep walking for days without having eaten anything or slept, but you won't be able to run at any pace that resembles a jog. I know, because I have.
What's the point of all this? It's to illustrate that muscle "exhaustion" is a misnomer. Muscle exhaustion is, in reality, a depletion of power-generating fuels leading to a state of minimal exertion.
Coincidentally, that's the mode the heart operates in 24/7.
Coincidentally, the heart has an asinine amount of mitochondria to fuel non-stop fatty acid oxidation.
The heart doesn't fatigue because it's not physically possible. It doesn't need creatine phosphate or glycogen (very very little) to pump blood. It's not a strenuous task.
An aside: if you had as much mitochondria in all your cells, as you do in your heart's, you would waste away.