VENOUS SYSTEM 4 – Fetal Circulation Lecture – Anatomy Lectures – MBBS Lectures

Objectives:

By the end of this session, you should be able to :
1.ExplainFetalCirculationwithspecialreferencetodifferentshunts
2.Enumeratethesitesinthefetuswheremixingofoxygenated&deoxygenatedbloodoccur
3.ExplainNeonatalcirculationandidentifytheChangesinfetalcirculationafterbirthwhichinclude:
•Stoppage of placental blood flow
•Beginning of respiration
•Closure of prenatal shunts
•Closure of ductus arteriosus
•Closure of foramen ovale
•Closure of ductus venosus

Fetal Circulation

• In the fetus highly oxygenated (about 80% saturated with oxygen), nutrient-rich blood returns under high pressure from the placenta through the left umbilical vein, which delivers most of this blood to the ductus venosus of the liver.
• Blood flow through the DV is regulated by a sphincter mechanism close to the umbilical vein.
• Rest of this blood mixes with a small amount of deoxygenated portal blood and then enters the inferior vena cava, where it mixes with deoxygenated blood returning from the trunk and lower limb.
• The inferior vena cava delivers this blood into the right atrium, where it is divided by lower free margin of the septum secundum (crista dividens) into two streams:
• most of this well-oxygenated blood passes directly into the left atrium through the foramen ovale.
• The other stream of the inferior vena caval blood joins the deoxygenated blood returning from the head, neck and upper limbs through the superior vena cava and passes from the right atrium to the right ventricle.

FETAL CIRCULATION BEFORE BIRTH

• The oxygenated blood entering the left atrium through foramen ovale mixes with a small amount of deoxygenated blood returning from the lungs through the pulmonary veins.
• From the left atrium the blood passes into the left ventricle.
• This well oxygenated blood is then propelled into the aorta.
• The coronary, brachiocephalic, left common carotid and left subclavian arteries all receive highly oxygenated blood which is distributed to the heart wall, head and neck, and upper limbs.
• Liver also receives well-oxygenated blood from umbilical vein.
• The medium oxygenated blood entering the right ventricle (well-oxygenated fromIVC & poorly oxygenated from SVC & coronary sinus) is pushed into the pulmonary trunk.
• Approximately 10% of this blood goes to the lungs.
• As the resistance in pulmonary vasculature is high during intra-uterine life, most of the blood passing into the pulmonary trunk is shunted through the ductus arteriosus into the descending aorta where it mixes with the oxygenated blood coming from the proximal aorta.
• This medium oxygenated blood (35%) is then distributed to the abdomen, pelvis and lower limbs.
• After the descending aorta has distributed blood to the trunk and legs, the remaining blood (65%) enters the umbilical arteries (with 58% oxygen saturation) and returns to the placenta for oxygenation.

The places where mixing of blood occurs

• Liver
• Inferior vena cava
• Right atrium
• Left atrium
• Entrance of ductus arteriosus

Changes in Circulation after Birth

1. Stoppage of placental blood flow due to spontaneous constriction (or obstetrical clamping) of the umbilical blood vessels.

After closing down functionally, the umbilical vessels become actually fibrosed within two to three months after birth.

2. Beginning of respiration, due to which constricted pulmonary vessels open and resistance of pulmonary vasculature drops rapidly. Aeration of lungs at birth is associated with:

• Decrease in pulmonary resistance
• Increase in pulmonary blood flow
• Progressive thinning of walls of pulmonary arteries due to stretching of lungs with first few breaths

Closure of umbilical arteries

• Due to contraction of smooth muscles caused by:
• Thermal & mechanical stimuli
• Change in oxygen tension
• Functionally close a few minutes after birth
• Actual obliteration after about one year
• The fibrosed distal parts of the intra abdominal umbilical arteries form the medial umbilical ligaments; however, the proximal portions of these arteries remain patent and persist as superior vesical arteries.

Closure of umbilical vein

The umbilical vein also becomes converted into a fibrous ligament called round ligament of liver or ligamentum teres hepatis which lies in the free margin of the falciform ligament.

3). Closure of prenatal shunts in the fetal circulation after birth initially functionally and then structurally.

i)       Closure of the ductus arteriosus.

• The DA constricts at birth but there is often a small shunt of blood via DA from

aorta to pulmonary trunk for 24-48 hrs in a normal full-term infant.

• At the end of 24 hrs, 20% of duct is functionally closed, 82% by 48 hrs, & 100% at 96 hrs.
• In premature infants & in those with persistent hypoxia, the DA may remain open much longer.
• In full-term infants, oxygen is the most important factor in controlling closure of DA & appears to be mediated by bradykinin which is released from the lungs during initial inflation ( it has a potent contractile effect on smooth muscle).
• Action of this substance appears to be dependent on high oxygen content of blood in aorta (PO2 50 mm Hg).
• Transforming growth factor β (TGF β) is probably involved in anatomic closure of DA after birth.
• Effect of oxygen on smooth muscle may be indirect by its effect on PGE₂ secretion.
• Patency of DA before birth is controlled by lower content of oxygen in blood & by prostaglandins that act on smooth muscle in the wall of DA.
• The PGs cause DA to relax.
• Inhibitors of PG synthesis such as indomethacin, can cause constriction of a patent DA in premature infants.
•   Ultimately, the ductus arteriosus is converted into a fibrous cord called ligamentum arteriosum.

1. Closure of the foramen ovale.
2. It occurs due to reversal in pressure between the two atria.
3. As the pulmonary circulation begins, sudden increase in the pulmonary blood return raises the pressure in the left atrium.
4. On the other hand, cessation of umbilical flow and opening of pulmonary vasculature reduce the pressure in the right atrium.
5. The higher pressure in the left atrium presses the flexible septum primum against the relatively rigid septum secundum, functionally closing the foramen ovale.
6. During the first days of life, this closure is reversible; crying of the baby creates a shunt from right to left, responsible for cyanotic period in the newborn.

•        Constant apposition leads to fusion of the two septa in about 1 year.

1. Closure of the ductus venosus.
2. This channel closes within a few hours after birth.
3. Later, it become fibrosed and forms the ligamentum venosum which lies in a fissure on the visceral surface of the liver.

• The right ventricular wall is thicker in fetuses & new-born infants because right ventricle is working harder in utero.
• By the end of first month, the left ventricular wall thickness is greater than the right because the left ventricle is now working harder.