system
Given the following post, respond as if you're commenting on Reddit:
human
What causes the heart of a human fetus to start beating?
gpt
ECGs of the fetal heart show similar electrical activity to those of adults. The sinoatrial (SA) node sends electrical signals for the heart to beat. The SA node is present early on, and it is identifiable approximately 35 days of development. The electrical signals are similar but the mechanism is very different.

The fetus does not depend on its lungs and liver (gas exchange and nutrients come from the placenta). The two organs are still underdeveloped and the blood is shunted away from the organs as they develop. The ductus arteriosus allows blood to bypass the lungs, and the liver is bypassed via the ductus venosus.

In addition, there is a one-way valve, the foramen ovale, that connects the right and left atrium, allowing oxygenated blood to be pumped into systemic circulation directly, allowing the brain to receive the blood with the most oxygen and the foramen ovale closes after birth. (Unlike adult circulation, the right side of the fetal heart has a higher pressure due to this; after birth this pressure reverses, closing this valve)