04/09/2026
The human heart starts beating around the fourth week of pregnancy (about 22 to 23 days after fertilization).
When and How the Heart StartsTiming:
The embryonic heart begins to pulse when the embryo is just a few weeks old.
Trigger: Heart cells fire electrical and calcium signals spontaneously, linking up to beat together in a coordinated rhythm.
Why the Heart Has So Much Energy Mitochondria Power:
Heart muscle cells are packed with mitochondria (the energy factories of cells), making up about 40% of the cell's space.
Fuel Efficiency: The heart burns fatty acids, glucose, and lactic acid very efficiently to produce a continuous supply of ATP (cellular energy).
Fetal Circulation Before birth, a fetus cannot use its lungs to breathe, so the placenta handles oxygen and waste removal.
Umbilical Cord: One umbilical vein brings oxygen-rich blood from the placenta to the baby, while two umbilical arteries carry waste-heavy blood back.
Three Shunts: The fetal body uses special bypass pathways to route blood past the unneeded lungs and liver:
Ductus Venosus: Bypasses the liver.
Foramen Ovale: A hole between the right and left upper heart chambers that sends oxygenated blood straight to the brain and body.
Ductus Arteriosus: A blood vessel that routes blood away from the lungs into the aorta.
