Fetal circulation
Also known as: prenatal circulation, transitional circulation, circulatory changes at birth
Before birth, the placenta, not the lungs, oxygenates the blood. Three shunts (the ductus venosus, foramen ovale and ductus arteriosus) send the best-oxygenated blood to the heart and brain and bypass the liver and the fluid-filled lungs.
Overview
The route of blood. Oxygenated blood leaves the placenta in the umbilical vein. This is the most oxygenated blood in the fetus, with a PO₂ of about 30–35 mmHg and a saturation of about 80%. Some of it perfuses the liver. The rest bypasses the liver through the ductus venosus into the IVC.
In the right atrium, the valve of the IVC and the edge of the septum secundum (the crista dividens) direct this oxygen-rich stream across the foramen ovale into the left atrium. From there it passes to the left ventricle and the ascending aorta, so the coronary arteries and the brain receive the best-oxygenated blood.
Deoxygenated blood returning in the SVC goes mainly through the tricuspid valve into the right ventricle and pulmonary trunk. Because pulmonary vascular resistance is very high in the fluid-filled, hypoxic lungs, most of this right ventricular output crosses the ductus arteriosus into the descending aorta. It then supplies the lower body and returns to the placenta through the two umbilical arteries. Only a small fraction of the combined ventricular output passes through the lungs.
Other adaptations. The fetus is helped by high-affinity HbF, a high cardiac output per kilogram and a high haemoglobin concentration (about 170 g/L at term).
Changes at birth. With the first breaths, the lungs expand and are oxygenated. Pulmonary vascular resistance falls sharply and pulmonary blood flow rises. More blood returns to the left atrium and its pressure rises. At the same time, clamping the cord removes the low-resistance placenta. Systemic resistance rises and IVC return falls, so right atrial pressure falls.
With left atrial pressure now higher than right, the septum primum is pressed against the septum secundum, functionally closing the foramen ovale. The rise in PaO₂ and the loss of placental prostaglandins make the ductus arteriosus constrict, closing it functionally within about 12–24 hours in term babies. The ductus venosus and the umbilical vessels also close. Fibrosis then turns these shunts into ligaments over the following weeks to months.
Parts & subdivisions
- One vein carries oxygenated blood to the fetus, and two arteries carry deoxygenated blood back to the placenta.
- Bypasses the liver, carrying blood from the umbilical vein to the IVC.
- Right-to-left shunt between the atria.
- Shunt from the pulmonary trunk to the aorta.
Function
- Delivers placental oxygen and nutrients preferentially to the heart and brain.
- Bypasses the non-functioning lungs and much of the liver.
- Changes, at birth, into the adult series circulation within minutes to hours.
Clinical significance
Persistent pulmonary hypertension of the newborn
If pulmonary vascular resistance fails to fall after birth, for example after meconium aspiration, sepsis or asphyxia, blood keeps shunting from right to left through the ductus arteriosus and foramen ovale. The baby is severely hypoxaemic. Treatment includes ventilation, inhaled nitric oxide and, in severe cases, ECMO.
Duct-dependent congenital heart disease
In some heart defects the baby's circulation depends on an open ductus arteriosus. Examples are hypoplastic left heart, critical coarctation, pulmonary atresia and transposition of the great arteries. When the ductus closes in the first days of life, the baby collapses or becomes cyanosed. A prostaglandin E1 infusion (alprostadil) keeps the ductus open until surgery.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.