I'd consider "controlling" something to be more direct, but that's semantics I guess.

If I heard "control heartbeat", I'd think more like breathing, where one could "hold their heart" like they "hold their breath".

If meditation lowers heartbeat and temperature, I imagine that it's "control" in the strictest sense, but I'd use "influence" or some other term if I was personally describing it.

You can actually up your heartbeat and temperature too, but you are right, it's more influencing that direct control.

A muscle is instant and precise.

Have you already felt being conscious of your breathing and feeling like being unable to let it go back to automatic control ? Well …

> one could "hold their heart"

… now that sounds creepy.

Not all muscles have a spark plug at the tip delivering its own signal to pulse. The sinoatrial node does this in the heart, and it is a reciprocal system like a v8 engine where one side firing begets the other side firing next. When these signals to fire go out of whack, you get fibril movement, which is when a defibrillator is deployed, it shocks the SA node forcing a 'reboot' of sorts and if all goes well, normal sinus rhythm resumes.