Fetal circulation

Fetal circulation 

As a fetus’s lungs, kidneys and GI tract are not functional before birth, the fetus receives oxygen, nutrients and removes wastes via the maternal circulatory system. This is done through the placenta where the maternal blood and the fetus’ blood are bought in close contact. Due to a fetus’ different oxygen needs and supply, the fetal circulation is adapted such that unused parts of the body do not receive excess blood whist other areas are depleted. This is occurs via 3 shunts:

l          Ductus venosus

l          Foramen ovale

l          Ductus Arteriosus

1.          Two placental arteries bring deoxygenated blood to the placenta where wastes diffuse out and useful substance diffuses in. The blood then returns to the fetus via the placental vein.

2.          Oxygenated blood partly drains into the hepatic portal vein to supply the liver, however about half of it drains into the first shunt, the ductus venosus, and directly into the inferior vena cava.

3.          Deoxygenated blood from the general circulation mixes with the oxygenated blood in the inferior vena cava and the right atrium.

4.          As the lungs are not yet functional, most of this mixed blood is channeled to the left atrium to be pumped directly to the rest of the body rather than the pulmonary system. This occurs through the foramen ovale, found on the interatrial septum. The septum primum prevents blood from flowing back.

5.          Mixed blood is then pumped to the rest of the body, especially the brain and the carotid arteries.

6.          The blood which is not channeled to the left atrium flows into the right ventricles. This blood is usually slightly lower in oxygen as blood from the superior vena cava mainly drains here rather than through the foramen ovale. Here, a third shunt, the ductus arteriosus allows blood to bypass the pulmonary circulation and enter directly into the descending aortic arch. The high pulmonary vascular resistance means that the blood flows primarily through the ductus arteriosus into the aorta. The position of the shunt also means that this blood, which is slightly lower in oxygen, is not pumped to the brain or the heart.

The ductus arteriosus also prevents the right ventricle from pumping against high vascular resistance which may damage the heart leading to right ventricular failure.

7.          Blood then supplies the rest of the body and a proportion is pumped back into the umbilical arteries branching from the internal iliac arteries to the placenta.

Changes after birth

Two main events happen at birth leading to the closure of the three shunts. Firstly the baby’s first breath greatly decreases pulmonary vascular resistance, and umbilical blood flow ceases as the umbilica is clamped off.

When the umbilical cord is tied off, blood ceases to flow through the umbilical arteries, and they fill up with connective tissue. The distal portions become the medial umbilical ligaments. The complete closure of the artery may take two to three months.

The umbilical vein ceases to receive blood from the placenta and collapses. It becomes the ligamentum teres. The ductus venosus also ceases to receive blood and collapses to becomes the ligamentum venosum.

As pulmonary vascular resistance decreases, blood flow to the left atrium from the lungs increases. This in turn leads to an increase in pressure in the left atrium, forcing the valve of the foramen ovale closed. This closes permanently in around a year becoming the fossa ovalis.

After the first breath, the lungs secrete bradykinins which closes the ductus arteriosus by vasoconstriction. This becomes the ligamentum arteriosusm.

Failure of the ductus arteriosus to close leads to a patent ductus arteriosus (PDA). In this condition, additional blood flows from the aorta to the pulmonary arteries increasing the fluid flowing to the lungs. This increases lung pressures, and leads to shortness of breath (as the oxygenated blood is mixed with deoxygenated blood).