What causes heart rate oscillations?

I set out to make a reasonably realistic sound of a human heart beat for an art project (my background is computational neurosci and I also do sound art). I expected this to be fairly easy - they just go boomboom boomboom, and they get a bit faster or slower depending on mood or exertion. Fine - I can easily make stochastic "mood" and "exertion" parameters on a simple virtual human. What I didn't know is that heart rate oscillates (over a range of around 30bpm) with a period of around 10 seconds (Mayer waves) - and it seems nobody knows what causes it. That can't be right. It can't just be unknown can it? - surely heart rate has been studied for centuries and every time-series and signal processing tool ever must have been thrown at it. So why do papers like this still have big question marks over the cause of heart rate oscillations?