Fetal circulation
The fetus receives blood via the umbilical vein. Once this enters the body, it travels through the liver as the ductus venosus before joining the inferior vena cava. It allows 80% of fetal blood to bypass the liver on return from the placenta.
- It closes functionally within minutes of birth, and structurally within a couple of weeks (may take longer in pre-term babies)
- Becomes the ligamentus venosum
The ductus arteriosus is a small connection between the pulmonary veins and the aortic arch. It allows blood to bypass the pulmonary circulation, and thus keeps blood pressure in the lungs lows
- The ductus arteriosus closes in response to high PO2. The PO2 high enough to close it only occurs once normal neonate breathing occurs. It usually closes fully after 1-2 days.
- When a baby takes the first breath, the lungs expand, forcing the pulmonary vessels to do the same. This drops the pressure in the pulmonary vessel sufficiency to allow blood to flow to the lungs (6x more than in the fetus). This then allows for oxygenation of the blood, and closure of the ductus arteriosus.
- The volume of blood returning to the right atrium also falls slightly, as the placenta is removed from circulation.
- Some babies with congenital defects rely on the ductus arteriosus to allow enough blood to reach the peripheral circulation. These babies are said to have duct-dependent circulation. In these babies, the clinical presentation is likely to worsen when the ductus arteriosus closes at about 1-2 days, sometimes later.
The foramen ovale is a hole in the atria, covered by a flap on the left side. In the fetus, the pressure in the left atrium is lower than in the right, and thus blood returning from the peripheral tissues can bypass the lungs, and pass from the right to the left atrium, to return to the peripheral tissues
- However, after birth, more blood goes to the lungs, as the ductus arteriosus shuts, and as a result more blood returns from the lungs into the left atrium – thus the pressure in the left atrium rises, and the valvular flap over the foramen ovale closes. Thus will initially be a functional closure, and over time, will become structurally permanent, and leave a depression in the wall of the atria, known as the fossa ovalis.
The three vessels described above are sometimes referred to as the fetal shunts.