Circulation through the heart

The fetal circulation through the heart can be divided into blood entering the right side of the heart from the:

• superior vena cava
• inferior vena cava

As the lungs are collapsed during fetal life, resistance through the pulmonary arteries is high and blood flow is small. Consequently, the pulmonary veins return relatively little blood to the left atrium. Conversely, the pressure within the right atrium is relatively high; this forces the septum primum to the left with flow of blood through the foramen ovale. The difference in pressure between the atria is reversed after birth.

As there is relatively little mixing of placental blood from the umbilical veins to the arch of the aorta, the head and upper limbs receive relatively oxygenated blood. This is facilitated by shunts such as the ductus venosus and the foramen ovale. Potential sites of mixing of saturated and desaturated blood include the:

• liver: mixing or umbilical vein and portal vein blood
• inferior vena cava: mixing of ductus venosus blood and blood from the peripheries e.g. the lower extremities
• right atrium: mixing superior and inferior vena cavae blood
• left atrium: mixing of right atrial and pulmonary vein blood
• descending aorta: mixing of blood from the proximal aorta (left ventricular blood) and ductus arteriosus