The major partitions of the heart are completed by around week 7 of development.
Circulation through the fetus utilizes a number of shunts to bypass the right side of the heart, or pulmonary circuit as the lungs are filled with fluid and still developing. These shunts include:
- Ductus venosus - shunt to bypass the liver
- Foramen ovale - shunt between right and left atria to bypass right ventricle & pulmonary trunk
- Ductus arteriosus - shunt between pulmonary trunk and aorta to bypass lungs
Oxygenation of the fetal blood occurs in the placenta. This oxygenated blood is carried from the placenta to the fetus through the umbilical vein, shunted through the liver and into the IVC. Blood entering the heart through the IVC is shunted from right to left atrium, bypassing the pulmonary circuit.
At birth, pressure changes between the right and left sides of the heart cause closure of these shunts and the complete separation of the right and left sides of the heart and opening of the pulmonary circuit of the heart.
Remnants of the fetal vessels and evidence of the shunts can be found in the adult. Roll over the image
The vestiges of these fetal structures are:
Umbilical vein - ligamentum teres hepatis
Ductus venous - ligamentum venosum
Foramen ovale - fossa ovalis
Ductus arteriosus - ligamentum arteriosum
Umbilical arteries - medial umbilical ligaments
Circulation in the Fetus
Oxygenated blood is carried to the fetus from the placenta by the umbilical vein. This blood is shunted through the liver by the ductus venosus and into the inferior vena cava (IVC).
As the IVC opens into the right atrium it is in alignment with the foramen ovale so that oxygenated fetal blood mostly shunts past the right atrium through the foramen ovale and into the left atrium and left ventricle.
This blood, which is 80% saturated with oxygen as it enters the fetus, does mix with small amounts of deoxygenated blood as it passes to and through the heart and the fetus.
- In the liver with deoxygenated blood from the hepatic sinusoids
- In the inferior vena cava (IVC) with deoxygenated blood from lower limbs & abdominal organs
- In the right atrium with deoxygenated blood returning from head and upper limbs via superior vena cava (SVC)
- In the left atrium with deoxygenated blood returning from the fetal lungs
- In the aorta at the opening of the ductus arteriosus with blood from the right ventricle
In spite of this mixing, the level of oxygen saturation in the blood is still adequate to support the developing organs and tissues as it it pumped through the aorta out to the body.
Umbilical arteries return fetal blood to the placenta for oxygenation in the placental villi and completing the circuit.
Changes at Birth
At birth, once the newborn has taken its first breath, pressure in the left atrium becomes greater than that in the right atrium, causing septum premium to cover foramen ovale converting it to fossa ovalis.
Blood entering the right atrium via the superior and inferior vena cavae can now only go into the right ventricle and into the pulmonary circulation. Upon oxygenation in the lungs, the blood returns to the left atrium and left ventricle for distribution to all the body tissues via the aorta.
Since the umbilical cord is cut, the umbilical vein and umbilical arteries are clamped so they collapse. The umbilical vein becomes the ligamentum teres hepatis and the ductus venosus becomes the ligamentum venosum.
The umbilical arteries also atrophy to become the medial umbilical ligaments.
Within a few days postpartum ductus arteriosus obliterates to become the ligamentum arteriosum due to release of bradykinin from the lungs.
Clinical Correlates
Patent Ductus Arteriosus
Patent ductus arterioles (PDA) is a condition in which the ductus arteriosus fails to close leaving an abnormal communication between the arch of the aorta and pulmonary trunk.
Bradykinins from the lung, decrease in prostaglandins due to loss of the placenta and changes in gas levels in the blood usually prompt the closure of the ductus arteriosus shortly after birth though complete closure may not happen until 2.5 weeks after birth. Should this fail to occur, a patent ductus arteriosus (PDA) exists. Its presence may go undetected for many years, but at any point may result in clinical symptoms, such as shortness of breath or intolerance to exercise.
