Importance

• It is necessary in serving the fetus and to prepare the circulatory  system to function after birth

Three Important Parts of the Fetal Circulation

• Ductus Venosus
  • shunts a significant majority (80%) of the blood flow of the umbilical vein directly to the inferior vena cava
  • it allows oxygenated blood from the placenta to bypass the liver
  • in conjunction with the other fetal shunts, tit plays a critical role in preferentially shunting oxygenated blood to  the fetal brain
  • Foramen Ovale
    • it is one of two fetal cardiac shunts
    • allows blood to enter the left atrium from the right atrium, thereby allowing oxygenated blood to bypass the pulmonary system
    • Ductus Arteriosus
      • is a cardiac shunt connecting the pulmonary artery to the aortic arch
      • it allows most of the blood from the right ventricle to bypass the fetus’ fluid-filled lungs, protecting the lungs from being overworked and allowing the left ventricle to strengthen

The Fetal Circulation

1. Oxygen from the placenta travels to the umbilical vein bringing oxygen and nutrients.
2. Some of the blood flows to the hepatic circulation, others bypass the liver and pass through the ductus venosus.
3. The blood from the lower parts of the body together with the blood in the ductus venosus flows towards the inferior vena cava.
4. Then it goes to the right atrium
5. Some of the blood from the right atrium goes to the right ventricle via the tricuspid valve while others pass the foramen ovale leading to the left atrium.
6. From the left atrium, it goes towards the left ventricle, mixing with the poorly oxygenated blood from the lungs and then pumped towards the ascending aorta.
7. From the ascending aorta, the blood is pumped to the upper parts of the body like the heart, neck, head and upper limbs.
8. Then perfuse to the placenta via the two umbilical arteries.
9. Meanwhile the blood that enters the right ventricle (from No. 5) together with the poorly oxygenated blood from the head and upper extremities returns to the right side of the heart by the way of the superior venacava then, passes through the pulmonary artery wherein 10% enters the lungs, most of the blood bypasses the lungs which is then pumped to the ductus arteriosus going to the descending aorta.
10. The blood is the pumped and perfused to other parts of the fetus.
11. The blood then returns to the placenta via the two umbilical arteries.

Transition after Birth

1. The infant takes first breath, causing the mechanical expansion of the lungs. (Increasing uptake of oxygen by lungs induces a vasoconstriction of ductus venosus and ductus arteriosis.)
2. Rapid decrease in pulmonary vascular resistance (pulmonary vasodilation that is produced by initiation of ventilation) occurs.
3. The pressure in the pulmonary circulation and the right side of the heart fall as the fetal lung fluid is replaced by air and as lung expansion decreases the pressure transmitted to the pulmonary blood vessels.
4. With lung inflation, the alveolar oxygen tension increases, causing reversal of the hypoxemia-induced pulmonary vasoconstriction of the fetal circulation.
5. Clamping of the umbilical cord causes removal of the low-resistance placental circulation and produces an increase in systemic vascular resistance and left ventrical pressure, which further closes the ductus venosus.
6. The resultant decrease in right atrial pressure and increase in left atrial pressure produce closure of the foramen ovale.
7. A decrease in pulmonary vascular resistance and an increase in systemic vascular resistance causes a left to right shunting and increasing PO2 (constricts ductal smooth muscle) which further closes the ductus arteriosus.

Adult Derivatives of Fetal Vascular Structures

• Because of certain changes in the cardiovascular system at birth, certain vessels and structures are no longer required.
• Over a period of months these fetal vessels form nonfunctional ligaments, and fetal structures such as the foramen ovale persist as anatomic vestiges of the prenatal circulatory system.