Fetal Heart Sketch
A sketch of a normal fetal heart at about 20-weeks, showing major features and circulation;
Not to scale; some features may be exaggerated for visibility;
Key to labels
Ao: aorta
AoD: descending aorta
arch: Aortic arch
duct: ductal arch ("ductus arteriosis")
FO: oval foramen ("foramen ovale")
IVC: inferior caval vein
LA, RA: left, right atrium
LPA, RPA: left, right pulmonary artery
LPV, RPV: left, right pulmonary veins
LV, RV: left, right ventricle
PA: pulmonary artery
SVC: superior caval vein
Valves (location)
Mitral valve: between the LA and LV
Aortic valve: between the LV and Aorta (Ao)
Tricuspid valve: between the RA and RV
Pulmonary valve: between RV and Pulmonary Artery (PA)
Circulation (arrows)
Filled (dark) arrows: oxygen-rich blood from placenta
Unfilled arrows: venous return from upper & lower body via SVC & IVC
Cross-hatched arrows: mixed venous and placental circulation
Minimal pulmonary circulation through lungs (not shown)
Brief Description
The normal fetal heart has three important differences, from the heart after birth, due to the reliance on the mother's blood supply via the placenta for nutrition, oxygen and blood flow:
1. The oval foramen ("foramen ovale"), labelled FO, is covered by a "flap" that allows "red", oxygenated blood from the placenta to enter the left atrium (LA). After birth, the FO flap closes due to lack of flow from the placenta and high pressure in the left atrium.
If the FO flap fails to close after birth (or has a defect so that it cannot seal the FO) this is known as an atrial septal defect (ASD), and results in mixing of red and blue blood.
2. The duct ("ductus arteriosis") is a bypass, so that "blue", less oxygen rich blood from the veins does not enter the lungs, but returns to the placenta. At birth, a baby takes its first breath, the lungs inflate and the duct slowly closes to allow the lungs to take over the oxygenation of blood.
However, if there is "duct-dependent" heart disease, the closing of the duct may reduce the supply of oxygenated blood to the body causing "blueness" or breathlessness.
It is important to detect these conditions before birth to prevent collapse at home, as the duct can be kept open the drug prostaglandin, until the problem can be corrected.
3. The placenta supplies nutrition and oxygen as "red", oxygenated blood from the mother and also removes waste as "blue", less oxygenated blood. After birth the umbilical cord to the placenta is cut and baby is no longer reliant on mother's blood supply.
Note:
The fetal heart works at much lower pressure than the heart after birth and there is much more mixing of "red" and "blue" blood through the FO flap and duct. This mixing allows a baby to survive even if there is heart disease blocking a valve or a problem with the connections of the chambers and vessels.