system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If a heart needs itself to beat, how does it start?
gpt
Heart muscle beats automatically.

Each cell will spontaneously trigger to contract on its own. The trick is that in the heart there are lots of cells next to each other. When one triggers it also triggers the ones next to it, which then trigger the ones next, and so on until the whole thing contracts in an even wave.

After a cell has triggered, it has a "dead time" when it can't be triggered. After the dead time, the each cell either waits for a trigger, or if one doesn't come, then it can spontaneously trigger.

During development, the embryo is small enough that oxygen and nutrients can get to the heart cells without the need for circulating blood - it just needs the placenta to be close enough. The heart cells start forming, and start beating at this time.

Eventually, as the embryo grows, the body gets big enough that it needs the blood to circulate, but by that time, the heart is already beating and starting to pump.